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Substituent effects on the rates of pyridinium ylid formation

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Title:
Substituent effects on the rates of pyridinium ylid formation
Creator:
Cross, Robert Edward, 1942-
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English
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xi, 98 leaves. : illus. ; 28 cm.

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Subjects / Keywords:
Pyridinium compounds ( lcsh )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis--University of Florida, 1971.
Bibliography:
Bibliography: leaves 94-97.
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Manuscript copy.
General Note:
Vita.

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University of Florida
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University of Florida
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Copyright Robert Edward Cross. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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030437427 ( ALEPH )
17015278 ( OCLC )

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Full Text
Substituent Effects on the Rates
of Pyridinium Ylid Formation
By
ROBERT E. CROSS
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA IN PARTIAL
FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA 1971




To My Wife, Eileen Kay




ACKNOWLEDGEMENT
The author will always be indebted to Dr. John A.
Zoltewicz, Chairman of his Supervisory Committee, for his enthusiastic guidance, patience, and support throughout the course of this research. Appreciation is also extended to the other members of his Committee, Dr. M. A. Battiste, Dr. W. R. Dolbier, Jr., Dr. R. C. Stoufer, and Dr. E. G. Sander.
A particular debt of gratitude is due his wife, Eileen, for her unfailing patience, understanding, and encouragement during these years of long days and nights.
The friendship and assistance of his fellow graduate students will always be gratefully remembered.
Special thanks are extended to Mrs. Judi Curtin for her patience during the typing of this dissertation.
Financial support from the Chemistry Department and
Graduate School of the University of Florida is gratefully acknowledged.
iii




TABLE OF CONTENTS
Page
ACKNOWLEDGEMENT ........................................ iii
LIST OF TABLES.......................................... vi
LIST OF FIGURES........................................ viii
ABSTRACT.............................................. x
CHAPTER
1. INTRODUCTION.................................... 1
2. RESULTS AND DISCUSSION......................... 5
Hydrogen-Deuterium Exchange at the 2- and 6Positions of 3-Substituted-l-Methylpyridinium
Ions............................................ 5
Hydrogen-Deuterium Exchange at the 2- and 6Positions of 3-Substituted Pyridines......... 27
3. EXPERIMENTAL..................... ............. 41
Instrumentation............................... 41
Chemicals...................................... 41
Stock Solutions............................... 41
Substrates.................................... ... 43
Pyridinium Salts.............................. 45
Pyridinium Betaines........................... 50
Preparation of Solutions for Kinetic Runs.... 50 Kinetic Runs................................... 51
Kinetics of Hydrogen-Deuterium Exchange in
3-Substituted-1-Methylpyridinium Ions........ 55
Kinetics of Hydrogen-Deuterium Exchange in
3-Substituted Pyridines....................... 77
iv




Page
pD Measurements .............................. 89
Control Runs ............................ ..... 92
BIBLIOGRAPHY .......................................... 94
BIOGRAPHICAL SKETCH ................................... 98
v




LIST OF TABLES
Table Page
1. Rate Constants for the Deuteroxide Ion-Catalyzed
Formation of 3-Substituted-l-Methylpyridinium
Ylids in Deuterium Oxide at 75.00.50............... 11
2. Resonance and Inductive Constants for the
Sulfonate and Carboxylate Substituents in Weakly
Protonic Solvents................................. 24
3. Relative Rates for H-D Exchange at the 2- and
6-Positions of Pyridine and 3-Chloropyridine in
3:17 (V:V) Dioxane-D20 at 197.50.50............. 29
4. Observed Dependence of the Rate of H-D Exchange
(197.50.50) at the 2- and 6-Positions of
Partially Neutralized 3-Chloropyridine on the
Degree of Neutralization... ..................... 33
5. pKa Values (250) for the Conjugate Acids of
Pyridine and 3-Chloropyridine................... 34
6. Chlorine Rate Factors............................. 34
7. Substituent Effects on Hydrogen-Deuterium
Exchange at the 2- and 6-Positions of a Series of
3-Substituted-l-Methylpyridinium lIons and
3-Substituted Pyridines........................... 38
8. Experimental Data for the Preparation of a Series
of 3-Substituted-l-Methylpyridinium Salts........ 46
9. Rates of H-D Exchange at the 2-Position of
3-Substituted-l-Methylpyridinium Ions in D20 at
75.00.50 and 1.0 Ionic Strength................. 56
10. Rates of H-D Exchange at the 6-Position of
3-Substituted-l-Methylpyridinium Ions in Da0 at
75.00.5* and 1.0 Ionic Strength................. 59
11. Kinetic Data for H-D Exchange at the 2- and
6-Positions of 1-Methylpyridinium-3-Oxide Betaine
in KOD Solution at 100.00.50 ...... ............ 72
vi




Table Page
12. Rate Constants for H-D Exchange at the 2- and
6-Positions of 1-Methylpyridinium and 3-Chloro1-Methylpyridinium Ions in 0.030M DC1 at
197.50.5 ...... ................................ 76
13. Rates of H-D Exchange at the 2- and 6-Positions
of 3-Substituted Pyridines at 197.50.50 ........ 78
14. Rates of H-D Exchange at the 2- and 6-Positions
of 3-Substituted Pyridines at 217.90.50 ........ 84
15. Rate Constants for Hydrogen-Deuterium Exchange
at the 2- and 4-Positions of Quinazoline in D20
Solution at 164.70.50.............. ............ 90
vii




LIST OF FIGURES
Figure Page
1. Mechanism for Base-Catalyzed Exchange at the
2- and 6-Positions of 3-Substituted-l-Methylpyridinium Ions..................................... 8
2. Plot of log k2 for H-D Exchange at the 2-Position
of 1-Methylpy idinium-3-Sulfonate Betaine at
75.00.50 vs pD..................................... 9
3. Taft Plot for H-D Exchange at the 2-Position of
3-Substituted-1-Methylpyridinium Ions in Buffered
D20 Solution at 75.00.5 ......................... 13
4. Correlation of H-D Exchange Rates at the 2Position of 3-Substituted-l-Methylpyridinium Ions
in Buffered D20 Solution at 75.00.50 by the
Extended Hammett Equation......................... 15
5. Correlation of Exchange Rates at the 6-Position
of 3-Substituted-l-Methylpyridinium Ions in
Buffered D20 Solution at 75.00.50 with the Oo
Substituent Parameter................................ 16
6. Correlation of H-D Exchange Rates at the 6Position of 3-Substituted-l-Methylpyridinium Ions
in Buffered D20 Solution at 75.00.50 by the
Extended Hammett Equation......................... 17
7. Correlation of Amide Ion-Catalyzed Rates of
Exchange at the para-Positions of Monosubstituted
Benzenes in Liquid Ammonia by the Extended
Hammett Equation.................................. 21
8. Relative Rates of Exchange at the 2- and 6Positions of l-Methylpyridinium-3-Oxide Betaine
and l-Methyl-4-Pyridone in OD /D20 at 100.00 ..... 26
9. Proposed Pathway for Base-Catalyzed HydrogenDeuterium Exchange at the 2,6-Positions of
1-Methyl-4-Pyridone............................... 27
viii




Figure Page
10. Proposed Mechanism for H-D Exchange at the 2and 6-Positions of 3-Substituted Pyridines in
Neutral D20 Solution............................ 28
11. Positional Rate Ratios for H-D Exchange in
3-Chloropyridine (D20) and 3-Chloro-l-Methylpyridinium Ion (0.030M DC1) at 197.5+0.50 ...... 31
ix




Abstract of Dissertation Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
SUBSTITUENT EFFECTS ON THE RATES
OF PYRIDINIUM YLID FORMATION
by
Robert E. Cross
June, 1971
Chairman: Dr. John A. Zoltewicz Major Department: Chemistry
Rates of hydrogen-deuterium exchange at the 2- and 6positions of a series of 3-substituted-l-methylpyridinium ions in buffered D20 solutions at 75.00.50 were obtained by the use of an nmr method in order to determine the effects of substituents on the rates of formation of the corresponding ylid intermediates. Deprotonation is catalyzed by deuteroxide ion. Substituents are observed to exert a profound influence on reactivity; rate spreads of 10' and 10' were obtained for the 2- and 6-positions respectively. Rates of exchange at the 2- and 6-positions are-correlated by the extended Hammett equation. From an evaluation of the data by this treatment, it is concluded that both inductive and resonance effects of the substituents influence reactivity. The inductive effect is found to predominate at position 2; this effect is diminished, while the resonance effect assumes a more important role at position 6.
X




Rates of hydrogen-deuterium exchange at the 2- and
6-positions of a series of 3-substituted pyridines in D20 or 3:17 (V:V) dioxane-D20 were obtained at 197.50.50 or 217.90.50 by the use of this nmr method in order to determine the effects of substituents on the exchange rates in the free bases. A two-step mechanism, involving an equilibrium protonation of nitrogen followed by a ratedetermining C-H ionization, was demonstrated for these reactions. The rate spreads encountered are smaller than those observed for exchange in the 3-substituted-1-methylpyridinium ions. The results are explained in terms of the effects of substituents on each step of the two-step mechanism.
xi




CHAPTER 1
INTRODUCTION
Substituent effect studies on the rates of basecatalyzed hydrogen-deuterium exchange reactions have been reported for several aromatic systems. Several groups of workers have independently studied the rates of amide ion catalyzed H-D exchange at the various positions of monosubstituted benzenes in liquid ammonia.1,2,3 Correlations
of exchange rates at the ortho and meta positions of these substrates were found with the Taft al substituent parameters and the Hammett am substituent constants respectively. I Exchange rates at the para positions of-these compounds were not correlated by the Hammett a parameters.' It
P
has recently been shown, however, that meta and para reactivities in the benzene series are best correlated by the a m* and a p
m p
substituent constants respectively.
Rates of isotope exchange reactions at the 4-positions
of a series of 3-substituted pyridines ( 1 ),5 the 2-positions of 3-substituted-pyridine-l-oxides ( 2 ),4 6 7 and the 2-positions of 3-substituted thiophenes ( 3 )8 were, in each case, found to be correlated by the Taft a, parameters. Methoxide ion catalyzed rates of exchange at the S-position
1




2
D
O G G
0"
N N S D
I+
O'
1 2 3
G
D S D G
4 5
of 3-substituted thiophenes (4)., and at the 5-position of 2-substituted thiophenes (5), were found to be correlated by the am o and a o parameters respectively.8
It should be pointed out, however, that in the case of the amide ion catalyzed exchange studies in liquid ammonia, competing side reactions were enountered in several instances,I some of the datawere not reproducible,9 and the number of substituents investigated was limited.' Furthermore, the methoxide ion catalyzed exchange studies in methanol often involved temperature extrapolations because of the large reactivity span encountered.
All of these substituent effect studies have been




3
interpreted in terms of a mechanism involving simple deprotonation by base at an annular position of an aromatic carbon acid.
Hydrogen-deuterium exchange in a number of heterocycles has also been studied in buffered D20 solution. Zoltewicz and Smith have demonstrated that, under these conditons, pyridine undergoes exchange at the 2,6-positions by a mechanism involving an equilibrium protonation on nitrogen, followed by a rate-determining C-H ionization.o This mechanism has also been demonstrated for exchange at the 2-positions of thiazole12 ( 6 ) and imidazole2 ( 7 ), and at the 3,5positions of pyrazole'3 ( 8 ) under similar conditions.
N QD
D D D
\ H
H
6 7 8
The effects of substituents on this two-step mechanism have, however, never been determined.
The study reported in the first part of Chapter 2
was carried out in order to determine the effects of substituents on the rates of hydrogen-deuterium exchange (pyridiniumn ylid formation) at the 2- and 6-positions of a series of 3-substituted-l-methylpyridinium ions in buffered D20 solutions. Under these conditions, previous preliminary work indicated that large reactivity spreads could be expected14,15 which could be conveniently investigated at a




4
constant temperature. An obvious advantage of employing D20 as the solvent is that the ultimate goal in these and similar studies is the determination of carbon acidity in aqueous media.
The study reported in the second section of Chapter 2
was carried out in order to demonstrate the operation of the two-step mechanism for exchange at the 2- and 6-positions of a series of 3-substituted pyridines in D20 solution, and to determine the effects of substituents on the exchange rates in terms of this mechanism. Since a pyridinium ion is a proposed intermediate in the two-step mechanism, it was hoped that the information obtained from the study on the substituted pyridinium ions would prove useful in interpreting the effects of substituents on the rates of exchange in the free bases, where the two-step mechanism would presumably operate.




CHAPTER 2
RESULTS AND DISCUSSION
Hydrogen-Deuterium Exchange at the 2- and 6-Positions of 3-Substituted-1-Meth_ypridinium Ions
The kinetics of 1I-D exchange at the 2- and 6-positions of a series of 3-substituted-l-methylpyridinium ions (9, Figure 1) in buffered D20 solutions of 1.0 ionic strength were measured at 75.00.50. The exchange reactions were followed by measuring the change in the integrated area of the appropriate nmr signals. The nmr spectra of most 3-substituted pyridines have been reported, and all proton signals assigned.16 Signals for those substrates for which spectra have not been reported were assigned by analogy, and the assignments supported by the changes in appearance of the spectra during exchange. Signals for the annular protons of 3-substituted-l-methylpyridinium ions are broadened and shifted downfield to varying degrees, relative to the signals for the annular protons of the free bases.17,18 Otherwise, the spectra are similar!'18
Attempts to follow the rate of exchange at the 6-position
of several of the substrates where G was an electron-withdrawing group met with failure. Hydrolysis of the nitrile group of the 3-cyano-l-methylpyridinium ion has been reported,19 and accounts for the lack of success with this substrate.
The 3-nitro-l-methylpyridinium ion apparently undergoes
5




6
a reaction with deuteroxide ion resulting in the formation of a product, the nmr spectrum of which suggests that it is either 3-nitro-1-methyl-4-pyridone (10) or 5-nitro-l-methyl2-pyridone (11). The latter possibility appears to be the most
0
NO2 NO2
N O
Me Me
10 11
likely on the basis of the rather large meta coupling constant (~3Hz) observed in the nmr. Coupling is not expected to be nearly so strong in the case of 10,where a nitrogen atom is positioned between the carbon atoms bearing the two interacting nuclei.16 Such products could conceivably result from nucleophilic attack by deuteroxide ion on the pyridinium ring, followed by oxidation of the resulting pseudo-base.20
The 3-acetyl-l-methylpyridinium ion reacts at room tempemrature with basic buffer. Possible side reactions include aldol condensation as well as attack by enolate anion on a pyridinium ring to give stable adduct such as 12 or 13.21
Attempts at measuring the rate of exchange at the
6-position of 3-methylsulfonyl-l-methyipyridinium iodide resulted in curved kinetic plots. The pD of the reaction mixture decreased steadily with time, but no extraneous peaks were apparent in the nmr spectrum.




7
0 0
Q H, H
C~z / CCs
CH3 7C
o HC
00 CH 3
12 13
During the remainder of the kinetic runs using deuterium
oxide and during control runs using proteo water, no appreciable degradation of substrate could be detected.
Results of these kinetic studies are reported in Tables 9 and 10. Under the conditions employed, no other annular
protons underwent detectable exchange.
1-Alkylpyridinium ions undergo base-catalyzed hydrogendeuterium exchange by simple deprotonation to give intermediates such as 14 and 15 (Figure 1).14,1s,22 28
Because the base concentration remains constant during a given run, the kinetics are pseudo-first order. The rates of deprotonation are then described by the equation
kg [PyrCH ] = k2[PyrCH3+]aODwhere k@ is the observed pseudo-first order rate constant. The second order rate constant k2 may then be calculated from kg and the activity of deuteroxide ion as determined from
pD measurements at 75.00.50.




G
CH
0-0
H 'N + H N
H +
I I
Me Me
G 14
O + OD
H NN H
Me G
9 00
N N~ H
+ D I+
Me Me
15
Figure 1. Mechanism for Base-Catalyzed H-D Exchange at the 2- and 6-Positions of 3-Substituted-l-Methylpyridinium Ions.
Several of the substrates were studied at two or more pD values; in these instances, exchange was demonstrated to be first order in deuteroxide ion. For example, a plot of log k vs pD for the three kinetic runs on 1-methylpyridium3-sulfonate betaine"9 resulted in a straight line with a slope of 1.05 (Figure 2 ).
It is also apparent (Tables 9 and 10 ) that, in the
several cases studied, any catalysis by buffer is marginal. This is seen from a comparison of the second order rate constant k2 with [DA] or [A], the concentration of buffer acid




9
so
10
0
u
5 0 -j
Y
LO
C:) Slope=l. 05
1.0
0 5
7.00 7,50 8.00 8,50
PD Figure 2. Plot of log k 2 for H-D Exchanae at the 2-Position
of 1-Methyl Py idiniu'-3-Sulfoonate Betaine at
75.00.5* vs PD.




10
or base. Moreover, undissociated water does not appear to act as a kinetically detectable base in these cases. It is likely that the Bronsted 8 value for these reactions should therefore be ~1.14,3o
The results reported in Table 1 indicate how substituents of 3-substituted-l-methyl pyridinium ions influence the rates of ylid formation at the 2- and 6-positions (Figure 1 ). Here the effects of substituents bonded to a carbon atom are on ortho and para centers. The effects on reactivity are observed to be large, with the 2-position being the more sensitive. This follows from a consideration of the kG/kH ratios, which compare the effect of a substituent relative to the effect of hydrogen. The 2-position is deactivated by
0 Me, and CO2 substituents, while activated by ND2, MeO, S03 I, Cl, CN, and NO2 groups. The 6-position is deactivated by 0 Me, MeO, and ND2 substituents, while activated by CO2 S03 I, and Cl groups. As previously mentioned, the effect of CN and NO2 groups on the reactivity of the 6-position was not obtained, owing to side-reactions during hydrogen-deuterium exchange studies.
If the effects of one'substituent acting on both positions
2 and 6 are compared in Table 1, it is evident that all possible combinations of activation-deactivation are found. Thus, substituents which activate both 2- and 6-positions include SOs I, and Cl; substituents which deactivate. both positions include 0 and Me. The only group which deactivates
2 and activates 6 is CO2 Groups which activate 2 and deactivate 6 include ND2 and OMe.




Table 1 Rate Constants for the Deuteroxide Ion-Catalyzed Formation of 3-Substituted1-Methylpyridinium Ylids (14 and 15) in Deuterium Oxide at 75.0+0.50.
k2, M-' sec-I
3-G 2-Position 6-Position kG/kH2' kG/k k2/k6
-H 1.99 x 10-1 1.99 x 10- 1.00 1.00 1.00
-0-a [2 x 10-4] [2 x 10-4] [1 x 10 -'] [1 x 10-] ~1
-Me 8.87 x 10-2 8.87 x 10-2 4.45 x 10' 4.45 x 10-' -1
-CO2 9.90 x 10 2 3.22 x 10-' 4.98 x 10' 1.62 3.08 x 10-1
-ND2 2.76 x 10 1.23 x 10 1.39 6.19 x 102 2.24 x 10'
-OMe 1.23 x 101 1.64 x 102 6,19 x 101 8.25 x 10- 7.50 x 101
-SO3 4.24 x 101 2.79 2.13 x 102 1.40 x 101 1.52.x 101
-I 8.85 x 101 2.25 4.45 x 102 1.13 x 101 3.93 x 10'
-Cl 5.25 x 102 2.30 2.64 x 103 1.16 x 101 2.28 x 102
-CN 1.05 x 104 -- 5.28 x 104 -- --NO2 1.87 x 10s -- 9.40 x 10s .
uncertain value




12
A correlation exists between log k' and the Taft inductive parameter aI, obtained from F19 shielding data for meta-substituted fluorobenzenes in weakly protonic solvents (aI values for some polar groups are solvent dependent).31 This is shown in Figure 3. The least-squares slope p, of this plot is 9.4, with a correlation coefficient r of 0.974 and a standard deviation of the slope of 0.7.32 The standard deviation of the intercept is 0.8.32 Values of aI for the Cl and I substituents were not available from the fluorine nmr data.31 Accordingly, the values +0.46 (Cl) and +0.39 (I), obtained from Taft's latest compilation based on chemical reactivities, were employed.33 This appears to be justified by the fact that the aI values for the F and Br substituents are independent of their source (chemical reactivity data, or fluorine nmr data in a variety of solvent systems).31 The value of -0.35, tabulated in Taft's paper as the a constant for the CO2 substituent in weakly protonic solvents, appears to be in error.31 A recalculation of the chemical shift data indicates that this value should be -0.05 instead.
The data for exchange at the 2-position may also be considered in terms of the extended Hammett equation
G kH
log k2/k2 =piaI + PR'Ro,
which separates the effects of substituents into their inductive (piai) and resonance (PRGRo) components.34,3s This four-parameter equation may be applied to the data more conveniently when it is expressed in the form
(log kG/k)/aRo = pI(aI/R) + PR'




13
12.0
0 NO
10.0 CN0
oci
8.0 01
So3 0
0
OOMe
6.0
HO 0 ND2 P1 = 94
me 0oo
r = 0.974
4.0
00
2.01 L.. __ ____-0.20 0.00 +0,20 +Q1,40 *Q.,60
a'
Figure 3. Taft Plot for H1-D Exchange at the 2-Position
of 3-Substituted-1-M',ethylpyridinium Ions in
Buffered D20 Solution at 75.00.50.




14
G H)/
A plot of (log k2/k2 )/R vs GI/oR should then resultinastraight line, from which the intercept gives PR and the slope pi.16
Such a plot for exchange at the 2-position of the
various substrates is illustrated in Figure 4 The leastsquares slope p, of this plot is 8.27; the intercept PR is
1.94. The success of this equation in correlating the data is indicated by a correlation coefficient of 0.998; the standard deviations of the slope and intercept are 1.5 and
0.2 respectively.32 The sulfonate and carboxylate substituents were omitted from the analysis because of the lack of tested OR0 values for these groups.
The data for exchange at the 6-position of 3-substituted1-methylpyridinium ions give a poor correlation with the a (r=0.928) or on (r=0.929) substituent constants.32,35'37 p p
An excellent correlation.(Figure 5 ) is obtained, however, with the a o0 parameter.s. The least-squares slope p of this
p
plot is 3.39; the correlation coefficient r is 0.993.2 The standard deviations of the slope and intercept are 0.1 and 0.2 respectively.32 The oxide substituent was eliminated from all of the plots, because the appropriate substituent constants for this group are not known. For this reason, the SOs substituent was also eliminated from the on and a o plots, p p
as well as the CO2 substituent from the a o0 plot.
The data for exchange at the 6-position may also be correlated by the extended Hammett equation. This is illustrated in Figure 6 The least-squares slope p, of this plot is 3.84; the intercept PR is 2.91.32 The correlation




40 NO20
OCN
30
20
10
Oo
0 e
0 OND2 PI = 8.27
p, = "1.94 OOMe PR r 0. 998
-10
0 Cl
Io
-20
-2 0 +2 +4 +6
Figure 4. Correlation of H-D Exchange Rates at the 2-Position
of 3-Substituted-l-Methylpyridiniumn Ions in
Buffered D20 Solution at 75.0+0.50 by the Extended
Hammett Equation.




16
+1.0
C
I
0.0
0 H
oMeO 0
-1.0 C/
Me p = 3.39
r = 0.993
-2.0 () ND2 L
-0.40 -0.20 0.00 +0.20
P
Figure 5. Correlation of Exchange Rates at the 6-Position
of 3-Substituted-1-Methylpyridinium Ions in
Buffered D20 Solution at 75.0+0.50 with the a o
Substituent Parameter.




17
0o
+4.0
0 Me ND 20
+2.0
Q OMe
0.0
0~ 0
0
-2.0
pI = 3.84
-4.0 PR = 2.91
Cl r = 0.987
-6.0
I
-2.0 -1,0 0.0 +1.0
0I/OR*
ar /aoRo Figure 6. Correlation of H-D Exchange Rates at the 6-Position
of 3-Substituted-l-Methylpyridinium Ions in Buffered
D20 Solution at 75.00.50 by the Extended Hammett
Equation.




18
coefficient r is 0.987, and the standard deviations of the slope and intercept are 0.8 and 0.3 respectively.32
The correlation between Taft's I values and the rates of deuteroxide ion catalyzed hydrogen-deuterium exchange at the 2-positions of 3-substituted-l-methylpyridinium ions (pi=9.4, 75.00.50), Figure 3 as well as that between the op0 substituent parameter and the corresponding rates of exchange at the 6-positions of these ions (p=3.4, 75.00.50) Figure 5, provides additional strong evidence for the formation of pyridinium ylid intermediates 14 and 15 during exchange. Furthermore, the effects of these substituents are large.
The reactivity of these ions may be considered to be
influenced both by the 3-substituent itself, acting on ortho and para reactive centers in the cases of exchange at the 2- and 6-positions, respectively, as well as by the positively charged nitrogen atom. A measure of the activating effect of the positively charged nitrogen atom may be obtained from a comparison of the reactivity of the pyridinium ion 16
I+
D
16
with that of benzene. Calculations, which involve several extrapolations and the neglect of small temperature and solvent differences, have been performed which indicate that the positively charged annular nitrogen atom activates an aromatic ring for deprotonation via ylid formation by an enormous factor of approximately 1014 to 1016 14




19
In spite of the very large activation by the positively charged nitrogen atom, substituents at the 3-position also exert a large influence on the rates of anion formation. The rate spread for exchange at the 2-position between substrates having NO2 and 0 substituents, which represent the extreme cases for this position, is a factor of approximately 10. The corresponding rate spread for the 6-position, where the SO3 and 0 substituents represent the extremes of reactivity, is a factor of approximately 104.
The exchange rates at the 2- and 6- positions of 3-substituted-l-mnethylpyridiniumn ions are explicable in terms of a reaction involving the generation of a considerable amount of negative charge. Substituents are observed to exert a profound effect on thc stability of this negative charge in the transition and intermediate.states. It seems likely that substituents exert approximately a constant effect on the positive charge throughout the reaction.
In the case of exchange at the 2-position, correlation of the rate data with Taft's a, parameters indicates that effects of substituents on the stability of the negative charge at this position are largely inductive in nature. Figure 4 illustrates, however, that a better correlation of the data results when the extended Hammett equation is employed. These results indicate that approximately 20 percent of the total substituent effect is due to a resonance component. Furthermore, this observation-does not appear to be unique. Similar treatments of data available from the




20
literature concerning the effects of ortho substituents on the rates and equilibria of reactions involving the a electron framework of aromatic systems, for which correlations with SI have been obtained, illustrate the operation of this small, but persistently detectable resonance effect. For example, amide-catalyzed rates of deprotonation at ortho positions in the benzene series in liquid ammonia are correlated by the four-parameter equation (Figure 7 ), and show a 12 percent resonance effect.' Similarly, substituent effects on the rates of H-D exchange at the 2-positions of 3-substitutedpyrazine-l-oxides in D20 at 310, 6 as well as on the acidities of 2-substituted pyridinium ions in H20,38 contain a resonance component comprising 16 percent of the total effect in each case, as determined from this treatment. However, exchange rates at the 2,6-positions of 1-substituted pyridinium ions, where the substituents are bonded to the positively charged nitrogen atom, appear to be governed only by an inductive effect. 14
Rates of exchange reactions at the 6-positions of the 3-substituted-l-methylpyridinium ions are found to be correlated by the a o substituent constants (Figure 5 ).as
P
As previously mentioned, this set of substituent constants has also been used to correlate the rates of amide-catalyzed deprotonation reactions at the para position in the benzene series in liquid ammonia.
Furthermore, Swain and Lupton have presented a treatment of substituent effects, analogous to the extended Hammett




21
+6,0Q CF3
+4.0
Oo
0
o Me
/
0.0- /ONMe2
4)o 0 OMe I)"p = 13.1
OOl~
P R = 1.75
-2.0 F
1 1' !
-2.0 0.0 +2,0 +4,0 +6,0
Figure 7. Correlation of Amide Ion-Catalyzed Rates of Exchange
at the para-Positions of Monosubstituted Benzenes
in Liquid Ammonial by the Extended Hammett Equation.




22
equation, in which a large number of substituent constants are broken down into their respective field (inductive) and resonance components.39 Their analysis suggests a 422 percent resonance component for the ap substituent parameter, a result consistent with the 43 percent resonance component calculated from treatment of the data for exchange at the 6-position by the extended Hammett equation (Figure 6).
The correlation with a0p is also consistent with the
nature of these substituent constants, which are supposed to reflect the absence of direct conjugation between the substituent and the reaction center.3s This is expected to be the case for deprotonation of annular carbon acids, where transition and intermediate states involve the formation of essentially localized electron pairs on carbon which are part of the a and not the fr electron framework. The poor correlation of the data for exchange at position 6 with the an substituent
P
parameters probably results from an overcompensation for strong donor groups, such as NH2. This has presumably arisen since, although aniline reactions were omitted, phenol and thiophenol reactions were employed to derive a Ps For p"
other substituents, the a and n values are very similar.
p p
From the above analysis, it may be concluded that, in the case of exchange at the 2-positions of 3-substituted-I methylpyridinium ions, the effect of the 3-substituent ortho to the reaction center is to influence the kinetic acidity of this position primarily by an inductive effect; a small




23
resonance effect, however, is clearly present. Analysis of data for related systems supports this conclusion. The relative importance of the resonance effect increases, however, at position 6, while the inductive effect falls off with increasing distance between reaction center and substituent. This conclusion results from a comparison of the PR aid pI values in the appropriate equations:
GH
(ortho) 2-Position: log k2/k'2 = 8.27aI + 1.94aRO G H
(para) 6-Position: log k6/k6 = 3.84aI + 2.91aR Rates of exchange at the 6-position are also correlated by
G H
log k6/k6 = 3.39 a p.
p
From the data given in Tables 9 and 10 and these correlation equations, the effects of other substituents on the rates of pyridinium ylid formation may be predicted with confidence.
The a I and OR* values for the S03 and CO2 substituents are not included in the basic sets of these values, as given by Taft.33 Because these substituents are charged, these values are also expected to be solvent dependent.31,40 From the known rates of exchange at the 2- and 6-positions of the corresponding substrates, the two four-parameter equations given above may be solved for the appropriate substituent parameters. Such calculations give aI and ORo values for these two substituents which are in fair agreement with those calculated from F" chemical shift or infrared intensity data in weakly protonic solvents (Table 2 ). 31,40,41
It should be noted that the mechanism of the




24
Table 2. Resonance and Inductive Constants for the Sulfonate
and Carboxylate Substituents in Weakly Protonic
Solvents.
-X o (calcd)a OR (calcd)a 0I aR0
-CO2 -0.08 +0.17 -0.05b +0.11c
-S03- +0.27 +0.04 +0.25b +0.07c
0. 00d
calculated from rate data and correlation equations for exchange at the 2- and 6-positions of 3-substituted-l-methylpyridinium ions.
bCalculatd from data in Ref. 31. CCalculated from data in Ref. 40. dRef. 41.




25
above exchange reactions has been considered in terms of simple deprotonation at carbon by base. However, it may well be that the rates of deprotonation of these pyridinium ions are influenced by internal return. That is, the rate of backprotonation of the hydrogen-bonded carbanion may favorably compete with the rate of replacement of proton by deuteron from solvent at the carbanion site. Rates of hydrogen exchange then do not solely reflect rates of deprotonation, and the pseudo-first order rate constants k are composites of rate constants for deprotonation, back-protonation, and separation of the hydrogen-bonded complex. If this is so, then the linear free energy relationship found for the deprotonation of these pyridinium ions assumes added importance. The correlation shows not only the effects of substituents, but also the effects of internal return on reactivity. 14
Furthermore, as mentioned previously, it seems likely that the a value in the Bronsted relationship
log k2= 8 log Ka + B
for these deprotonation reactions is large and close to unity, because of the lack of appreciable buffer catalysis.3 If this is the case, then the following relationship
G H = lo KH
log k2/k2 = log K /KH
holds, and the hydrogen exchange rate constant ratio is the equilibrium acidity constant ratio. A determination of K
a
for one member of the series then amounts to a determination of K for them all.
a




26
The kinetics of hydrogen-deuterium exchange at the 2and 6-positions of l-methylpyridinium-3-oxide betaine were determined at 100.00.50 in 0.06M and 0.30M KOD. The experimental data are reported in Table 11ii. Both positions were observed to undergo exchange at approximately the same rate. A comparison of the values obtained from the two runs for k 6 (Table 11), the second order rate constant based on the activity of deuteroxide ion at 250, illustrates the fact that the exchange reaction is first order in deuteroxide ion. A comparison of the more accurate value (see Experimental) for k2'6 the second order rate constant based on the calculated concentration of deuteroxide ion, obtained from the run in 0.30M KOD (Table 11), with the corresponding second order rate constant reported by Beak and Bonham for exchange at the equivalent 2,6-positions of l-methyl-4-pyridone (17, Figure 8 ) reveals that, interestingly enough, exchange in the pyridinium ion is only 2.6 times faster than in the pyridone.25
0
11 0
N H(1) (~2.6) H N+ H (-2.6)
17 J I
~~ Me Me
Figure 8 Relative Rates of Exchange at the 2- and 6Positions of 1-Methylpyridinium-3-Oxide Betaine and l-Methyl-4-Pyridone2 s in OD-/D20 at 100.0?. Although, because of the lack of data on appropriate model compounds, an interpretation of the similarity in rates for these two substrates cannot be made at this time, the results




27
are not inconsistent with the formulation of transition and intermediate states for exchange in the case of 17 which possess a considerable degree of ylidic character (Figure 9 ).2s
0 0 0 0
OD
N H N N N D
I I I I
Me Me Me Me
Figure 9 Proposed Pathway for Base-Catalyzed HydrogenDeuterium Exchange at the 2,6-Positions of
1-Methyl-4-Pyridone.
Hydrogen-Deuterium Exchange at the 2- and 6-Positions of 3-Substituted Pyridines
In order to determine the effects of substituents on
the rates of hydrogen-deuterium exchange reactions at the 2and 6-positions of 3-substituted pyridines, a series of kinetic studies were carried out on these substrates at elevated temperatures in DO20 solutions in the absence of added base. Again, exchange reactions were followed by measuring the change in the integrated area of the appropriate nmr signals.16 All exchange reactions were observed to follow good pseudofirst order kinetics.
The compound studied most extensively, for purposes of
comparison with pyridine itself, was 3-chloropyridine. Because of the limited solubility of this compound in D20, the kinetics of H-D exchange at the 2- and 6-positionswere measured in a 3:17 (V:V) dioxane-D20 solution. Pyridine was also studied




28
in this solvent system. Convenient rates were obtained at 197.50.50, and the appropriate kinetic data are reported in Table 13. A comparison of the pseudo-first order rate constants for the two systems is given in Table 3, and illustratesthe fact that the effect of the chlorine atom in the 3-position is to increase the rate of exchange at the 2-position, relative to pyridine, by a factor of 8.6, while making the 6-position 7.7 times less reactive than the equivalent 2,6-positions of pyridine. An attempt will be made to explain these and other results in terms of the mechanism illustrated in Figure 10, which has been demonstrated for exchange at the 2,6-positions of pyridine in D20 solution in the absence of added base.10 This is a two-step mechanism.
G Kb G
+ DpO + ODH H H HN
I+
D
k\ k
18
GG
0.0
H N+ H
D D
19 20
Figure 10. Proposed Mechanism for H-D Exchange at the 2and 6-Positions of 3-Substituted Pyridines in
Neutral D20 Solution.




29
Table 3. Relative Rates for H-D Exchange at the 2- and
6-Positions of Pyridine and 3-Chloropyridine
in 3:17 (V:V) Dioxane-D20 at 197.50.50.
3-G Position k sec1 krel
___ __ __ __rel
-H 2,6 3.19 x 10-s 1
-Cl 2 2.73 x 10 8.6
-Cl 6 4.16 x 10-6 0.13




30
The first step, an equilibrium, involves initial abstraction of a deuteron from a solvent molecule by the nitrogen atom of the pyridine, with the resultant formation of pyridinium and deuteroxide ions. The second, slow step involves abstraction of the proton from the 2- or 6-positions of the pyridinium ion by deuteroxide ion, resulting in formation of the corresponding ylid, 19 or 20 .
Since one of the reactive species proposed in this
mechanism is the pyridinium ion 18 an exchange experiment was carried out on the 3-chloro-l-methylpyridinium ion in
0.030M DC1 solution at 197.50.50 (Table 12 ). It is important to note that, in accordance with the proposed mechanism, this study had to be carried out in an acidic solution, the exchange reaction being much too fast to measure in neutral solution at this temperature. This is because the concentration of the substrate is, in this case, also the concentration of the pyridinium ion intermediate. Hence, in the case of this substrate, the first step of the two-step mechanism does not take place, and the results obtained reflect the effect of substituents on the second step only. In the case of the exchange study on 3-chloropyridine, the importance of the first step in the two-step mechanism may also be eliminated by comparing positional reactivities within the molecule, since the first step is common to formation of both ylids 19 and 20 In the case of exchange at the 2- and 6-positions of the l-methyl-3-chloropyridinium ion, the same order of reactivity is found as was observed in the case of




31
exchange in 3-chloropyridine (Tables 12 and 13). That is, the 2-position is considerably more reactive than the 6-position in both cases. More importantly, however, the similarity between the observed positional rate ratios (Figure 11) provides additional support for the intermediacy of pyridinium ion 18 during the course of exchange in the free base.
Cl Cl
0 0
N N
1+
Me
k/k= 66 k /k = 52
Figure 11. Positional Rate Ratios for H-D Exchange in
3-Chloropyridine (DO20) and 3-Chloro-l-Methylpyridinium Ion (0.030M DCI) at 197.50.50.
The rate expression for the proposed two-step mechanism may be derived from the scheme outlined in Figure 10. On the basis of this mechanism, the rate is given by the product of a second order rate constant k2 times the product of the concentrations of pyridinium and deuteroxide ions:
rate = k2[PyrD ][OD-].
Replacing the concentrations product by the product of the base ionization constant times the concentration of free pyridine gives
rate = k2Kb[Pyr].
The rate actually measured, however, is equal to an observed pseudo-first order rate constant k times.the total substrate concentration:




32
rate = k ([PyrD+] + [Pyr]).
If the latter two rate expressions are set equal to one another,
k = k2K [Pyr]
[PyrD+] + [Pyr]
Rearranging the above rate constant equation gives
[y D ] + ____r
k [Pyr = k2Kb,
which says that the product of the observed pseudo-first order rate constant times the ratio of the total concentration of substrate to the concentration of substrate present as the free base is equal to a constant.
In order to test this prediction, hydrogen-deuterium
exchange was studied at the 2- and 6-positions of 3-chloropyridine at 197.50.50 in D20 solutions of the partially neutralized (DC1) substrate (Table 13). The results reported in Table 4 indicate that this prediction was realized, and verify the proposed mechanistic scheme.
The effect of the chlorine atom on the rates of exchange at the 2- and 6-positions of 3-chloropyridine relative to pyridine may be explained in terms of the two-step mechanism (Figure 10). The first step involves the formation of a pyridinium ion, and hence will depend on the basicity of the pyridine molecule. The pKa values for the conjugate acids of pyridine and 3-chloropyridine are reported in Table 5 .2 Although these values were obtained at 250, they nevertheless




33
Table 4. Observed Dependence of the Rate of H-D Exchange
(197.5+0.50) at the 2- and 6-Positions of
Partially Neutralized 3-Chloropyridine on the
Degree of Neutralization.
[Pyr] + [Pyr D ]
Position 106k., sec-1 [Pyr] 106k2Kb
2 580 1.0 580
340 1.5 510
220 2.0 440
130 4.0 520
46 10 460
6 7.0 1.0 7.0
2.9 2.0 5.8




34
Table 5. pKa Values (250) for the Conjugate Acids of
Pyridine and 3-Chloropyr-idine.12
Compound pKa
Pyridine 5.25
3-Chloropyridine 2.84
Table 6. Chlorine Rate Factorsa
C1 H
Position Chlorine Rate Factor (k /k )
Ortho (2-) 403
Para (6-) 7.76
aDetermined from the rates of H-D exchange at the 2- and 6-positions of 1-methylpyridinium and 3-chloro-l-methylpyridinium ions in 0.030M DC1 at 197.5+0.50.




35
present the trend to be expected at higher temperatures; namely, that pyridine is a significantly stronger base than 3-chloropyridine. The effect of chlorine on the first step of the reaction, then, is to decrease the basicity of the annular nitrogen atom.
The second step in the reaction scheme (Figure 10)
involves removal of a proton from the 2- or 6-positions of the pyridinium ion. In this case, the effect of the substituent is on the acidity of a proton bound to carbon. An estimate of the effect of chlorine on this step of the reaction may be obtained from a comparison of the rates of exchange at the 2- and 6-positions of the 3-chloro-l-methylpyridinium ion with the rate of exchange at the equivalent 2,6-positions of the 1-methylpyridinium ion itself, as determined from the studies in 0.030M DC1 at 197.50.50. This comparison is reported as a pair of chlorine rate factors (Table 6) which reflect the ability of the chlorine atom to stabilize the carbanionic center of the ylid developing at ortho and para centers. The stabilizing effect of the chlorine atom is observed to be much stronger at the ortho position.
These observations allow the chlorine substituent effect observed in the exchange reactions of the free base (Table 3) to be explained in terms of the two-step mechanism. In the first step, 3-chloropyridine is deactivated relative to pyridine because of the base-weakening effect of the chlorine atom (Table 5 ). In the second step, in. the case of exchange at the 2-position, this initial deactivation is more than




36
compensated for by the activation resulting from the significant stabilization of the developing carbanionic center at this position (Table 6). The net effect is activation.
In the case of exchange at the 6-position, however, the stabilizing effect of chlorine on the negative charge developing at the para position is weak (Table 6), and the net effect is deactivation relative to pyridine.
The results of these H-D exchange studies on pyridine
and 3-chloropyridine are then explained in terms of a two-step mechanism (Figure 10) involving pyridinium ion and pyridinium ylid intermediates, and the opposing effects of chlorine on nitrogen basicity and carbon acidity.
The rates of hydrogen-deuterium exchange at the 2- and 6-positions of several other 3-substituted pyridines were measured in D20 solution at 197.50.5 or 217.9 0.50. Exchange at the 2,6-positions of pyridine itself was followed at both temperatures for purposes of comparison. In the cases of nicotinic acid and 3-pyridinesulfonic acid, sodium carbonate was added to insure that the substituents were in the anionic form. The pseudo-first order rate constants and conditions under which they were obtained are reported in Tables 13 and 14.
The ionic strengths of the various sample solutions were
not held constant, since, in many cases, the limited solubility of the substrate in D20 precluded the addition of supporting electrolytes. The similarity between rate constants for exchange at the 2,6-positions of pyridine, obtained in D20




37
solution at 197.50.5 or 217.90.50, with those obtained in .0OM NaCl (D20), however, indicate that ionic strength has little effect on the reaction rates in the case of the two-step
mechanism (Tables 13 and 14).
The rate constant equation
k = k2Kb [Pyr]
[PyrD+] + [Pyr]
previously derived for the two-step mechanism, predicts that in neutral or slightly basic solution, where the concentration of pyridinium ion is very small, the pseudo-first order rate constant should be independent of the base concentration; that is,
k = k2Kb.
This was demonstrated by the constancy observed in the pseudofirst order rate constants k2 and k over a pD range of 7-10 in the case of the nicotinate anion (Table 14).
Summaries of the effects of substituents on pyridinium ylid formation, obtained from the data for exchange at the 2- and 6-positions of 3-substituted-l-methylpyridinium ions at 75.00.50, and of the effects of substituents on the twostep mechanism, obtained from the data for exchange at the 2- and 6-positions of 3-substituted pyridines at elevated temperature, are reported in Table 7 as a series of rate ratios. The effects of ortho and para substituents, relative to hydrogen, on the rates of 1-methylpyridinium ylid formation have been explained in the previous section in terms of the




Table 7 Substituent Effects on Hydrogen-Deuterium Exchange at the 2- and 6-Positions
of a Serigscof 3-Substituted-1-Methylpyridinium Ionsa and 3-Substituted
Pyridines .
G G
0 '
D N D D N D
Me
-G k G/kH kG/kH k/k kG/k/ H k/ k. kH
-Cl 2700 12 230 8.6 b 0.13b 66
-CO2 0.50 1.6 0.31 0.39c 1. 2c 0.33
-SO3 210 14 15 1.8c 0.42c 4.4
-OMe 60 0.80 75 13bc [0.44]bd [29]d
-ND2 1.4 0.060 23 73b,c 0.31c 25
aData obtained from experiments at 75.00.50 in buffered D20 solutions of 1.0 ionic strength. bData obtained from experiments at 197.50.50 in D20 or 3:17 (V:V) dioxane-D20 solution. cData obtained from experiments at 217.90.50 in D20 solution. dUncertain
O




39
inductive and resonance effects of the substituent relative to hydrogen in each case. Note that this information pertains only to the second step of the two-step mechanism. In the case of exchange in the free bases, however, where the two-step mechanism has been shown to operate, the total substituent effect on ortho and para centers, relative to hydrogen, reflects not only the effect of the substituent on ylid formation (second step), but also its effect on the basicity of the pyridine (first step). The effect of the 3-chloro substituent has been discussed in this context earlier. Note, for example, that the effect of the amino group on 1-methylpyridinium ylid formation is activating at the ortho position and deactivating at the para position. The basestrengthening effect of the amino group,.2 that is, its activating effect relative to hydrogen on the first step of the two-step mechanism, is reflected by a more pronounced activating effect at the ortho position as well as a decrease in the deactivating effect at the para position in the case of the two-step mechanism.
Note also that the effects of substituents are much more pronounced in the case of 1-methylpyridinium ylid formation than in the case of the two-step mechanism. This results not only from the normal compression of reactivities which accompanies an increase in temperature, but also because the effect of a particular substituent on each step of the two-step mechanism generally works in opposite directions. For example, an electron-withdrawinggroup would make the pyridine




40
a weaker base, so that less of the substrate would be in the reactive pyridinium ion form relative to pyridine itself. The effect of this group would generally stabilize the carbanionic center of the incipient pyridinium ylid, however, and the effect on the second step of the mechanism would then be activating relative to pyridine itself. A similar discussion would apply for electron-releasing groups.
The effects of substituents on an ortho relative to a para reactive center in the cases of 1-methylpyridinium ylid formation and the two-step mechanism are seen from a comparison of the third columns in each of the two series (Table 7). In every case, the relative effects of substituents at the two positions are in the same direction, the C02 group being the only substituent studied which activates the para relative to the ortho position. This provides further support for the two-step mechanism, where a comparison of positional rate ratios within a given substrate amounts to a comparison of the substituent effect on the second step (pyridinium ylid formation) only. The ratios are, of course, smaller in the case of the two-step mechanism, a result consistent with the fact that these reactions were studied at a much higher temperature.




CHAPTER 3
EXPERIMENTAL
Instrumentation
Proton nmr spectra were recorded on a Varian Associates Model A-60A instrument. Melting points were determined in a Thomas-Hoover Unimelt melting point apparatus. Measurements of pD were made employing a Beckman Model 1019 Research pH Meter equipped with either a Sargent-Welch (S-30070-10) or Corning (476050) high-temperature, high-alkalinity miniature combination electrode. Kinetic runs at 75.00 were carried out in a Haake Model F constant temperature circulator. Microanalyses were performed by Galbraith Laboratories, Inc., Knoxville, Tennessee.
Chemicals
All common laboratory chemicals, unless specified to
the contrary, were reagent grade and from various suppliers. Deuterium oxide (99.7 percent) was obtained from Columbia Organic Chemicals.
Stock Solutions
Tetramethylammonium chloride (TMAC), sodium isobutyrate,
and tris-Chydroxymethyl-)aminomethane (THAM) were dried at 1000 overnight and stored in a pesiccator before use. These hygroscopic materials, as well as potassium hydroxide, were
41




42
handled in a dry box.
Deuterated stock solutions of acetic acid, boric acid, citric acid, potassium mono- and dihydrogen phosphate, sodium and potassium carbonate, sodium and potassium chloride, TMAC, sodium isobutyrate, and t-butyl alcohol were prepared by dissolving an appropriate weight of the analytical reagent or primary standard grade material in deuterium oxide and diluting to mark in a volumetric flask.
A 3:17 (V:V) dioxane-D20 solution was prepared by
mixing 1.5 ml of reagent grade dioxane with 8.5 ml of deuterium oxide.
Dilute DC1 was prepared by diluting commercial
concentrated HC1 with D20. The solution was standardized by potentiometric titration vs THAM.
Stock sodium deuteroxide solution was prepared by dissolving freshly cut sodium in D20 under a nitrogen atmosphere. Stock potassium deuteroxide solution was prepared by dissolving a weighed quantity of reagent grade KOH in D20. Both solutions were standardized by potentiometric titration vs standardized HC1.
The analogous proteo stock solutions for control runs were prepared in identical fashion, employing distilled water in place of D20.
Deoxygenated solutions, when they were required, were prepared in the following manner: The appropriate solution was refluxed under a continuous stream of nitrogen for 30 minutes, and then allowed to cool to approximately 00 in an




43
ice bath under a nitrogen atmosphere. The solutions were stored under nitrogen in tightly stoppered bottles. Substrates
Free Bases
Fisher reagent grade pyridine, which had been dried over KOH pellets, was used directly. 3-Chloropyridine was obtained from Aldrich Chemical Company, Inc., and was redistilled (bp 149-150'; lit."3 1480(744 mm)) before use. 3-Pyridinesulfonic acid was purchased from Aldrich, and was treated with Norit and recrystallized three times from hot water (mp >3000; lit.3 3570(dec)). Nicotinic acid was obtained from Nutritional Biochemicals, Inc., and was recrystallized twice from hot water (mp 235.5-236.00; lit.4 236-2370) prior to use. 3-Aminopyridine was purchased from Nepera Chemical Company and was purified by vacuum sublimation at 50-60CO0.1 mm); mp 63.5-64.00 (lit.)4 63-650). 3-Methoxypyridine45 was redistilled (bp 80.0-80.5'(25 mm); lit.46 70-710(12 mm)) prior to use. Quinazoline, purchased from Aldrich, was purified by vacuum sublimation at 400(0.03 mm); mp 48.00 (lit.43 48.0-48.50).
Methyl nicotinate.- This compound, mp 41.0-41.50
(lit."4 42.3-43.5*) was prepared in 88 percent yield by esterification of nicotinic acid.47
3-Nitropyridine.- This compound was prepared by
reduction of 2-chloro-3-nitropyridine, obtained from Aldrich, with copper powder in molten benzoic acid according to the procedure of Kirby and Varvoglis.'8 A mixture of 35.0 g of




44
benzoic acid and 16.8 g (0.106 moles) of 2-ckloro-3-nitropyridine was heated to 1500, and, after the mixture had melted, 25.0 g (0.395 moles) of copper powder was added in portions over a S-minute period while the mixture was stirred magnetically. After several additional minutes of heating, the melt was cooled to 90-1000, 140 ml of 20 percent Na2CO3 was added, and the mixture heated on the steam bath while the larger lumps of melt were broken up with a spatula. The slurry was then transferred to a mortar and the remaining
larger particles were pulverized. The slurry was heated for an additional 20 minutes on the steam bath and suctionfiltered. The filtrate was extracted four times with 150 ml portions of methylene chloride. The residue was stirred thoroughly with 50 ml of methylene chloride and filtered. The CHzCl2 extracts were combined and dried over anhydrous sodium sulfate overnight, and the solvent was removed on a rotary evaporator. The product, an oil which crystallized in yellow needles on cooling in an ice bath, weighed 7.83 g (60 percent yield). It was used without further purification to prepare the methiodide.
3-Pyridyl methyl sulfone.- The method used to prepare this compound by permanganate oxidation of 3-methylthiopyridine was adapted from the procedure described by Barlin and Brown 4 for the preparation of the 2- and 4-pyridyl methyl sulfones. A 3 percent aqueous solution of KMnO4 was added slowly with stirring to a solution of 2.00 g (16.0 mmoles) of 3-methylthiopyridineso in 150 ml of 20 percent




45
acetic acid until a permanent pink color appeared. The excess permanganate was destroyed by bubbling sulfur dioxide through the slurry until the pink color was discharged. The water and acetic acid were removed on a rotary evaporator, and 50 ml of 2N NaOli was added to the residue. The slurry was extracted with 50 ml of ether and filtered. The solid residue was treated with an additional 50 ml of 2N NaOH, then 50 ml of ether, and filtered again. The combined water-ether mixture was transferred to a separatory funnel and separated. The aqueous layer was extracted twice more with 100 ml portions of ether and the extracts combined and dried over barium oxide. The drying agent was filtered off and the ether removed on a rotary evaporator. There remained 1.73 g (69 percent) of white crystalline product which was used directly to prepare the methiodide.
Pyridinium Salts
The pyridinium iodides, chlorides, and perchlorates
employed in this study were, for the most part, prepared by one of the general methods described below. Exceptions are described in greater detail in the following text. Experimental details are summarized in Table 8,
Pyridinium iodides.- These salts were prepared by heating a solution of 50 mmoles of the 3-substituted pyridine and 4.0 ml (9.1 g, 64 mmoles) of methyl iodide in a small volume (10-30 ml) of an appropriate solvent (Table 8) at 70' for 3-4 hours. The mixture was cooled and the crystals filtered and recrystallized from an appropriate




Table 8. Experimental Data for the Preparatibn of a Series of 3-Substituted-l-Methylpyridinium Salts.
Reaction Recrystallization Yield, M.P., 0C
3-G X Solvent Solvent %
Found Lit.
-CO2Me I Absolute 50% CV:V) 57 130.0- 129.5-a
Methanol Methanol-Ether 131.5 130.2
-Cl I Absolute Absolute 86 138.0- 141Ethanol Ethanol 138.5 142
-OMe I Absolute Absolute 74 155.5- c
Methanol Ethanol 156.5
-I I Absolute 80% (V:V) 66 -Ethanol Aqueous Methanol
-NHz I Absolute Absolute 49 120.0- 123d
Ethanol Ethanol 121.0
-OHn I Acetone Acetone 64 111.5- 114-e
114.5 116
-Men I Absolute Acetone 72 96.0- 95
Methanol 97.0
-COMe I Absolute Absolute 39 165.0- 162
Methanol Methanol 166.5
-S02Me I Absolute 90% CV:V) 70 213.0- g
Methanol Aqueous Ethanol 214.0




Table 8. Continued.
Reaction Recrystallization Yield, M.P., 0C
3-G X Solvent Solvent %
Found Lit.
-C1n Cl Absolute 68
Methanol
-I C1 85% CV:V) Absolute 76 257.5
Aqueous Ethanol (dec) --Methanol
-NO2 Cl 85% (V:V) 75% Ethanol- 74 255Aqueous 25% Ether 256 -Methanol CV:V) (dec)
-COMe C1 Absolute 50% (V:V) 82 197.5- i
Methanol Ethanol-Ether 198.5
-CN C10O ---- 95% Ethanol 31 128.0- 135-jk
128.5 137
-Cl C104 ---- 95% Ethanol 59 95.5- 1
96.0
-OMe C104 ---- 95% Ethanol 49 55.056.0
-H C104 ---- 95% Ethanol 75 136.5- 135.0m
137.5
aL. Bradlow and C. A. VanderWerf, J. Org. Chem., 16, 1143 (1951).




Table 8. Continued.
b
M. Liveris and J. Miller, Australian J. Chem., 11, 297 (1958). CAnalysis: Calcd: C, 33.49; H, 4.01; N, 5.58. Found: C, 33.68; H, 3.97; N, 5.56. dN. F. Turitsyna and A. F. Vompe, Dokiady Akad. Nauk S.S.S.R., 74, 509 (1950); Chem. Abstr., 45, 3846h (1951).
eK. Mecklenborg and M. Orchin, J. Org. Chem., 23, 1591 (1958). fRef. 51.
gAnalysis: Calcd: C, 28.11; H, 3.37; N, 4.68. Found: C, 27.94; H, 3.45; N, 4.80.
h
Analysis: Calcd: C, 28.21; H, 2.76; N, 5.48. Found: C, 28.00; H, 2.74; N, 5.38. Analysis: Calcd: C, 55.99; H, 5.87;.N, 8.16. Found: C, 55.69; H, 5.90; N, 8.14. JRef. 19.
kAnalysis: Calcd: C, 38.46; H, 3.23; N, 12.81. Found: C, 38.40; H, 3.23; N, 12.77. Analysis: Calcd: C, 31.60; H, 3.09; N, 6.14. Found: C, 31.82; H, 3.04; N, 5.91. mS. Ukai and K. Hirose, Chem. Pharm. BuZll. (Tokyo), 16, 195 (1968). nExtremely hygroscopic.




49
solvent (Table 8). The final product was dried under vacuum over P0Os in a drying pistol for 24 hours at room temperature.
3-Nitro-1-methylpyridinium iodide.- This salt was prepared in 72 percent yield according to the method described by Pfleiderer, Sann, and Stock;s5' mp 217.00 (lit.s5 2150).
Pyridinium chlorides.- These compounds were prepared
from the corresponding iodides by halide exchange. A solution of 45 mmoles of the 3-substituted-l-methylpyridinium iodide
in the minimum volume of an appropriate solvent (Table 8) was refluxed with 22.6 g (158 mmoles) of silver chloride
while stirring the mixture mechanically. The mixture was then cooled, the silver salts removed by filtration, and the solvent removed on a rotary evaporator. The pyridinium chloride was then recrystallized from an appropriate solvent and dried under vacuum over P20s in a drying pistol for 24 hours at room temperature.
Pyridinium perchlorates.- The general method used to prepare these compounds was adapted from that reported by Eisenthal and Katritzky s2 for the preparation of 1-methoxypyridinium perchlorate. A mixture of 50 mmoles of the pyridine and 6.30 g (50 mmoles) of dimethyl sulfate was heated on the steam bath for 2 hours. After cooling to room temperature, 8 ml of absolute ethanol, 5 ml of 70 percent perchloric acid, and 36 ml of ethyl acetate were added, and the resulting solution cooled thoroughly in an ice bath. The crystalline perchlorate which separated was recrystallized twice from the minimum volume of hot 95 percent ethanol, washed with anhydrous ether, and dried in a pistol over




50
P20s under vacuum for 12 hours at room temperature.
Pyridinium Betaines
1-Methylpyridinium-3-sulfonate betaine, *- The procedure for the preparation of this compound was adapted from that reported for the synthesis of the 2- and 4- isomers.53 A mixture of 7.95 g (50 mmoles) of 3-pyridinesulfonic acid and 20 ml of dimethyl sulfate was heated at 150-1600 for 2 hours with magnetic stirring. After cooling, 50 ml of anhydrous ether was added to the reaction mixture and the
solid material thoroughly pulverized with a stirring rod. The solid was then removed by filtration and washed four
more times with 50 ml portions of ether. The crude product was recrystallized three times from the minimum volume of
hot 50 percent aqueous ethanol. The final crop was washed with ether, filtered, and dried overnight under vacuum in a pistol over P205 at room temperature. The pure product weighed 3.60 g (42 percent yield); mp >3000 (lit.54 5ss 1300). Analysis: Calcd for C6H7NO3S: C, 41.61; H, 4.07; N, 8.09. Found: C, 41.79; H, 4.17; N, 7.95.
l-Metlhylpyridinium-3-carboxylate betaine." This compound was prepared in 83 percent yield from methyl nicotinate methiodide by employing the method of Kosower and Patton;s7 mp 234.5-235.5' (lit.s57 230-2330).
Preparation of Solutions for Kinetic Runs
Volumes of one to five milliliters of solution were prepared for each run. When solubility permitted, the




51
solutions were approximately 0.SM in substrate. An appropriate amount of the substrate Cpyridine or pyridinium salt) was transferred to a tared volumetric flask and weighed accurately on an analytical balance. When hygroscopic substrates were used, a dry bag, filled with nitrogen and holding an open container of P20s, was employed for these manipulations.
Accurate volumes of stock acid, base, buffer, internal standard, or supporting electrolyte solutions were then delivered to the flasks by means of Hamilton Microliter syringes, and the solution finally diluted to mark with D20 or stock 3:17 (V:V) dioxane-D20 solution. Deoxygenated solutions were prepared under a nitrogen atmosphere.
Approximately one milliliter of the final solution was
then transferred to an nmr tube which was sealed immediately. The remainder of the solution was generally stored under nitrogen in a septum-stoppered test tube for later comparisons or pD measurements.
Kinetic Runs
Methods
Kinetics were obtained by heating the sample solutions in sealed nmr tubes. For kinetic runs at 75.0 + 0.50, the temperature was maintained by immersing the nmr tube in a Haake Model F constant-temperature circulating bath. The temperature of this bath was checked periodically using a National Bureau of Standards Certified thermometer. For runs at higher temperatures, the inmar tube was immersed in a re-




52
fluxing vapor batli. Distilled water was used to maintain 100.0 0 0.5, mesitylene for 164.7 -F 0.5', ethylene glycol for 197.5 + 0.5., and naphthalene for 217.9 + 0 5 From time to time, the temperatures of these baths were checked with an ordinary 76 mm immersion thermometer. In some of the earlier runs at elevated temperature, the variations in temperatures of the various vapor baths exceeded those reported above. In no case, however, did individual variations exceed + 1.50, even over extended periods of time.
Periodically, the nmr tube was removed from the bath, quenched, and the proton nmr spectrum of the solution) recorded. Reactions were followed by measuring the change in the integrated area of the nmr signal of the proton of interest with respect to that of a non-exchanging proton in the reaction mixture. The integrals of proton signals were measured in six successive sweeps, three in each direction, and the average value was taken. In favorable cases, a ring proton of the substrate served as the area reference. In others, an internal standard external to the substrate was used.
The internal standards employed included methanol,
t-butyl alcohol, TMAC, sodium isobutyrate, and diethyl ether. The choice of an internal standard for a particular sample solution was dictated largely by the position of its nmr signal(s) relative to those of the substrate or cosolvent, its stability under the conditions of the kinetic run, and its solubility in the sample solution.




53
The deuteration reactions studied may be expressed in
the following general form:
k1JOD']
PyrH + D20 PyrD + HOD
k1JOD]
Experimentally, because the concentration of OD catalyst remains unchanged, pseudo-first order kinetics are observed. Furthermore, since deuteration of the intermediate hydrogenbonded carbanion (ylid) occurs after the rate-determining step, 14 only the fraction of solvent molecules containing the isotope of interest appears in the rate expression. It is reasonable to assume that the kinetic isotope effect in these systems is negligible, due to extensive internal re14,58
turn, s so that k, = k 1. The rate of appearance of deuterated substrate is then:
d[PyrD] = k[PyrH][OD ] [H] +[D] k_ 1[PyrD]O [] [D]
-dt [[H +kDl-IH + [
where [H] and [D] represent the concentrations of the lighter and heavier isotopes, respectively, in the solvent pool. It should be noted that fil] represents the total concentration of the protium isotope, whatever its source(s); these include contamination of the deuterium pool by H-D exchange at the
position of interest, prior exchange at a more reactive position, use of a substrate possessing functional groups




54
which undergo rapid exchange, the protium impurity (0.3%) present in the commercially available D20, and the use of stock buffer solutions containing the lighter isotope.
Particularly unfavorable circumstances may be imagined. Consider exchange approaching completion at the 6-position
of the 3-amino-l-methylpyridinium ion in a citric acidcitrate (four exchangeable hydrogens) buffer. The deuterium pool becomes contaminated by 2 equivalents of protium from the amino group of the substrate, 2 more equivalents from exchange at the 6- and more reactive 2-positions, as well as from the buffer and normal solvent impurity. Even in this extreme case, the fraction of deuterated solvent amounts to only 2-3 mole percent of the total at the substrate and buffer concentrations generally employed. Therefore, the concentration of H in the denominator of the first term as well as the entire second term of the above equation may be regarded as insignificant, and the rate expression simplifies
to
d[PyrD] kdPyrH] =dt d rH]d- ki [PyrH][OD ] = k[PyrH],
where k., the pseudo-first order rate constant, is defined
as
k kLOD ].
Integrating,
[PyrH]l
Jn kt
[PyrlH] t




55
Practically, the pseudo-first order rate constant k was obtained by determining the half-life directly from a plot of IPyrHl]o/[PyrH] t vs t on semilogarithmic paper, and applying the relationship
k 0.693
t 1
Kinetics of Hydrogen-Deuterium Exchange in 3-Substituted1-Methylpyridinium Ions
Hydrogen-Deuterium exchange experiments at 75.0 1 0.5.The kinetics of hydrogen-deuterium exchange at the 2- and 6-positions of a series of 3-substituted-l-methylpyridinium ions in buffered D20 solutions of 1.0 ionic strength were studied at 75.0 + 0.50. The experimental data are summarized in Tables 9 and 10. With the exception of the 3-methylsulfonyl- and 3-acetyl-l-methylpyridinium ions, where deuteration of the methyl groups of the -S02CH3 and -COCH3 substituents was observed, exchange occurred exclusively at the 2- and/or 6-positions of the substrates under these conditions. An average of 15 points constituted each kinetic plot, and each run was followed for an average of 2.6 halflives.
The particular pyridinium salt (chloride, iodide, or perchlorate) which was employed was determined largely by the solubility of the particular salt, the reactivity of the counterion (iodides were avoided when acidic buffers were employed, owing to air oxidation of the iodide ion to free iodine), and ease of preparation and handling (perchlorates




Table 9. Rates of H-D Exchange at the 2-Position. of 3-Substituted-1-Methylpyridinium
Ions in D20 at 75.0 + 0.50 and 1.0 Ionic Strength.
3-G k2 sec-1 pD a pD k2, M-1 sec-1 b [DA], Mc A], Md
-NO2 4.84 x 10-4 e,h 4.942 4.937 1.87 x 10s 0.026j 0.033j
-CN 3.18 x 10 fi 6.017 6.005 1.05 x I0 0.053" 0.006
-Cl 9.38 x 10-s e,h 6.788 6.777 5.25 x 102 0.047k 0.012k
-I 2.75 x I0- e,h 7.035 7.017 8.85 x 101 0.035k 0.024k
1 11
-So 2.88 x 10-4 h 8.370 8.332 4.49 x 101 0.0501 0.0091
7.70 x 10-s h 7.939 7.774 4.32 x 101 0.0551 0.0041
8.02 x 10-6 h 6.854 6840 3.90 x 101 0.035k 0.024k
Avg 4.24 x 101
-OMe 3.98 x 10-'i 9.054 9.035 1.23 x 101 0.0291 0.0301
-ND2n 2.50 x 10-sg,h 9.444 9.483 2.76 x 10' 0.0201 0.0391
-H 1.84 x 104fi 10.498 10.490 1.99 x 10-1 0.001m 0.049m
-CO2 n 1.20 x 10-h 10.627 10.624 9.50 x 10 0.001m 0.049m
1.35 x 10-s5h 9.738 9.641 1.03 x 10' 0.0121 0.0481
Avg 9.90 x 10




Table 9. Continued.
3-G k2, sec-I pD a pD k2, M-1 sec -1b [DA] Mc [A-], Md
W 0
-Me 3.74 x 10 g'P 11.121 11.116 9.54 x 102 0.000m 0.132m
2.35 x 10-4 g,h 10.998 10.981 8.19 x 102 0.004m 0.096m
Avg 8.87 x 102
-0- o,q 1.67 x 10- g 10.974 10.527s 9.87 x 10- r 0.000m 0.154m
4.98 x 10-6 g,h 12.292 11.021 3.67 x r t t
Avg 2.33 x 104
apD at 75.0 + 0.5' of unheated portion of original solution. bCalculated from k2 and aOD- (based on the measured pD of the recovered solution).
'OD
CBuffer acid. dBuffer base. eSubstrate counterion Cl. fSubstrate counterion C104.
f
OSubstrate counterion I. Supporting electrolyte KC1.




Table 9. Continued.
iSupporting electrolyte NaCl.
-Deuterocitric acid-deuterocitrate. kD2PO -DP04.
1D
D3BO3-D2B03 -.
mDCO3 CO32
nSolution becomes yellow during the course of the run. oExchange appears to occur at the 2- and 6-positions at approximately the same rate. PA small amount of exchange occurred initially at room temperature. qMeoD internal standard. rCalculated from k2 and aOD- (based on the average of pD and pD ).
OD 0
sSignificant etching occurred during the course of the run. tNo buffer system was employed. The run was carried out in 0.1M KOD.
00




Table 10. Rates of H-D Exchange at the 6-Position of 3-Sub.stituted-l-Methylpyridinium
Ions in D20 at 75.0 + 0.50 and 1.0 Ionic Strength.
3-G k, sec- pDo a pD k, M-, sec-, b IDA] Mc [A-], Md
-SO3- m 7.55 x i0-4 h ---- 9.954 2.81 0.000i 0.059i
3.59 x 10-4 h 9.522 9.594 3.07 0.011 0.0481
1.30 x 10-4 h,l 9.373 9.277 2.30 0.023i 0.036
6.60 x 10-s h,1 8.977 8.873 2.96 0.035J 0.024j
Avg 2.79
-Cl m 1.79 x 104 e,i 9.475 9.419 2.28 0.017j 0.0427.00 x 10-5 f,h 9.026 9.004 2.32 0.035j 0.024i
Avg 2.30
-I 2.25 x 10-4 f,h 9.679 9.525 2.25 0.051j 0.009j
-C02 m 3.98 x 10-4 h 10.627 10.624 3.16 x 10-' 0.001k 0.049k
4.28 x 10-s h 9.738 9.641 3.27 x 10-' 0.012i 0.0483
Avg 3.22 x 107
-H 1.84 x 10-4 e,i 10.498 10.490 1.99 x 10- 0.001k 0.049k
-0Me m 3.87 x 10"4 g,h 11.123 10.896 1.64 x 10-1 0.000k 0.100k




Table 10. Continued.
3-G a6 -ec c d
3-G k6, sec pDa pD kG, M-' sec [DA], Mc [A ], M
-Me n 3.74 x 10-4 g,o 11.121 11.116 9.54 x 10- 0.000k 0.132k
2.35 x 10-4 g,h 10.998 10.981 8.19 x 10-2 0.004k 0.096k
Avg 8.87 x 10-2
-ND2 m 4.00 x 10-s g 11.214 11.037 1.23 x 102 0.000k 0159k
-0-" n,p 1.67 x 10-7 g 10.974 10.527r 9.87 x 10- q 0.000k 0.154k
4.98 x 10. g,h 12.292 11.021r 3.67 x 10-- q s s
Avg 2.33 x 10-4
-CN e,i ------------t t t t 0.053J 0.006j
-NO2 g,h,l -----------t t ----t t 0.005 0.055ss
f h- - - t -- t -- t - -- t
-COMe f,h t t t t 0.055J 0.005
-SO2Me g,h t 8.201u 6.104u t 0.052i 0.007i
apD at 75.0 + 0.50 of unheated portion of original solution. bCalculated from k6 and aD- (based on the measured pD of the recovered solution).
OD
CBuffer acid.




Table 10. Continued.
dBuffer base.
eSubstrate counterion CIOt.
f
Substrate counterion Cl. gSubstrate counterion I. supporting electrolyte KCl. iSupporting electrolyte NaC. ISupporting electrolyte NaC1.
JD3BO3-D2BO3.
kDC03 -C03 .
1Deoxygenated solution. mSolution becomes yellow during the course of the run. nExchange appears to occur at the 2- and 6-positions at approximately the same. rate. oA small amount of exchange occurred initially at room temperature. PMeOD internal standard. Calculated from k and aD- (based on the average of pD and pDo).
qcalc~ate frmk ODr Significant etching occurred during the course of the run. SNo buffer system was employed. The run was carried out in 0.1M KOD.




Table 10. Continued.
tNo data were obtained, owing to the occurrence of side reactions involving the consumption of base and substrate. UpD measurements made at 25.




63
are generally much less hygroscopic than the corresponding chlorides).
The ionic strength of the solutions was maintained at 1.0 by addition of a standard stock solution of KC1 or NaCI in D20. It was necessary to employ KC1 for solutions with pD >10 in order to minimize the sodium ion error encountered in making pD measurements. Sodium chloride was used when the substrate was a perchlorate because of the low solubility of potassium perchlorate.
Buffered solutions were employed in order to insure constant pD; in several instances, the buffer ratio was varied for purposes of detecting the possible operation of general base catalysis.
Except in the case of 3-cyano-l-methylpyridinium perchlorate, where the substrate concentration employed was approximately 0.3M because of low solubility, all substrate concentrations were approximately 0.5M.
The nmr spectra for most 3-substituted-l-methylpyridinium ions (G NO2, CN, SO2Me, S03, C02 Cl, I) exhibit the same general pattern for the annular protons. In the order of increasing field, they appear as a rather broad singlet for the 2-proton, a series of overlapped peaks for the 4- and 6-protons, and a broad triplet for the 5-proton. As the 2-position undergoes exchange, its signal decreases in intensity; the appearance of the signals for the other annular protons is altered only slightly. As exchange approaches




64
completion at the 6-position, the signals for the 4- and 5-protons emerge as a sharp and broad doublet respectively. For exchange at the 2- and 6-positions of these substrates, the 5-proton was used as the internal standard, so that
[H-2]0 [H-5]t
[H-2]t [H-2]t
and
[H-6] [H-5]t
[H-6]t ([H-4]t + [H-6]t) [H--5]t
The signals for the annular protons of the 1-methylpyridinium ion itself appear as a broad multiplet representing the overlapped signals for the 2,6- and 4protons and, at slightly higher field, a broad peak for the 3,5-protons. As exchange occurs at the 2,6-positions, the 4position emerges as a well-defined triplet slightly downfield from the broad doublet for the 3,5-protons. In this case, the signal for the 3,5-protons was used as the area reference; the appropriate concentration ratio, then, is defined by
[H-2,6]o [H-3,5]t
[H-2,6]t ([H-2,6]t + [H-4]t) 0.5[H-3,5]t
As the electron-releasing ability of the 3-substituent is enhanced, the appearance of the nmr spectrum undergoes notable changes. The nmr spectrum of the 1,3-dimethylpyridinium ion exhibits itself as a set of overlapped peaks




65
for the 2- and 6-protons at lowest field, followed by a broad doublet for the 4-proton at slightly higher field, and finally a broad triplet for the 5-proton at highest field. In this case, the 5-position was used as the area reference. That exchange was occurring at the 2- and 6-positions of this substrate at approximately equal rates was illustrated by the fact that a plot of
log ([H-2,6] /[H-2,6] )= log (2[H-5S] /[H-2 ,6] t) vs t was linear up to 3.7 half-lives. A series of theoretical kinetic plots for two parallel first order reactions with rate constants k and k' were constructed for k=nk' (n=1,2,3,4). As expected, the curvature in these plots becomes more pronounced and deviates more severely from that where k=k' with increasing n. It is apparent from these plots, however, that for n<3, the degree of curvature in the plot could conceivably escape detection. Hence, the uncertainty in the rate constants for the 2,6-positions of this substrate are larger than those for which well-defined reactivity differences exist between the 2- and 6-positions.
The nmr spectra of the 3-methoxy- and 3-amino-l-methylpyridinium ions appear as a downfield set of overlapped peaks for the 2- and 6-positions and another set of overlapped signals for the 4- and 5-positions at slightly higher field. As exchange approaches completion at the 2-position, the signal for the 6-proton reveals itself as a broad doublet. As the 6-position exchanges, a pair of sharp, partially overlapped doublets emerge as the signals for the 4- and 5-protons. In these cases, the




66
integral of the combined signals for the 4- and 5-protons was used as the area reference, giving
[H-2]0 0.5([H-4]t + [H-S]t)
[H-2]t ([H-2]t + [H-6]t-) 0.5([H-4]t + [H-5]t)
and
[H-6] 0.5([H-4]t + [H-5]t)
[H-6]t [H-6]t
In the case of 1-methylpyridinium-3-oxide betaine, the
signals for the 2- and 6-protons appear as a set of overlapped peaks slightly downfield and partially overlapped with a second set of overlapped peaks representing the combined signals for the 4- and 5-protons. In this case, the signal for the 1-methyl- protons was used as the area reference. That these protons did not undergo exchange was evidenced by the fact that the ratio of the area of their signal to that of the signal for the methyl protons of methanol, which was used as an internal standard, remained constant throughout the course of the run. A plot of
log 11-2]o + [H-6]o ,which was defined by
[H-2]t + [H-6]t
0.667 [1-CtH3]
log
S [H-2t]+ [H-6]t + [H-4]t + [H-5]t 0.667 [1-CHa]
vs t was linear for up to 3.2 half-lives in an unbuffered run at 100.0+0.50 (see next section), indicating that, as in the




67
case of the 1,3-dimethylpyridinium ion, exchange was occurring at approximately equal rates at the 2- and 6-positions. As mentioned previously, however, these rates may actually differ by as much as a factor of two or three, and this difference still remainsundetectable.
Two kinetic runs were made on this substrate at 75.0+0.50.
In both cases, since the material weighed was 3-hydroxy-l-methylpyridinium iodide, the calculated amount of dilute KOD was added to completely neutralize the substrate.
In one of the runs, a DC03 -C032 buffer system was
employed. Because of the extreme sluggishness of the reaction (t112=69,300 minutes, approximately 1.5 months), exchange was only followed for 0.7 half-lives. At that time, the nmr tube was cracked open, pD measurements made, and the solution acidified to pD 1-2 with dilute DCI. Neutralization of the 3-oxide group has the effect of separating the signals for the 2- and 6-protons from those for the 4- and 5-protons, and the final point in the kinetic plot, which fell on the line established by the previous points, was determined by using the 4- and 5-positions as the area reference, so that
[H-2] + [H-6]0 [H-4]t + [H-5]t
[H-2]t + [H-6]t [H-2]t + [H-6]t
It should be pointed out that this plot exhibited a considerable degree of scatter because only a small fraction of exchange had occurred by the end of the run.
Attempts to increase the rate of this reaction by
increasing the buffer base strength met with failure. Amine




68
buffers, generally the systems of choice at 250, are useless at 750 because the pronounced temperature dependence of their pK's4Z makes them little stronger than carbonate buffers at this temperature. The use of a DP04 2 -P04- buffer resulted in severe etching of the glass walls of the nmr tube.
A second run at 75.00.5 was carried out in 0.1M KOD. In this more strongly basic medium, exchange occurred at a much faster rate (t1/2 =2,320 minutes) than in the buffered run, and was followed for 1.5 half-lives. Curvature was evident in the 7-point kinetic plot, however, owing to a continuous decrease in pD due to etching.
After the completion of each of the kinetic runs at
75.0', the pD of the solution recovered from the nmr tube as well as that of a portion of the original solution (pD ) were determined at 75.00.50 (see section on pD measurements). The activity of the deuteroxide ion was obtained from the relation
a K D20, 750
OD- a D30+
where aDO+ =antilog (-pD) and KD20, 750 = 2.985 x 10-14 was calculated from recently reported data. '59)60 In most cases, pD and pD0 agreed to within 0.02 pD units.
The activity of deuteroxide ion so obtained and the pseudofirst order rate constant were then used to calculate the bimolecular rate constant k for I--D exchange at a particular




69
position from the rate constant equation k=
aOD
in all of the kinetic runs except those on the 1-methylpyridinium-3-oxide betaine, aD30+ was obtained from the measured pD of the recovered solution. Because of the drastic pD changes (1.271 pD units during the run in 0.1M KOD, and 0.447 pD units during the buffered run) and curved kinetic plots encountered during kinetic runs on this substrate, it was arbitrarily decided to calculate aOD- in each case from the average of the pD measurements on original and recovered solutions. The two bimolecular rate constants obtained from these two runs differ by a factor of 3.7. This agreement appears remarkable in view of the large experimental uncertainties (pD changes, limited number of points, curved kinetic plots) involved.
Attempts to follow the rates of H-D exchange at the
6-positions of the 3-cyano-, 3-nitro-, 3-acetyl, and 3-methylsulfonylpyridinium ions met with failure, owing to the occurrence of side reactions involving the consumption of base and substrate.
Heating a solution of 3-cyano-l-methylpyridinium perchlorate in a D3B03-D2BO- buffer at 75.00.5* for a total of 250 minutes resulted in partial exchange at the 6-position. Pronounced curvature in the kinetic plot began to become apparent after about 100 minutes, however, and new peaks were observed to arise in the aromatic region of the nmr spectrum. The solution gradually became dark brown as heating progressed.




70
The addition of basic buffer to a solution of 3-nitro-lmethylpyridinium iodide resulted in the immediate formation of a deep red color in the solution. The initial nmr spectra revealed considerably broadened peaks in the aromatic region, which sharpened after several minutes of heating at 75.00.50. This first heating period was accompanied by considerable darkening of the solution; further heating produced little color change. The spectra after several minutes of heating were found to be superimposable on those for solutions of the substrate in neutral or acidic solution. Little, if any, exchange occurred at the 6-position, even after prolonged heating. Dissolving the substrate in a D3BO3-D2B03 buffer solution of pD~10 at room temperature resulted in a rapid decrease in pD to about 8, as evidenced by testing a portion of the resulting solution with Hydrion pH paper. Use of deoxygenated solutions did not eliminate the color or pD changes. A crude experiment was carried out in which an 0.5M solution of 3-nitro-l-methylpyridinium iodide in HC03--CO32- buffer was heated for 15 minutes at 75.00. The aqueous solution was extracted with ether, and the extract dried over anhydrous Na2SO4. Its nmr spectrum revealed four sets of peaks in a 1:1:1:3 ratio: a doublet, farthest downfield, with JAB~3Hz; a pair of doublets (JAB~3Hz; JBC~ 10 Hz) at a position 34.5 Hz upfield from this peak; a doublet (JBc~10Hz), appearing at a position 126 Hz upfield from the peak at lowest field; and a 3-proton singlet 300 Hz upfield from the signal at lowest field.
Addition of basic buffer to a solution of 3-acetyl-lmethylpyridinium chloride or iodide resulted in the intermediate




71
formation of a dark red color. The nmr spectrum of the initial solution revealed that exchange of the methyl protons of the acetyl group had occurred to a large extent at room temperature, as well as considerable decomposition of the substrate, as evidenced by the appearance of extraneous peaks in the aromatic and aliphatic regions, some of which were partially overlapped with the substrate and water peaks. Exchange did not occur at the 6-position, even after prolonged periods of heating.
A solution of 3-methylsulfonyl-l-methylpyridinium iodide in D3BO3-D2B03 buffer was heated for a total of 1400 minutes at 75.00.50. H-D exchange of the methyl protons of the methylsulfonyl group occurred at room temperature, and was essentially complete after 15 minutes'heating. A small amount of exchange was observed at the 6-position, but curvature in the kinetic plot became increasingly severe with time. The solution darkened slightly during the course of heating, but no new peaks were observed to arise in the nmr spectra. pD measurements made at room temperature on portions of the original and recovered solutions indicated that the pD had dropped by more than 2 pD units during the course of heating.
Kinetics of deuteration at the 2- and 6-positions of
l-methylpyridinium-3-oxide betaine at 100.00.50.- The kinetics of H-D exchange at the 2- and 6-positions of 1-methylpyridinium3-oxide betai.Lne were determined at 100.00.50 in 0.06M and
0.30M KOD. The experimental data are given in Table 11. The ionic strengths of the two solutions were.different. Methanol was added as an internal standard in both cases; the ratio of the




Table 11. Kinetic Data for H-D Exchange at the 2- and 6-Positions of 1-Methylpyridinium3-Oxide Betaine in KOD Solution at 100.0 + 0.50
[OD'], Ma k2'6 sec-1 pD b pDc a0D-, Md ki'6 M- sec-1 e k26, M- sec-1 f
0.30 g 3.26 x 10 14.292 14.195 2.12 x 10 1.09 x 10 1.54 x 10
0.06 h 3.82 x 10-5 13.300 13.284 2.60 x 10-2 6.36 x 10-4 1.47 x 10aConcentration based on measured volume of stock KOD solution added by analytical syringe.
bpD of portion of original unheated solution at 25". CpD of portion of recovered solution at 250.
d
Activity of deuteroxide ion at 250, calculated from pD of recovered solution. eSecond order rate constant, based on measured [OD-]. fSecond order rate constant, based on aOD-.
gIonic strength 0.95.
hIonic strength 0.65.




73
area of the nmr signal for its methyl protons to that for the
1-methyl- protons of the substrate remained constant throughout the course of each run. The 1-methyl- signal was used as the area reference, and in both cases a plot of t vs [1-2] 0+ [H-6]0
log 0
[H-2]t + [H-6]t
was linear up to as much as 3.2 half-lives, indicating that the rates of exchange at the 2- and the 6-positions did not differ by more than a factor of two or three. An average of 15 points constituted each kinetic plot. After the completion of each run, pD measurements were made at 250 on portions of original and recovered solutions. The activity of deuteroxide ion at 250 was then determined from the relation
_ KD20, 250
aOD a D30+
where aD30+ =antilog (-pD) and KD20 25'=1 .351 x 10-1s was calculated from available literature data.59,60 Second order rate constants k2'6 and k2'6 are reported in Tablell, and
2C 2a
were calculated from the following expressions:
k26 2 6 k2 6
2C and k2a,6 = '
[OD] a aODwhere [OD-] represents the calculated concentration of deuteroxide ion (based on the volume of stock KOD solution added by syringe) and a OD- is the activity of deuteroxide ion as determined from the pD of the recovered solution. The apparent discrepancy between the values obtained for k2,6 2C
from the two runs is probably due to the error encountered in accurately syringing the rather large quantities of stock




74
KOD solution necessary for neutralization of the hydroxypyridinium ion, as the values obtained for k2'6 are in excellent 2a
agreement. The syringing error is expected to be more important in the preparation of the less basic solution, so that the value obtained for k2'6 from the run in 0.30M KOD 2c
should be regarded as the more accurate value.
The nmr signals for all of the annular protons of
1-methylpyridinium-3-oxide betaine overlap to a greater or lesser extent. It is therefore impossible to say with absolute certainty that exchange occurs at the 2- and 6-positions exclusively, simply from the appearance of the nmr spectra. After the pD measurement had been made on the recovered solution from the kinetic run in 0.30M KOD (presumably an 0.65M solution of l-methylpyridinium-2,6-d2-3-oxide betaine of 90 percent isotopic purity), the solution was neutralized (pD~7) with stock DC1 solution. The neutral solution was stirred with
0.420 g (i5mmoles) of sodium bicarbonate and 570 P1 (6 mmoles) of dimethyl sulfate at room temperature for 8 hours, the mixture filtered, and the nmr spectrum of the filtrate recorded. The signals in the aromatic region of this spectrum possessed a different appearance and occurred at lower field (relative to the methyl protons of CH3OD) compared to the corresponding signals in the last spectrum of the kinetic run. That exchange had occurred exclusively at the 2- and 6-positions of the 3-oxide was evidenced by the fact that the nmr spectrum of the methylation reaction mixture was identical to that of a solution of 3-methoxy-l-methylpyridinium-2,6-d2 iodide of




75
90 percent isotopic purity, obtained at the end of the kinetic run on this substrate at 75.00.50.
Kinetics of deuteration at the 2- and 6-positions of 1-methylpyridinium and 3-chloro-1-methylpyridinium ions in 0.030M DC1 at 197.50.50.- Kinetics of H-D exchange at the 2,6-positions of 1-methylpyridinium ion and at the 2- and 6-positions of 3-chloro-l-methylpyridinium ion in 0.03M DC1 were determined at 197.50.50. Pseudo-first order rate constants are reported in Table 12. The concentration of substrate was approximately 0.5M in all runs. Acetic acid was found to be unsuitable as an internal standard because its methyl protons undergo exchange under these conditions on a
time scale similar to that for exchange of the 6-proton of the 3-chloro-l-methylpyridinium ion. The methyl protons of isobutyric acid were found to be suitable for this purpose. Weighed quantities of sodium acetate or sodium isobutyrate were employed in preparing solutions, and the volume of stock DC1 solution necessary to completely neutralize the acetate or isobutyrate ions and to produce a D30+ concentration of 0.030M was added by analytical syringe. The 5-position was used as the area reference'in the case of the 3-chloro-lmethylpyridinium ion, and the 3,5-positions in the case of the 1-methylpyridinium ion. Appropriate area ratios were obtained as described previously. That these protons did not undergo exchange under the conditions of the run was evidenced by the fact that the ratio of the area of their signal to that of the methyl protons of the isobutyric acid




Table 12. Rate Constants for H-D Exchange at the 2- and 6-Positions of 1-Methylpyridinium
and 3-Chloro-1-Methylpyridinium Ions in 0.030M DC1 at 197.5 + 0.50*.
3-G k sec-1 k sec' pD a pD b
-Cl c 1.70 x 10-4 d,e 1.82 x 10-6 d,e
1.60 x 10 g fg----------- --- --2.07 x 10-4 g,h,i 3.45 x 10-6 g,h,i 1.635 1.937
Avg 1.79 x 10-'
-H c 4.45 x 10-7 g,h,i 4.45 x 10- 7g,h,i 1.556 1.915
apD of portion of original unheated solution at 250.
b
pD of portion of recovered solution at 25'. cSubstrate counterion Cl-.
dTemperature maintained at 197.5 + 1.00. eMethanol formed by solvolysis of the substrate. fAcetic acid, used as the internal standard, underwent exchange on a time scale similar to that for exchange at the 6-position of the substrate. gMethanol added to suppress solvolysis of the substrate. hDeoxygenated solutions employed. iIsobutyric acid internal standard.




77
internal standard remained constant. Reactions were followed for an average of 2.1 half-lives at each position. An average of 14 points constituted each kinetic plot.
An early run on the 3-chloro-l-methylpyridinium ion indicated that an equilibrium concentration of methanol was formed by solvolysis of the substrate on a time scale comparable to that for exchange at the 6-position. Solutions for subsequent kinetic runs were 0.32M in methanol. The addition of methanol apparently had the effect of successfully suppressing this solvolysis reaction, as the ratio of the area of the signal for the methyl protons of methanol to that for the methyl protons of the isobutyric acid internal standard remained constant. The effect of the solvolysis reaction is to increase the acidity of the solution, so that the value for k in Table 12, obtained from the run in the presence of added methanol, should be regarded as the more accurate va-lue. Kinetics of Hydrogen-Deuterium Exchange in 3-Substituted Pyridines
Hydrogen-deuterium exchange experiments at 197.50.5o.The kinetics of hydrogen-deuterium exchange at the 2- and/or 6-positions of pyridine and several 3-substituted pyridines in D20 or 3:17 (V:V) dioxane-D20 solution were studied at 197.50.50. Pseudo-first order rate constants, and the conditions under which they were obtained, are reported in Table 13. The presence of suitable internal standards verified the fact that, under these conditions, only the 2- and 6positions of the various substrates undergo exchange. An average of 11 points constituted each kinetic plot, and each




Table 13. Rates of H-D Exchange at the 2- and 6-Positions of 3-Substituted Pyridines
at 197.5 + 0.50.
3-G Solvent k2, sec-' k6 sec-1
-H D20 2.64 x 10-s a 2.64 x 10-s a
D20 b,c 4.11 x 10-s e 4.11 x 10-s e
Avg 3.38 x 10-s Avg 3.38 x 10-s
D20, p = 1.00 (NaCl) 2.30 x 10-s 2.30 x 10-s
3:17 [V:V) Dioxane-D20 b,c 3.19 x 10-s e 3.19 x 10-s e
3:17 (V:V) Dioxane-D20 3.19 x 10-s 3.19 x 10-s
-Cl D20 b 5.80 x 10-4 a 7.00 x 10-6 a
3:17 CV:V) Dioxane-D20 b 2.26 x 10-4 e 3.69 x 10-6 e
3:17 (V:V) Dioxane-D20 b,d 2.63 x 10-4 e '3.87 x 10-6 e
3:17 (V:V) Dioxane-D20 b,d 3.30 x 10-4 4.92 x 10-6
Avg 2.73 x 10-4 Avg 4.16 x 10-6
DC1 (35% Neutralized) b 3.44 x 10-4 a
DC1 (50% Neutralized) b 2.21 x 10-4 a 2.87 x 10-s a
DC1 (75% Neutralized) b 1.30 x 10-4 a
Oo




Table 13. Continued.
3-G Solvent k2, sec-1 k6, sec1
-Cl DC1 (90% Neutralized) b 4.62 x 10-4 a
-ND2 D20 c 2.41 x 10-4
-0Me D20 f 4.36 x 10-4
aRate constant based on three-point kinetic plot. bSolution deoxygenated. cTMAC internal standard.
d
Diethyl ether internal standard. eTemperature controlled to 197.5 + 1.00.
f
t-Butyl alcohol internal standard.




80
run was followed for an average of 2.4 half-lives.
Pseudo-first order rate constants for exchange at the 2- and 6-positions of 3-chloropyridine were determined in 3:17 (V:V) dioxane-D20 solution. The use of a cosolvent was necessary because of the low solubility of this substrate in D20. The rate of exchange at the 2,6-positions of pyridine was also determined in this solvent system for purposes of comparison.
The kinetics of H-D exchange at the 2,6-positions of
pyridinewere also studied in 1.OM NaCl (D20) in order to determine the effect of ionic strength on the reaction rate.
Preliminary experiments with 3-chloropyridine indicated that the presence of oxygen in the system resulted in serious decomposition on a time scale comparable to tiat for exchange at the 6-position of this substrate. Later runs were carried out in deoxygenated solutions.
All sample solutions were approximately O.SM in substrate, with one exception. In order to study the rate of exchange at the 2- and 6-positions of 3-chloropyridine in D20, it was necessary to employ dilute (approximately 0.2M) solutions so that solubility of the substrate was effected. At this low concentration, however, the signal-to-noise ratio in the nmr spectrum diminishes to an extent which makes accurate integration an impossibility. To circumvent this problem, rate data wcre obtained by heating a portion of the dilute solution in a sealed tube for a certain period of time. After quenching, the tube was cracked open, the D20 solution saturated with




sodium chloride, and the substrate concentrated by extraction with a small volume of carbon tetrachloride. The extracts were dried over anhydrous sodium sulfate, the drying agent was removed, and the nmr spectrum of the carbon tetrachloride solution was recorded. Appropriate area ratios were then obtained from these spectra. Only two heating periods, generally covering about 60-70 percent of the reaction, were employed for each position. The positional rate constants obtained using this technique, then, result from kinetic plots consisting of only three points (the origin plus two experimental points), and are therefore to be regarded as only approximate.
A series of experiments were carried out in which the
rates of exchange at the 2- and 6-positions of 3-chloropyridine were studied as a function of the degree of neutralization of the substrate. An appropriate volume of deoxygenated stock DC1 solution was added by analytical syringe to give the desired percent neutralization, and the solution divided into two equal portions. Since partial neutralization of this substrate results in considerable broadening and overlap of peaks in the nmr spectrum, rate data were obtained by employing the extraction technique described previously, after prior neutralization of the acid by addition of 1.5M Na2CO3. Again, these rate constants are only approximate, since they result from kinetic plots consisting of only three points.
The nmr spectrum of pyridine in D20 or 3:17 (V:V)
dioxane-D20 appears, in the order of increasing field, as a




82
series of complex multiplets representing signals for the 2,6-, 4-, and 3,5-protons. As exchafige approaches completion at the 2,6-positions, the signals for the 4- and 3,5-protons appear as a sharp triplet and doublet respectively. In this case, the signal for the 3,5-protons was used as the area reference, and the appropriate area ratio is then defined by
[H-2,6] [H-3,5]t
[H-2,6]t [H-2,6]t
The nmr spectrum of 3-chloropyridine in 3:17 (V:V) dioxaneD20 appears basically as a set of overlapped peaks for the 2- and 6-protons at lowest field, followed at slightly higher field by a doublet for the 4-proton, and finally a quartet for the 5-proton at highest field, although a good deal of fine structure due to cross-ring coupling is also evident. As exchange occurs at the 2-position, the 6-position emerges as a doublet which is further split by cross-ring coupling. As exchange at the 6-position approaches completion, the 4- and 5-protons appear as a pair of doublets. The signal for the 5-proton was used as the area reference for this substrate, so that
[H-2]0 [H-5]t and [H-6] [H-5]t
- and[H-2]t ([H-2]t + [H-6]t) [H-5]t [H-6]t TH-6]t
The nmr spectra of 3-methoxy- and 3-aminopyridine in D20 are very similar. The initial spectra both exhibit a complex multiplet representing overlapped signals for the 2- and 6-protons at lowest field followed by another complex multiplet




83
representing overlapped signals for the 4- and 5-protons at higher field. As exchange approaches completion at the 2-position, the 6-position reveals itself as a triplet. The overlapped signals for the 4- and 5-protons were used as the area references in these cases, with the appropriate ratio being given by
[H-2]0 0.5([H-4]t + [H-5]t)
[H-2]t ([H-2]t + [H-6]t) 0.5([H-4]t + [HI-5]t)
Hydrogen-deuterium exchange experiments at 217.9+0.50.The kinetics of hydrogen-deuterium exchange at the 2- and 6-positions of pyridine and several 3-substituted pyridines in D20 solution were studied at 217.9+0.50. Pseudo-first order rate constants, and the conditions under which they were obtained, are reported in Table l4. The presence of suitable internal standards verified the fact that, under these conditions, only .the 2- and 6-positions of the various substrates undergo exchange. Pseudo-first order kinetic plots were linear for up to an average of 2.4 half-lives, and were composed of an average of 10 points.
In order to determine the effect of ionic strength on
the rate of H-D exchange reactions at this temperature, a run with pyridine as the substrate was carried out in 1.0M NaCl (D20).
Solid sodium carbonate was added to D20 solutions of
3-pyridinesulfonic acid and nicotinic acid in order to insure that the substituents were in the anionic form. Kinetic runs with nicotinic acid were carried out at several different




Table 14. Rates of H-D Exchange at the 2- and 6-Positions of 3-Substituted Pyridines
at 217.9 + 0.5*.
3-G Solvent k2, sec- k sec- pD pD
-H D20 1.58 x 10-4 a 1.58 x:10-4-- ---D20 2.10 x 10- b 2.10 x 10- b
Avg 1.84 x 104 Avg 1.84 x 104
D20, .p = 1.00 (NaCI) 2.60 x 10-4 2.60 x 10-4 ---CO2 D20 7.50 x 10-5 2.30 x 10-4 ~7 d,e 7 d,e
D20 c 7.04 x 10-5 2.10 x 10-4 9.748 ,f 9.462 df
D20 c 7.23 x 10-s 2.29 x 10- 10.375 d,f 9.984 d,f
Avg 7.26 x 10- Avg 2.23 x 10-4
-SO3 D20 c 3.36 x 10- b 7.67 x 10 b 10.084 df 9.897 d,f
D20 2.46 x 10-s g 8.55 x 10-6 g---- ----ND2 D20 h 1.40 x 10- 5.72 x 10-5 11.108 f 11.063 f
-OMe D20 i 2.32 x 10- [8.15 x 10-s] 8.914 f 7.887 f
aC. L. Smith, Ph.D. Dissertation, University of Florida (1968).




Table 14. Continued.
bRate constant based on three-point kinetic plot. CMeOD internal standard. dpD adjusted by addition of solid sodium carbonate.
e
Approximate pD at 25' as determined by testing a portion of the solution with Hydrion pH paper.
pD at 250.
gun in the absence of added base. hTMAC internal standard. 1t-Butyl alcohol internal standard. iRate constant unreliable because of the occurrence of side reactions and a substantial pD change during the course of the run.
00
Lq




86
base concentrations. In these cases, pD measurements at 250 were made on portions of original and recovered solution. The kinetics of H-D exchange at the 2- and 6-positions of 3-pyridinesulfonic acid were also studied in the absence of added base.
TMAC, which was employed as the internal standard in the case of 3-aminopyridine, reacted under the conditions of the run, as evidenced by a continuous decrease in the area of its signal relative to those of the 4- and 5-protons of the substrate. This loss of signal, which occurred on a time scale comparable to that for exchange at the 6-position, was accompanied by the appearance of a new signal about 50 Hz upfield from that of the tetramethylammonium ion which increased in area with time, until, by the end of the run, both peaks were of equal area. The combined integral of these peaks did, however, maintain a practically constant ratio to that of the 4- and 5-protons. The strong, fishy odor characteristic of alkyl amines was apparent on opening the nmr tube at the end of the run; addition of a small amount of trimethylamine resulted in an increase in size of the peak farthest upfield. Acidification of the solution resulted in a downfield shift of both the tetramethylammonium and trimethylamine peaks; no other extraneous peaks were apparent. The nmr spectrum after addition of a small amount of 3-amino-l-methylpyridinium iodide indicated that alkylation of the annular nitrogen had not taken place. pD measurements on portions of original and recovered solution indicated that very little change had




87
taken place during the course of the run. The rate constant reported in Table 14 was in good agreement with that obtained from a preliminary run with t-butyl alcohol as the internal standard. It seems likely that the loss of tetramethylammonium ion results from displacement of Me3N by water, deuteroxide, or chloride ion. The substitution products, methyl alcohol or methyl chloride, could conceivably escape detection. Oxidation, eventually resulting in the formation of formate ion, could occur in the former case. Because of the high volatility and insolubility of methyl chloride in D20, it might also go undetected. In any event, this side reaction does not present a serious complication.
The kinetics of hydrogen-deuterium exchange at the
6-position of 3-methoxypyridine was complicated by several side-reactions. Ether cleavage took place, as evidenced by a gradual decrease in the area of the signal for the methyl protons of the substituent relative to that of the internal standard. The nmr spectra also were characterized by the simultaneous appearance of a new peak of unknown origin about 12-15 Hz downfield from the -OCH3 signal, as well as of a signal 28 Hz upfield from the -OCH3 signal which was shown to be due to methanol (by addition of a small amount of the same). Furthermore, the combined area of these three peaks, relative to that of the internal standard, decreased with time, so that a 22 percent reduction in signal had occurred by the end of the run. Ofthe signal remaining, approximately 10 percent was due to the unidentified upfield peak, while 13 percent was dbe tomethanol. Inaddition, the recovered solution was found to bea fullpD




88
unit more acidic than a portion of the original solution at 250. Although the combined area of the three peaks maintained a constant 3:2 ratio with that of the 4- and 5-protons of the substrate, and the pseudo-first order kinetic plot was linear up to 2.5 half-lives, the reliability of the rate constant reported in Table 14 must be regarded with skepticism because of the complicating factors just discussed.
The nmr spectra of pyridine and 3-amino- and 3-methoxypyridine were discussed in the previous section. In the latter two cases, exchange at the 6-position was followed by measuring the area ratio given by
[H-6]0 0.5([H-4]t + [H-5]t)
[H-6]t [H-6]t
The nmr spectra of the neutralized forms of 3-pyridinesulfonic acid and nicotinic acid are similar. Basically, a singlet for the 2-proton occurs at lowest field, followed by, in the order of increasing field, a pair of doublets and a quartet for the 6-, 4-, and 5-protons respectively, although, as in the case of most 3-substituted pyridines, significant cross-ring coupling is evident. As exchange approaches completion at the 2- and 6-positions, the 4- and 5-positions appear as a pair of sharp doublets. The 5-position was used as the area reference in both cases.
The signals for the 4- and 6-protons are overlapped in
the nmr spectrum of 3-pyridinesulfonic acid in the absence of added base. Exchange at the 6-position was followed by employing the area ratio defined by




89
[H-6] [H-5]t
[H-6]t ([H-6]t + [H-4]t) [Ii-5]t
Kinetics of hydrogen-deuterium exchange in quinazoline in D20 at 164.70.50. The kinetics of hydrogen-deuterium exchange at the 2- and 4-positions of quinazoline in D20 solution were studied at 164.7+0.50. Pseudo-first order rate constants are summarized in Table 15. Under these conditions, exchange occurred exclusively at these two positions.
The 2- and 4-protons appear as a pair of singlets in the
nmr spectrum of quinazoline, with the signal for the 4-position appearing at lowest field. A complex multiplet representing overlapped signals for the remaining annular protons appears at highest field. The area of this multiplet for the benzoprotons was used as the area reference, so thai
[H] 0.25 ([H-5] + [H-6] + [H-7] + [H-8])t
[Hi] t [H]t
represents the appropriate area ratio for the proton of interest. pD Measurements
Measurements of pD were performed on many of the solutions
employed in the various kinetic runs, both at 250 and at 75.0+0.50. NBS standard buffers were prepared as described by Bates.62
For pD measurements at 250, the meter was standardized
at pH 6.865 against the NBS phosphate buffer by adjusting the standardization control on the meter.62 When the pD of an acidic solution was being measured, the meter was linearized




Full Text

PAGE 1

Substituent Effects on the Rates of Pyridinium Ylid Formation By ROBERT E. CROSS A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1971

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To My Wife, Eileen Kay

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ACKNOWLEDGEMENT The author will always be indebted to Dr. John A. Zoltewicz, Chairman of his Supervisory Committee, for his enthusiastic guidance, patience, and support throughout the course of this research. Appreciation is also extended to the other members of his Committee, Dr. M.A. Battiste, Dr. W. R. Dolbier, Jr., Dr. R. C. Stoufer, and Dr. E. G. Sander. A particular debt of gratitude is due his wife, Eileen, for her unfailing patience, understanding, and encourageme11t during these years of long days and nights. The friendship and assistance of his fellow graduate students will always be gratefully remembered. Special thanks are extended to Mrs. Judi Curtin for her patience during the typing of this dissertation. Financial support from the Chemistry Department and Graduate School of the University of Florida is gratefully acknowledged. iii

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TABLE OF CONTENTS Page ACKNOWLEDGEMENT ................ . . . . . iii LIST OF TABLES........................................ vi LIST OF FIGURES ....................................... viii ARSTRAC r I' X CHAPTER 1. INTRODUC1., ION. . . . . . . . . 1 2. RESULTS AND DISCUSSION....................... S Hydrogen-Deuterium Exchange at the 2and 6Positions of 3-Substituted-1-Methylpyridinium Ions. . . . . . . . . . . 5 Hydrogen-Deuterium Exchange at the 2and 6Positions of 3-Substituted Pyridines......... 27 3. EXPERIJ\1ENT i \L ......... .......... : . . . 41 Instrumentation.............................. 41 Chemicals.................................... 41 Stock Solutions.............................. 41 Su bstrates....... . . . . . . . 43 Pyridiniurn Salts............................. 45 Pyridinium Betaines.......................... 50 Preparation of Solutions for Kinetic Runs. SO Kinetic Runs. . . . . . . . . 51 Kinetics of Hydrogen-Deuterium Exchange in 3-Substituted-1-Methylpyridinium Ions........ 55 Kinetics of Hydrogen-Deuteriu m Exchange in 3-Substituted Pyridines... . . . . . 77 iv

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Page pD ~1ea sur cmen ts . . . . . . . 8 9 Control Runs ............................ ..... 92 B I BL IO G R.r'\. P I-IY . . . . . . . . . 9 4 BIOGRAPH I CAL SKETCH. . 98 V

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LIST OF TABLES Table Page 1. Rate Constants for the Deuteroxide Ion-Catalyzed Formation of 3-Substitutecl-1-Methylpyridinium Ylids in Deuterium Oxide at 75.0.5............ 11 2. Resonance and Inductive Constants for the Sulfonate and Carboxylate Substituents in Weakly Protonic Sol vents................................ 24 3. Relative Rates for H-D Exchange at the 2and 6-Positions of Pyridine and 3-Chloropyridine in 3 : 1 7 (V : V) Di ox an e D 2 0 at 1 9 7 5 0 5 . . . 2 9 4. Observed Dependence of the Rate of H-D Exchange (197.5.5) at the 2and 6 Positions of Partially Neutralized 3-Chloropyridine on the Degree of Neutralization ... ;..................... 33 5. pK Values (25) for the Conjugate Acids of Py~idine and 3-Chloropyridine. ... .. ............. 34 6. Chlorine Rate Factors............................ 34 7. Substituent Effects on Hydrogen-Deuterium Exchange at the 2and 6-Positions of a Series of 3-Substituted-1-Methylpyridinium Ions and 3-Substituted Pyridines.. ... .... ... ..... ....... 38 8. Experimental Data for the Preparation of a Series of 3-Substituted-1-Methylpyridinium Salts........ 46 9. Rates of H-D Exchange at the 2-Position of 3-Substituted-1-Methylpyridiniurn Ions in D2O at 75.0.5 and 1.0 Ionic Strength................. 56 10. Rates of H-D Exchange at the 6-Position of 3-Substituted-1-Methylpyridiniurn Ions in D2.O at 75.0.5 and 1.0 Ionic Strength................. 59 11. Kinetic Data for H-D Exchange at the 2and 6-Pos it ions of 1-Methylpyridiniurn.,. 3-Oxide Betaine in KOD Solution at 100.0,5 ...... .......... ... 72 vi

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Table Page 12. Rate Constants for H-D Exchange at the 2and 6-Positions of 1-Methylpyridinium and 3-Chloro1-Methylpyridinium Ions in 0.030M DCl at 197. 5. Sb ........................ : . . . 76 13. Rates of H-D Exchange at the 2and 6-Positions of 3-Substitute
PAGE 8

LIST OF FIGURES Figure Page 1. Mechanism for Base-Catalyzed Exchange at the 2and 6-Positions of 3-Substituted-l-Methylpyridinit1111 Ions.................................. 8 2. Plot of log k~ for H-D Exchange at the 2-Position of l-Methylpytidinium-3-Sulfonate Betaine at 75.0.5 VS pD.................................. 9 3. Taft Plot for H-D Exchange at the 2-Position of 3-Substituted-1-Methylpyridinium Ions in Buffered D2O Solution at 75.0.5~... .. ......... ......... 13 4. Correlation of H-D Exchange Rates at the 2Position of 3-Substituted-1-Methylpyridinium Ions in Buffered D 2 O Solution at 75.0.5 by the Extended Hammett Equation........................ 15 5. Correlation of Exchange Rates it the 6-Position of 3-Substituted-1-Methylpyridinium Ions in Buffered D 2 O Solution at 75.0.5 with the a 0 Substituent Parameter ........................ ~... 16 6. Correlation of H-D Exchange Rates at the 6Position of 3-Substituted-1-Methylpyridinium Ions in Buffered D 2 O Solution at 75.0.5 by the Extended HanITTctt Equation........................ 17 7. Correlation of Amide Ion-Catalyzed Rates of Exchange at the para-Positions of Monosubstituted Benzenes in Liquid Ammonia by the Extended Hammett Equation................................. 21 8. Relative Rates of Exchange at the 2and 6Positions of l-Methylpyridinium-3-Oxide Betaine and l-Methyl-4-Pyridone in OD-/D 2 O at 100.0..... 26 9. Proposed Pathway for Base-Catalyzed Hydrogen Deuterium Exchange at the 2,6-Positions of l-Methyl-4-Pyridone.............................. 27 viii

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Figure Page 10. Proposed Mechanism for H-D Exchange at the 2and 6-Positions of 3-Substituted Pyri
PAGE 10

Abstract of Dissertation Presented to the Graduate Council of the University of Florida in Partial Pulfillment of the Requirements for the Degree of Doctor of Philosophy SUBSTITUENT EFFECTS ON THE RATES OF PYRIDINIUM YLID FORMATION by Robert E. Cross June, 1971 Chairman: Dr. John A. Zoltewic z Major Department: Chemistry Rates of hydrog e n-deuterium exchang e at the 2and 6positions of a series of 3-substituted-1-methylpyridinium ions in buffered D 2 0 solutions at 75.0i0.5 were obtained by the use of an nmr method in order to determine the effects of substituents on the rates of formation of the corresponding ylid intermediates. Deprotonation is catalyzed by deutcroxide ion. Substituents are observed to exert a profound influence on reactivity; rate spreads of 10 9 and 10 4 were obtained for the 2and 6-positions respectively. Rates of exchange at the 2and 6-positions are correlated by the extended Hammett equation. From an evaluation of the data by this treatment, it is concluded that both inductive and resonance effects of the substituents influence reactivity. The inductive effect is found to prednntinate at position 2; this effect is diminished, while the resonance effect assumes a more _important role at position 6. X

PAGE 11

Rates of hydrogen-deuterium exchange at the 2and 6-positions of a series of 3-substituted pyridines in D 2 0 or 3:17 (V:V) dioxane-D 2 0 were obtained at 197.5.5 or 217.9.5 by the use of this nmr method in order to determine the effects of substituents on the exchange rates in the free bases. A two-step mechanism, involving an equilibrium protonation of nitrogen followed by a rate determining C-H ionization, was demonstrated for these reactions. The rate spreads encountered are smaller than those observed for exchange in the 3-substituted-1-methylpyridinium ions. The results are explained in terms of the effects of substituents on each step of the two-step mechanism. xi

PAGE 12

CHAPTER 1 INTRODUCTION Substituent effect studies on the rates of base catalyzed hydrogen-deuterium exchange reactions have been reported for several aromatic systems. Several groups of workers have independently studied the rates of amide ion catalyzed H-D exchange at the various positions of monosubstituted benzenes in liquid ammonia. 1 2 3 Correlations of exchange rates at the ortho and meta positions of these substrates were found with the Taft a 1 substituent parameters and the Hammett am substituent constants respectively. 1 Exchange rates at the para positions of these compounds were not correlated. by the Hammett a parameters. 1 It p has recently been shown, however, that meta and para reactivities in the benzene series are best correlated by the substituent constants respectively.'+ a o m and a 0 p Rates of isotope exchange reactions at the 4-positions of a series of 3-substituted pyridines ( 1 ), 5 the 2-positions 6 7 of 3-substituted-pyridine-1-oxides ( 2 ),'+ 2-positions of 3-substituted thiophenes ( 3 ) 8 and the were, in each case, found to be correlated by the Taft o 1 parameters. Methoxide ion catalyzed rates of exchange at the 5-position 1

PAGE 13

G CT S D 1 3 G D D S -~ D S G 4 5 of 3-substituted thiophenes (4)~ and at the 5-position of 2-substituted thiophenes (5), were found to be correlated by the om 0 and op 0 parameters respectively. 8 It should be pointed out, however, that in the case of the amide ion catalyzed exchange studies in liquid ammonia, competing side reactions were enountered in several instances, 1 some of the data were not reproducible, 9 and 2 the number of substituents investigated was limited. 1 Furthermore, the methoxide ion catalyzed exchange studies in methanol often involved temperature extrapolations because of the large reactivity span encountered. All of these substituent effect studies have been

PAGE 14

interpreted in terms of a mechanism involving simple deprotonation by base at an annular position of an aromatic carbon acid. 3 Hydrogen-dPuterium exchange in a number of heterocycles has also been studied in buffered D20 solution. Zoltewicz and Smith have demonstrated that, under these conditons, pyridine undergoes exchange at the 2,6-positions by a mechanism i11volving an equilibrium protonation on nitrogen, followed by a rate-determining C-H ionization. 10 This mechanism has also been demonstrated for exchange at the 2-positions of thiazole 11 ( 6 ) and imidazole 1 2 ( 7 ) and at the 3, 5positions of pyrazole 13 ( 8) under similar conditions. 6 '.>D 7 \ H D)p)-n ~-N H 8 The effects of substituents on this two-step mechanism have, however, never been determined. The study reported in the first part of Chapter 2 was carried out in order to determine the effects of sub stituents on the rates of hydrogen-deuterium exchange (pyridiniura ylid formation) at the 2and 6-positions of a series of 3-substituted-1-methylpyridinium ions in buffered D 2 0 solutions. Under these conditions, previous preliminary work indicated that large reactivity spreads could be expectcd 14 15 which could be conveniently investigated at a

PAGE 15

4 constant temperature. An obvious advantage of employing D 2 0 as the solvent is that the ultimate goal in these and similar studies is the determination of carbon acidity in aqueous media. The study reported in the second section of Chapter 2 was carried out in order to demonstrate the operation of the two-step mechanism for exchange at the 2and 6-positions of a series of 3-substituted pyridines in D 2 0 solution, and to determine the effects of substituents on the exchange rates in terms of this mechanism. Since a pyridinium ion is a proposed intermediate in the two-step mechanism~ it was hoped that the information ohtained from the study on the substituted pyridinium ions would prove useful in interpreting the effects of substituents on the rates of exchange in the free bases, where the two-step mechanism would presumably operate.

PAGE 16

CHAPTER 2 RESULTS AND DISCUSSION t~~gen-Deuterium Exch a nge a~ thE: 2and 6 Positions of 3-Substituted-1-Methylpyridinium Ions The kinetics of ll-D exchange at the 2and 6-positions of a series of 3-substituted-1-methylpyridinium ions (9, Figure 1) in buffered D 2 0 solutions of 1.0 ionic strength were measured at 75.0.5. The exchange reactions were followed by measuring the change in the integrated area of the appro priate umr signals. The nmr spectra of most 3-substituted pyri
PAGE 17

6 a reaction with deuteroxide ion resulting in the formation of a product, the nmr spectrum of which suggests that it is either 3-nitro-l-methyl-4-pyridone C!Q) or 5-nitro-l-methyl2-pyridone Ul). The latter possibility appears to be the most 0 ONO, N J Me 10 ~ ~ 11 likely on the basis of the rather large meta coupling constant (~3Hz) observed in the nmr. Coupling is not expected to be nearly so strong in the case of 10, where a nitrogen a tom is -"' positioned between the carbon atoms bearing the two interacting nuclei. 16 Such products could conceivably result from nucleophilic attack by deuteroxide ion on the pyridinium ring, followed by oxidation of the resulting pseudo-base. 20 The 3-acetyl-1-methylpyridinium ion reacts at room tem perature with basic buffer. Possible side reactions include aldol condensation as well as attack by enolate anion on a pyridinium ring to give stable adduct such as 12 or 13. 21 Attempts at measuring the rate of exchange at the 6-position of 3-methylsulfonyl-1-methylpyridinium iodide resulted in curved kinetic plots. The pD of the reaction mixture decreased steadily with time, but no extraneous peaks were apparent in the nmr spectrum.

PAGE 18

7 0 0 12 13 During the remainder of the kinetic runs using deuterium oxide and during control runs using protea water, no appreciable degradation of substrate could be detected. Results of these kinetic studies are reported in Tables 9 and 10. Under the conditions employed, no other annular protons underwent detectable exchange. 1-Alkylpyridinium ions undergo base-catalyzed hydrogen deuterium exchange by simple deprotonation to give intermediates such as 14 and 15 (Figure 1). 14 15 ,22-2s Because the base concentration remains constant during a given run, the kinetics are pseudo-first order. The rates of deprotonation are then described by the equation + + k~ [PyrCH3] = kz[PyrCH3 ]aODwhere k~ is the observed pseudo-first order rate constant. The second orJer rate constant k 2 may then be calculated from k~ and the activity of deuteroxide ion as determined from pD measurements at 75.0.5.

PAGE 19

H ~G AY)=H 1+ @G+ OD N+ H I / Me 14 Me 9 15 Figure 1. Mechanism for Base-Catalyzed H-D Exchange at the 2and G Positions of 3-Substituted-1-Methyl pyridiniu m Ions. Several of the substrates were studied at two or more pD values; in these instances, exchange was demonstrated to be first order in deuteroxide ion. For example, a plot of 8 log ki pD for the three kinetic runs on l-methylpyri
PAGE 20

I u (!) V) 5 0 ----------10 5. 0 1.0 o. 5 ----r-7 .00 7,50 8.00 8,50 pD Figure 2. Plot of log k: for H~D Exchange at the 2-Po~ition of l-Methylpyridiniurn-3-Sulfonate Betaine at 75.0.5 vs pD. 9

PAGE 21

or base. Moreover, undissociated water does not appear to act as a kinetically detectable base in these cases. It is likely that the Brnsted B value for these reactions sho~ld therefore be ~I. 14 30 10 The results reported in Table 1 indicate how substituents of 3-substituted-1-methyl pyridinium ions influence the rates of ylid formation at the 2and 6 positions (Figure 1 ) Here the effects of substituents bonded to a carbon atom are on ortho and para centers. The effects on reactivity are observed to be large, with the 2-position being the more sensitive. This follows from a consideration of the kG/kH ratios, which compare the effect of a substituent relative to the effect of hydrogen. The 2-position is deactivated by 0-, Me, and CO 2 sub~tituents, while activated by ND 2 MeO, S0 3 -, I, Cl, CN, and N0 2 groups. The 6-position is deactivated by 0-, Me, MeO, and ND 2 substituents, while activated by CO 2 -, S0 3 -, I, and Cl groups. As previously mentioned, the effect of CN and N0 2 groups on the reactivity of the 6-position was not obtained, owing to side-reactions during hydrogen-deuterium exchange studies. If the effects of onesubstituent acting on both positions 2 and 6 are compared in Table 1, it is evident that all possible combinations of activation-deactivation are found. Thus, substituents which activate both 2and 6-positions include S0 3 -, I, and Cl; substituents which deactivate both positions include O and Me. The only grpup which deactivates 2 and activates 6 is CO 2 Groups which activate 2 and deactivate 6 include ND 2 and OMe.

PAGE 22

Table 1 Rate Constants for ~1e Deuteroxide Ion-Catalyzed Formation of 3-Substituted1-Methylpyridinium YliJs (14 and 15) in Deuterium Oxide at 75.0.5. k2, M-1 sec1 3-G 2-Position 6-Position k~/kI} k~ /kI] kz/kG -H 1.99 X 101 1.99 X 101 1.00 1.00 1.00 -a [2 X 10'+] [2 X 104 ] [l 103 ] [l 103 ] -1 -0 X X -Me 8.87 X 102 8.87 X 102 4.45 X 101 4.45 X 101 -1 -CO2 9.90 X 102 3.22 X 101 4.98 X 101 1. 62 3.08 X 101 -ND2 2.76 X 101 1.23 X 102 1. 39 6.19 X 102 2.24 X 10 1 -OMe 1.23 X 10 1 1.64 X 102 6.19 X 10 1 8.25 X 101 7.50 X 10 1 -SO 3 4.24 X 10 1 2.79 2.13 X 10 2 1.40 X 10 1 1. 52 X 10 1 I 8,85 X 10 1 2. 25 4.45 X 10 2 1.13 X 10 1 3.93 X 10 1 -Cl 5.25 X 10 2 2.30 2.64 X 10 3 1.16 X 10 l 2.28 X 10 2 -CN 1.05 X 10'+ 5.28 X 10 4 -NO2 1. 8 7 X 10 5 9.40 X 10 5 aUncertain value

PAGE 23

12 A correlation exists between log kl and the Taft inductive parameter o 1 obtained from F 19 shielding data for meta-sub stituted fluorobenzenes in weakly protonic solvents (o 1 values for some polar groups are solvent dependent). 31 This is shown in Figure 3. The least-squares slope p 1 of this plot is 9.4, with a correlation coefficient r of 0.974 and a standard deviation of the slope of 0.7. 32 The standard deviation of the intercept is 0.8. 32 Values of o 1 for the Cl and I substituents were not available from the fluorine nmr data. 31 Accordingly, the values +0.46 (Cl) and +0.39 (I), obtained from Taft's latest compilation based on chemical reactivities, were employed. 33 This appears to be justified by the fact that the o 1 values for the F and Br substituents are independent of their source (chemical reac+ivity data, or fluorine nmr data in a variety of solvent systems). 31 The value of -0.35, tabulated in Taft's paper as the o 1 constant for the CO 2 substituent in weakly protonic solvents, appears to be in error. 31 A recalculation of the chemical shift data indicates that this value should be -0.05 instead. The data for exchange at the 2-position may also be considered in terms of the extended Hammett equation log k~/k~=p 1 a 1 + pRoR 0 which separates the effects of substituents into their inductive (p 1 o 1 ) and resonance (pRoR~) components. 34 35 This four-parameter equation may be applied to the data more conveniently when it is expressed in the form (log k~/k~)/oR 0 P1Cor/~R 0 ) + PR

PAGE 24

NN bl) 0 rl + 13 12. 0 10.0 8. 0 6. 0 Pr = 9.4 r = 0.974 4.0 oo 2. o.._ _____ _._ _____ L_ __ ___ ___ ___ ~_ -0.20 0.00 +0,20 +Q,40 er I Figure 3. Taft Plot for H-D Exch a nge at the 2-Position of 3-Substituted-1-Methylpyridinium Ions in Buffered D 2 0 Solution a t 75.0.5.

PAGE 25

14 G H A plot of (log k 2 /k 2 )/aR 0 vs cr 1 /aR 0 should then resultb1 a straight line, from which the intercept gives pR and the slope p 1 36 Such a plot for exchange at the 2-position of the various substrates is illustrated in Figure 4 The least squares slope Pr of tl1is plot ts 8.27; the intercept pR is 1..94. The success of this equation in correlating the data is indicated by a correlatitin coefficient of 0.998; the standard deviations of the slope and intercept are 1.5 and 0.2 respectively. 32 The sulfonate and carboxylate substituents wire omitted from the analysis because of the lack of tested oR 0 values for these groups. The data for exchange at the 6-position of 3-substituted1-methylpyridinium ions give a poor correlation with the op (r=0.928) or a; (r=0.929) substituent constants. 32 35 37 An excellent correlation (Figure S) is obtained, however, with the a O paramcter. 35 The least-squares slope p of this p plot is 3.39; the correlation coefficient r is 0.993. 32 The standard deviations of the slope and intercept are 0.1 and 0.2 respectively. 32 The oxide substituent was eliminated from all of the plots, because the appropriate substituent constants for this group are not known. For this reason, the substituent was also eliminated from the a; and a o p plots, as well as the CO 2 substituent from the op 0 plot. The data for exchange at the 6-position may also be correlated by the extended Hammett equation. This is illustrated in Figure 6 The least-squares slope Pr of this plot is 3.84; the intercept pR is 2.91. 32 The correlation

PAGE 26

15 40 30 20 ::r::"' -....... (.'.JN b 0 rl 10 0 0 0 /<>Me Pr = 8.27 OND 2 I PR = 1.94 OQMe r 0.998 -10 '-' Cl -20 __ L -2 0 +2 +4 +6 oI/oR 0 Figure 4. Correlation of H-D Exchange Rates at the 2-Position of 3-Substituted-1-Methylpyridinium Ions in Buffered D 2 0 Solution at 75.0.5 by the Extended Hammett Equation.

PAGE 27

t~----..-r ,....._ .. .,.......,.., ,._.-..-....,., .,....._.., _._. ., .,_----.....,._,..,..,,._ ,..,. -.., ... __ .,. ______ __ .. .... +l.O 0.0 MeOO -1. 0 ( / > Me 2 0 ___ __ __ _L_ -0.40 -0.20 (J 0 p j o.oo p = 3.39 r = 0.993 _____ ..,__,,, ___ ..J +0.20 Figure 5. Correlation of Exchange Rates at the 6-Position of 3-Substituted-1-Methylpyridinium Ions in Buffered D 2 0 Solution at 75.0.5 with the o 0 Substituent Parameter. p 16

PAGE 28

17 +4.0 0 Me +2.0 ::r::> 0.0 -........ t=)"' 0 b co 0 rl -2.0 Pr = 3.84 -4. 0 PR = 2.91 t = 0.987 -6.0 --~--~---~-----L ____ _,_ __ .., ~2.0 -:1,0 0,0 +1.0 aI/aRo Figure 6. Correlation of H-D Exchange Rates at the 6-Position of 3-Substituted-1-M~thylpyridinium Ions in Buffered D2O Solution at 75.0.5 by the Extended Hammett Equation.

PAGE 29

coefficient r is 0.987, and the standard deviations of the slope and intercept are 0.8 and 0.3 respectively. 32 The correlation between Taft's a 1 values and the rates of deuteroxide ion catalyzed hydrogen-deuterium exchange at the 2-positions of 3-substituted-1-methylpyridinium ions (p 1 =9.4, 75.0.5), Figure 3, as well as that between the 18 crp 0 substituent parameter and the corresponding rates of exchange at the 6-positions of these ions (p=3.4, 75.0.5), Figure S, pr ov ides additional strong evidence for the formation 0 pyridiniu m ylid intermediates 14 and !~ during exchange. Furthermore, the effects of these substituents are large. The reactivity of these ions may be considered to be influenced both by the 3 substituent itself, acting on ortho and para reactive ce~ters in the cases of exchange at the 2and 6-positions, respectively, as well as by the positively charged nitrogen atom. A measure of the activating effect of the positively charged nitrogen atom may be obtained from a comparison of the reactivity of the pyridinium ion 16 16 --:v with that of benzene. Calculations, which involve several extrapolations and the neglect of small temperature and solvent differences, have been performed which indicate that the positively charged annular nitrogen atom activates an aromatic ring for deprotonation via ylid fo-rmation by an enormous factor of approximately 10 14 to 10 16 14

PAGE 30

19 In spite of the very large activ~tion by the positively charged nitrogen atom, substituents at the 3-position also exert a large influence on the rates of anion formation. The rate spread for exchange at the 2-position between substrates having NO 2 and O substituents, which represent the extreme cases for this position, is a factor of approximately 10 9 The corresponding rate spread for the 6-position, where the SO 3 and O substituents represent the extremes of reactivity, is a factor of approximately 10 4 The exchange rates at the 2and 6positions of 3-sub stituted-1-methylpyridinium ions are explicable in terms of a reaction involving the generation of a considerable amount of negative charge. Substituents are observed to exert a profound effect on the stability of this negative charge in the transition and intermediate states. It seems likely that substituents exert approximately a constant effect on the positive charge throughout the reaction. In the case of exchange at the 2-position, correlation of the rate data with Taft's oI parameters indicates that effects of substituents on the stability of the negative charge at this position are largely inductive in nature. Figure 4 illustrates, however, that a better correlation of the data results when the extended Hammett equation is employed. These results indicate that approximately 20 percent of the total substituent effect is due to a resonance component. Furthermore, this observation-does not appear to be unique. Similar treatments of data available from the

PAGE 31

20 literature concerning the effects of ortho substituents on the rates and equilibria of reactions involving the cr electron framework of aromatic systems, for which correlations with a 1 have been obtained, illustrate the operation of this small, but persistently detectable resonance effect. For example, amide-catalyzed rates of deprotonation at ortho positions in the benzene series in liquid ammonia are correlated by the four-parameter equation (Figure 7), and show a 12 percent resonance effect. 1 Similarly, substituent effects on the r~tes of ll-D exchange at the 2-positions of 3-substituted pyrazine-1-oxides in D 2 0 at 31, 6 as well as on the acidities of 2-substituted pyridinium ions in H 2 0, 38 contain a resonance component comprising 16 percent of the total effect in each case, as determined from this treatment. However, exchange rates at the 2,6-positions of 1-substituted pyridinium ions, where the substituents are bonded to the positively charged nitrogen atom, appear to be governed only by an inductive effect: 14 Rates of exchange reactions at the 6-positions of the 3-substituted-1-methylpyridinium ions are found to be correlated by the crp 0 substituent constants (Figure 5 ). 35 As previously mentioned, this set of substituent constants has also been used to correlate the rates of amide-catalyzed deprotonation reactions at the para position in the benzene series in liquid ammonia. 4 Furthermore, Swain and Lupton have presented a treatment of substituent effects, analogous to the extended Hammett

PAGE 32

+6,0 +4.0 0 be,:: + 2 0 0.0 2. 0 pl= 13.1 PR= 1.75 L---J.----''--------J ______ l __ ~2.0 0.0 +2,0 al/CF3 -t,fi, 0 Figure 7. Correlation of Amide Ion-Catalyzed Rates of Exchange at the para-Positions of Monosubstituted Benzenes in Liquid Arnrnonia 1 by the Extended .Hammett Equation.

PAGE 33

22 equation, in which a large number of substituent constants are broken do,in into their respectiv~ field (inductive) and resonance components. 39 Their analysis suggests a 42 per cent resonance component for the crp 0 substituent parameter, a result consistent with the 43 percent resonance component calculated from treatment of the data for exchange at the 6-position by the extended Hammett equation (Figure 6). The correlation with crp 0 is also consistent with the nature of these substituent constants, which are supposed to reflect the absence of direct conjugation between the sub stituent and the reaction center. 35 This is expected to be the case for deprotonation of annular carbon acids, where transition and intermediate states involve the formation of essentially localized electron pairs on carbon which are part of the a and not the TT electron framework. The poor correlation of the data for exchange at position 6 with the on substituent p parameters probably results from an overcompensation for strong donor groups, such as NH2, This has presumably arisen since, although aniline reactions were omitted, phenol d th h 1 t 1 d to der1ve n 35 F an 1op eno reac ions were emp oye op. or other substituents, the op 0 and a~ values are very similar. From the above analysis, it may be concluded that, in the case of exchange at the 2-positions of 3-substituted-l methylpyridinium ions, the effect of the 3-substituent ortho to the reaction center is to influence the kinetic acidity of this position primarily by an inductive effect; a small

PAGE 34

23 resonance effect, however, is clearly present. Analysis of data for related systems supports this conclusion. The relative importance of the resonance effect increases, however, at position 6, while the inductive effect falls off with increasing distance between reactio11 center and substituent. This conclusion results from a comparison of the pR arid p 1 values in the appropriate equations: (ortho) 2-Position: log k~/k~ = 8.270 1 + l.94oR 0 (para) 6-Position: log k~/k~ = 3.840 1 + 2.91oR 0 Rates of exchange at the 6-position are also correlated by log k~/k~ = 3.39 op 0 From the data given in Tables g and 10 and these correlation equations, the effects of other substituents o~ the rates of pyridinium ylid formation may be predicted with confidence. The o 1 and oR 0 values for the SO 3 and CO 2 substituents are not included in the basic sets of these values, as given by Taft. 33 Because these substituents are charged, these values are also expected to be solvent dependent. 31 40 From the known rates of exchange at the 2and 6-positions of the corresponding substrates, the two four-parameter equations given above may be solved for the appropriate substituent parameters. Such calculations give o 1 and oR 0 values for these two substituents which are in fair agreement with those calculated from F 19 chemical shift or infrared intensity data in weakly protonic solvents (Table 2 ). 31 40 41 It should be noted that the methanism of th e

PAGE 35

24 Table 2. Resonance and Inductive Constants for the Sulfonate and Carboxylate Substituents in Weakly Protonic Solvents. -x cr 1 (calcd) a crR 0 (calcd)a cr I crR 0 --CO2 -0.08 +0.17 -o.osh +O.llc -SO3 +0.27 +0.04 +o.2sh +0.07c; o.ood aCalculated from rate data and correlation equations cir exchange at the 2and 6-positions of 3-substituted-1-methyl pyridinium ions. bCalculated from data in Ref. 31. cCalculated from data in Ref. 40. d Ref. 41.

PAGE 36

25 above exchange reactions has been co11sidered in terms of simple deprotonation at carbon by base. However, it may well be that the rates of deprotonation of these pyridinium ions are influenced by internal return. That is, the rate of back protonation of the hydrogen-bonded carbanion may favorably compete with the rate of replacement of proton by deuteron from solvent at the carbanion site. Rates of hydrogen exchange then do not solely reflect rates of deprotonation, and the pseudo-first order rate constants k~ are composites of rate c~nstants for deprotonation, back-protonation, and separation of the hydrogen-bonded complex. If this is so, then the linear free energy relationship found for the deprotonation of these pyridinium ions assumes added importance. The correlation shows not only the effects of substituents, but also the effects of internal return on reactivity. 14 Furthermore, as mentioned previously, it seems likely that the B value in the Brnsted relationship log k2= Blog Ka + B for these deprotonation reactions is large and close to unity, because of the lack of appreciable buffer catalysis. 3 ~ If this is the case, then the following relationship log k~/k~ = holds, and the hydrogen exchange rate constant ratio is the equilibrium acidity constant ratio. A determination of K a for one member of the series then amounts to a determination of Ka for them all.

PAGE 37

26 The kinetics of hydrogen-deuterium exchange at the 2and 6-positions of 1-methylpyridiniurn-3-oxide betaine were determined at 100.0.5 in 0.06M and 0.30M KOD. The experi mental data are Teported in Table 11. B~th positions were observed to undergo exchange at approximately the same rate. A comparison of the values obtained from the two runs for klA 6 (Table 11), the second order rate constant based on the activity of deuteroxide ion at 25, illustrates the fact that the exchange reaction is first order in deuteroxide ion. A comparison of the more accurate value (see Experimental) for k!~ 6 the second order rate constant based on the calculated concentration of deuteroxide ion, ~btained from the run in 0. 30M KOD (Table 11), with the corresponding second order rate constant reported by Beak and Bonham for exchange at the equivalent 2,6-positions of 1-mcthyl-4-pyridone C!r, Figure 8) reveals that, interestingly enough, exchange in the pyridinium ion is only 2.6 times faster than in the pyridone. 25 0 0 JXo N H(l) (~2.6) H N+ H (".' 2. 6) 17 J I Me Me Figure 8 Relative Rates of Exchange at the 2and 6Positions of l~Methylpyridinium-3-Oxide Betaine and 1-Methyl-4-Pyridone 25 in OD-/D 2 O at 100.0?. Although, because of the lack of data on appropriate model compounds, an interpretation of the similarity in rates for these two substrates cannot be made at this time, the results

PAGE 38

are not inconsistent with the formulation of transition and intermediate states for exchange in the case of 17 which 27 possess a considerable degree of ylidic character (Figure g ). 25 0 0 0 f )\ OD li-,@ N N+ I I I Me Me Me Me Figure 9 Proposed Pathway for Base-Catalyzed Hydrogen Deuterium Exchange at the 2,6-Positions of 1-Methyl-4-Pyridone. 25 Hydrogen-Deuterium Exchange at the 2and 6-Positions of 3-Substituted Pyridines In order to determine the effects of substituents on the rates of hydrogen-deuterium exchange reactions at the 2and 6-positions of 3-substituted pyridine~, a series of kinetic studies were carried out on these substrates at elevated temperatures in D 2 0 solutions in the absence of added base. Again, exchange reactions were followed by measuring the change in the integrated area of the appropriate nmr signals. 16 All exchange reactions were observed to follow good pseudo first order kinetics. The compound studied most extensively, for purposes of comparison with pyridine itself, was 3-chloropyridine. Because of the limited solubility of this compound in D20, the kinetics of H-D exchange at the 2and 6-positionswere measured in a 3:17 (V:V) dioxane-D 2 0 solution. Pyridine was also studied

PAGE 39

28 in this solvent system. Convenient rates were obtained at 197.5.5, and the appropriate kinetic data are reported in Table 13. A comparison of the pseudo-first order rate constants for the two systems is given in Table 3 and illustratesthe fact that the effect of the chlorine atom in the 3-position is to increase the rate of exchange at the 2-position, relative to pyridine, by a factor of 8.6, while making the 6-position 7.7 times less reactive than the equivalent 2,6-positions of pyridine. An attempt will be ma_cle to explain these and other results in terms of the mech,:rnism illustrated in Figure 10, which has been demonstrated for exchange at the 2,6-positions of pyridine in D 2 0 solution in the absence of added base. 10 This is a two-step mechanism. @G+ D,O H N H ~G + OD 1+ D 18 -~G :~\:~H +, D 20 Figure 10. Proposed Mechanism for H-D Etchange at the 2and 6-Positions of 3-Substituted Pyridines in Neutral D 2 0 Solution.

PAGE 40

Table 3-G -H -Cl -Cl 3, Relative Rates for H-D Exchange at the 2and 6-Positions of Pyridine and 3-Chloropyridirre in 3:17 (V:V) Dioxane-D 2 0 at 197.5.5. 29 Position kt/!, sec1 krel 2,6 3.19 X 105 1 2 2.73 X 104 8.6 6 4.16 X 106 0.13

PAGE 41

30 The first step, an equilibrium, involves initial abstraction of a deuteron from a solvent molecul~ by the nitrogen atom of the pyridine, with the resultant formation of pyridinium and deuteroxide ions. The second, slow step involves abstraction of the proton from the 2or 6-positions of the pyridinium ion by deuteroxide ion, resulting in formation of the corresponding ylid, !~ or ~2 Since one of the reactive species proposed in this mechanism is the pyridinium ion!! an exchange experiment was carried out on the 3-chloro-1-methylpyridinium ion in O.O3OM DCl solution at 197.5.5 (Table 12 ). It is important to note that, in accordance with the proposed mechanism, this study had to be carried out in an acidic solution, the exchange reaction being much too fast to measure in neutral solution at this temperature. This is because the concentration of the substrate is, in this case, also the concentration of the pyridinium ion intermediate. Hence, in the case of this substrate, the first step of the two-step mechanism does not take place, and the results obtained reflect the effect of substituents on the second step only. In the case of the exchange study on 3-chloropyridine, the importance of the first step in the two-step mechanism may also be eliminated by comparing positional reactivities within the molecule, since the first step is comm0n to formation of both ylids !~ and ~2 In the case of ex~hange at the 2and 6-positions of the l-methyl-3-chloropyridinium ion, the same order of reactivity is found as was 9bserved in the case of

PAGE 42

31 exchange in 3-chloropyridine (Tables 12 and 13). That is, the 2-position is considerably more reactive than the 6-position in both cases. More importantly, however, the similarity between the observed positional rate ratios (Figure 11) provides additional support for the intermediacy of pyridinium ion 18 during the course of exchange in the free base. Figure 11. Positional Rate Ratios for H-D Exchange in 3-Chloropyridine (D2O) and 3-Chloro-1-~lcthyl pyridininm Ion (0.030M DCl) at 197.5.5. The rate expression for the proposed two-step mechanism may be derived from the scheme outlined in Figure 10. On the basis of this mechanism, the rate is given by the product of a second order rate constant k 2 times the product of the concentrations of pyridinium and deuteroxide ions: + rate= k 2 [PyrD] [OD]. Replacing the concentratioft product by the product of the base ionization constant times the concentration of free pyridine gives rate= k2Kb[Pyr]. The rate actually measured, however, is equal to an observed pseudo-first order rate constant kt/I times. the total substrate concentration:

PAGE 43

rate= k~ ([PyrD+] + [Pyr]). If the latter two rate expressions are set equal to one another, k~ = k2Kb I [Pyr] ] [PyrD] + [Pyr] Rearranging the above rate constant equation gives I [PyrD+] + [PL!J_l k~1 [Pyr] 32 which says that the product of the observed pseudo-first order rate constant times the ratio of the total concentration of substrate to the concentration of substrate present as the free base is equal to a constant. In order to test this prediction, hydrogen-deuterium exchange was studied at the 2and 6-positions of 3-chloro pyridine at 197.5.5 in D 2 0 solutions of the partially neutralized (DCl) substrate (Table 13). The results reported in Table 4 indicate that this prediction was realized, and verify the proposed mechanistic scheme. The effect of the chlorine atom on the rates of exchange at the 2and 6-positions of 3-chloropyridine relative to pyridine may be explained in terms of the two-step mechanism (Figure 10), The first step involves the formation of a pyridinium ion, and hence will depend on the basicity of the pyridine molecule. The pK values for the conjugate acids a of pyridine and 3-chloropyridine are reported in Table 5 .~ 2 Although these values were obtained at 25, they nevertheless

PAGE 44

Table Position 2 6 4. Observed Dependence of the Rate of H-D Exchange (197.5.5) at the 2and 6-Positions of Partially Neutralized 3-Chloropyridine on the Degree of Neutr a lization. 580 340 220 130 46 7.0 2.9 + [Pyr] + [Py~l [Pyr] 1.0 1.5 2.0 4.0 10 1.0 2. 0 580 510 440 520 460 7.0 5.8 33

PAGE 45

Table Table 5. pKa Values (25) for the Conjugate Acids of Pyridine and 3-Chloropyridine. 42 ~ompound Pyriaine 3-Chloropyridine 6. Chlorine Rate Factorsa Position Ortho (2-) Para (6-) pKa 5.25 2.84 403 7.76 aDetermined from the rates of H-D exchange at the 2and 6-positions of 1-methylpyridinium and 3-chloro-1-methyl pyridinium ions in 0.030M DCl at 197.5.5. 34

PAGE 46

present the tre1Ld to be expected at higher temperatures; namely~ that pyridine is a significantly stronger base than 3-chloropyridine. The effect of chlorine on the first step of the reaction, then, is to decrease the basicity of the annular nitrogen atom. The second step in the reaction scheme (Figure 10) involves removal of a proton from the 2or 6-positj ons of 35 the pyridinium ion. In this case, the effect of the substituent is on the acidity of a proton bound to carbon. An estimate of the effect of chlorine on this step of the reaction may be obtained from a comparison of the rates of exchange at the 2and 6-positions of the 3-chloro-1-methylpyridinium ion with the rate of exchange at the equivalent 2,6-positions of the 1-methylpyridinium ion itself, as determined from the studies in 0.030M DCI at 197.Si0.5. This comparison is reported as a pair of chlorine rate factors (Table 6) which reflect the ability of the chlorine atom to stabilize the carbanionic center of the ylid developing at ortho and para centers. The stabilizing effect of the chlorine atom is observed to be much stronger at the ortho position. These observations allow the chlorine substituent effect observed in the exchange reactions of the free base (Table 3) to be explained in terms of the two-step mechanism. In the first step, 3-chloropyridine is deactivated relative to pyridine because of the base-weakening effect of the chlorine a tom (Table 5 ) In the second step, in the case of exchange at the 2-position, this initial deactivation is more than

PAGE 47

36 compensated for by the activation resulting from the signif icant stabilization of the developing c~rbanionic center at this position (Table 6). The net effect is activation. In the case of exchange at the 6-position, however, the stabilizing effect of chlorine on the negative charge developing at the para position is weak (Table 6), and the net effect is deactivation relative to pyridine. The results of these H-D exchange studies on pyridine and 3-chloropyridine are then explained in terms of a two-step m~cha11ism (Figure 10) involving pyridinium ion and pyridinium ylicl intermediates, and the opposing effects of chlorine on nitrogen basicity and carbon acidity. The rates of hydrogen-deuterium exchange at the 2and 6-positions of several other 3-substituted pyridines were measured in D20 solution at 197.5.5 or 217.9 0.5. Exchange at the 2,6-positions of pyridine itself was followed at both temperatures for purposes of comparison. In the cases of nicotinic acid and 3-pyridinesulfonic acid, sodium carbonate was added to insure that the substituents were in the anionic form. The pseudo-first order rate constants and conditions under which they were obtained are reported in Tables 13 and 14. The ionic strengths of the various sample solutions were not held constant, since, in many cases, the limited solubility of the substrate in D 2 0 precluded the addition of supporting electrolytes. The similarity between rate constants for exchange at the 2,6-positions of pyridine, obtained in D20

PAGE 48

37 solution at 197.5.5 or 217.9.5, with those obtained in L OM NaCl (D 2 0), however, indicate that ionic strength has little effect on the reaction rates in the case of the two-step mechanism (Tables 13 and 14), The rate constant equation [ [Pyr] ] [PyrD ] + [Pyr] previously derived for the two-step mechanism, predicts that in neutral or slightly basic solution, where the concentration of pyridinium ion is very small, the pseudo-first order rate constant should be independent of the base concentration; that is, This was demonstrated by the constancy observed in the pseudo first order rate constants k; ~nd kt over a pD range of 7-10 in the case of the nicotinate anion (Table 14). Summaries of the effects of substituents on pyridinium ylid formation, obtained from the data for exchange at the 2and 6-positions of 3-substituted-1-methylpyridinium ions at 75.0.50, and of the effects of substituents on the two step mechanism, obtained from the data for exchange at the 2and 6-positions of 3-substituted pyridines at elevated temperature, are reported in Table 7 as a series of rate ratios. The effects of ortho and para substituents, relative to hydrogen, on the rates of 1-methylpyridinium ylid formation have been explained in the previous section in terms of the

PAGE 49

Table 7 Substituent Effects on Hydrogen-Deuterium Exchange at the 2and 6-Positions of a Serigs of 3-Substituted-1-Methylpyridinium Ionsa and 3-Substituted Pyridines ,c. ;Q:C D N D 1+ Me -G k~/k~ k~/k~ -Cl 2700 12 -CO2 a.so 1. 6 -S03 210 14 -OMe 60 0.80 -ND2 1. 4 0.060 aData obtained from experiments at bData obtained from experiments at cData obtained from experiments at dUncertain k~/k~ k2G;k2H 1/J 1/J ,.GG /k6H l\.1/J 1/J k2/k6 y 1/J 230 8.6 b 0.13b 66 0.31 0.39c 1. zC 0.33 15 1. 8c 0.42c 4.4 75 13b,c [0.44]b,d [29]d 23 7_3b,c 0.31c 25 75.0.5 in buffered D 2 0 solutions of 1.0 ionic strength. 197.5.5 in D 2 0 or 3:17 (V:V) dioxane-D 2 0 solution. 217.9.5 in D 2 0 solution. t,,I 00

PAGE 50

39 inductive and resonance effects of the substituent relative to hydrogen in each case. Note that this information pertains only to the second step of tl1e two-step mechanism. In the case of exchange in the free bases, however, where the two-step mechanism has been shown to op~rate, the total substituent effect on ortho and para centers, relative to hydrogen, reflects not only the effect of the substituent on ylid formation (second step), but also its ~feet on the basicity of the pyridine (first step). The effect of the 3-chloro substituent has been discussed in this context earlier. Note, for example, that the effect of the amino group on 1-methylpyridinium ylid formation is activating at the ortho position and deactivating at the para position. The base strengthening effect of the amin~ group,~ 2 that is, its activating effect relative to hydrogen on the first step of the two-step mechanism, is reflected by a more pronounced activating effect at the ortho position as well as a decrease in the deactivating effect at the para position in the case of the two-step mechanism. Note also that the effects of substituents are much more pronounced in the case of 1-methylpyridinium ylid formation than in the case of the two-step mechanism. This results not only from the normal compression of reactivities which accom panies an increase in temperature, bLt also because the effect of a particular substituent on each step of the two-step mechanism generally works in opposite directions. For example, an electron-withdrawingwoup would make the pyridine

PAGE 51

a weaker base, so that J ess of the substrate would be in the reactive pyridinium ion form relative to pyridine itself. The effect of this group would generally stabilize the carbanionic center of the incipient pyridinium ylid, however, and the effect on the second step of the mechanism would then be activating relative to pyridine itself. A similar discussion would apply for electron-releasing groups. The effects of substitucnts on an ortho relative to a para reactive center in the cases of 1-methylpyridinium ylid formation and the two-step mechanism are seen from a comparison of the third colunms in each of tl1e two series (Table 7), In every case, the relative effects of substituents at the two positions arc in the same direction, the CO 2 group being the only substituent studied which activates the para relative to the ortho position. This provides further support for the two-step mechanism, where a comparison of positional rate ratios within a given substrate amounts to a comparison of the substituent effect on the second step (pyridinium yli
PAGE 52

Instrumentation CHAPTER 3 EXPERIMENTAL Proton nmr spectra were recorded on a Varian Associates Model A-60A instrument. Melting points were determined in a Thomas-Hoover Unimelt melting point app a ratus. Measurements of pD were made employing a Beckman Model 1019 Research pH Meter equipped with either a Sargent-Welch (S-30070-10) oi Corning (476050) high-temperature, high-alkalinity miniature combination electrode. Kinetic runs at 75.0 were carried out in a Haake Model F constant temperature circulator. Microanalyses were performed by Galbraith Laboratories, Inc., Knoxville, Tennessee. Chemicals All common laboratory chemicals, unless specified to the contrary, were reagent grade and from various suppliers. Deuterium oxide (99.7 percent) was obtained from Columbia Organic Chemicals. Stock Solutions Tetramethylammonium chloride (TMAC), sodium isobutyrate, and !,ris-(hydroxymethyl-)aminomethane (THAM) were dried at 100 overnight and stored in a pesiccator before use. These hygroscopic materials, as well as potassium hydroxide, were 41

PAGE 53

handled in a dry box. Deuterated stock solutions of acetic acid, boric acid, citric acid, potassium monoand dihydrogen phosphate, sodium and potassium carbonate, sodium and potassium chloride, TMAC, sodD1m isobutyrate, and !-butyl alco]wl were prepared by dissolving an appropriate weight of the analytical reagent or primary standard grade material in deuterium oxide and diluting to mark in a volumetric flask. A 3:17 (V:V) dioxane-D 2 0 solution was prepared by mixing 1.5 ml of reagent grade dioxane with 8.5 ml of deuterium oxide. Dilute DCl was prepared by diluting commercial concentrated HCl with D 2 0. The solution was standardized by potentiometr ic titration ys_ THAM. 42 Stock sodium deuteroxide solution was prepared by dissolving freshly cut sodium in D 2 0 under a nitrogen atmosphere. Stock potassium deuteroxide solution was prepared by dissolving a weighed quantity of reagent grade KOH in D 2 0. Both solutions were standardized by potentiornetric titration vs standardized HCl. The analogous protea stock solutions for control runs were prepared in identical fashion, employing distilled water in place of D 2 0. Deoxygenated solutions, when they were required, were prepared in the following manner: The appropriate solution was refluxed under a continuous stream of nitrogen for 30 minutes, and then allowed to cool to approximately 0 in an

PAGE 54

ice bath under a nitrogen atmospIt ere. The solutions were stored under nitrogen in tightly stoppered bottles. Substrates ------Free Bases 43 Fisher reagent grade pyridine, which had been dried over KOH pellets, was used directly. 3-Chloropyridine was obtained from Aldrich Chemical Company, Inc., and was re distilled (bp 149-150; lit. 43 148(744 mm)) before use. 3-Pyridinesulfonic acid was purchased from Aldrich, and was treated with Norit and recrystallized three times from hot water (mp >300; lit. 43 357(dec)). Nicotinic acid was obtained from Nutritional Biochemicals, Inc., and was re crystallized twice from hot water (mp 235.5-236.0; lit. 43 236-237) prior to use. 3-Aminopyricline was purchased from Nepera Chemical Company and was purified by vacuum sublimation a t 5 0 6 0 ( O l mm) ; mp 6 3 S 6 4 O 0 ( 1 it 4 4 6 3 6 s O ) 3 Me t ho x y pyriclinelf 5 was redistilled (bp 80.0-80.5(25 mm); lit. 4 E 70-71(12 mm)) prior to use. Quinazoline, purchased from Aldrich, was purified by vacuum sublimation at 40(0.03 ~n); mp 4 S O O ( 1 it If 3 4 8 O 4 8 s O ) Methyl nicotinate.This compound, mp 41.0-41.5 (lit. 47 42.3-43.5) was prepared in 88 percent yield by esterification of nicotinic acid. 47 3-Nitro1:yridine. This compound was prepared by reduction of 2-chloro-3-nitropyridine, obtained from Aldrich, with copper powder in molten benzoic acid according to the procedure of Kirby and Varvoglis.~ 8 A mixture of 35.0 g of

PAGE 55

44 benzoic acid and 16.8 g (0.106 moles) of 2-chloro-3~nitro pyridine was heated to 150, and, after the mixture ha~ melted, 25.0 g (0.395 moles) of copper powder was added in portions over a 5-minute period while the mixture was stirred magnetically. After several additional minutes of heating, the melt was cooled to 90-100, 140 ml of 20 percent Na2CO3 was added, and the mixture heated on the steam bath while the larger lumps of melt were broken up with a spatula. The slurry was then transferred to a mortar and the remaining larger particles were pulverized, The slurry was heated for an additional 20 minutes on the steam bath and suction filtered. The filtrate was extracted four times with 150 ml portions of methylene chloride. The residue was stirred thoroughly with SO ml of methylen6 chloride and filtered. The CH 2 Cl2 extracts were combined and dried over anhydrous sodium sulfate overnight, and the solvent was removed on a rotary evaporator. The product, an oil which crystallized in yellow needles on cooling in an ice bath, weighed 7.83 g (60 percent yield). It was used without further purification to prepare the methiodide. 3~Pyridyl methyl sulfone.The method used to prepare this compound by permanganate oxicla tion of 3--methyl thio pyridine was adapted from the procedure described by Barlin and Brown 49 for the preparation of the 2and 4-pyri
PAGE 56

45 acetic acio. until a perma n.ent pink colo:c appeared. The excess permanganate was destroyed by bubbling sulfur dioxide through the slurry until the pink color was discharged. The water and acetic acid were removed on a rotary evaporator, and SO ml of 2N Na.OH uas added to the residue. The slurry was extracted with 50 ml of ether and filtered. The solid residue was treated with an additional SO ml of ZN NaOH, then 50 ml of ether, and filtered again. The combined water-ether mixture was transferred to a separatory funnel and separated. The aqueous layer was extracted twice more with 100 ml portions of ether and the extracts combined and dried over barium oxide. The drying agent was filtered off and the ether re moved on a rotary evaporator. There remained 1.73 g (69 per cent) of white crystalline product which was used directly to prepare the methiodide. Pyridinium Salts The pyridinium iodides, chlorides, and perchlorates employed in this study were, for the most part, prepared by one of the general methods described below. Exceptions are described in greater detail in the following text. Experi mental details are summarized in Table 8, fx.!.i
PAGE 57

Table 8. Experimental Data for the Preparation of a Series of 3-Substituted-1-Methyl pyridinium Salts Reaction Recrys t allization Yield, M. p ., oc 3-G X Solvent Solvent % Foun d L i t. -CO 2 Me I Absolute 50 % CV:V) 57 130.0129.5a Methanol MethanolE ther 131. 5 130.2 .,.cl I Absolute Absolute 86 138.0141b Ethanol Ethanol 13 3 .5 142 -OMe I Absolute Absolute 74 1 5 5 5 ..C Methanol E thanol 156.5 -I I Absolute 80 % (V:V) 66 E thanol Aqueous .M ethanol -NH2 T Absolute Absolute 49 120.0123d .,_ Ethanol Ethanol 121 0 -OHn I Acetone Acetone 6 4 111.5114e 114.5 116 -Me n I Absolute Acetone 72 96.095 Methanol 97.0 -COMe I Absolute Absolute 39 165.0162 Methanol Meth a nol 166.5 -SO2Me I Absolute 90% (V:V) 70 213.0___ g M ethanol Aqueous Eth a nol 214.0 .i::,.

PAGE 58

Table 8. Continued. Reaction Recrystallization Yield, M. p. oc 3 G X Solvent Solvent 0 o Found Lit. -Cln Cl Absolute 68 M ethanol .. ~_.,._.,._ .. r Cl 85 % (V:V) Absolute 76 257.5 h Aqueous Eth a nol (dee) M e thanol N02 Cl 85 % (V:V) 75 % Ethanol74 255Aqu e ous 25 % Ether 256 Methanol (V :V) ( d ee) -CO M e Cl Absolute 50 % (V:V) 82 197.5i Methanol EthanolE ther 198. 5 -CN Cl01t 95 % Ethanol 31 12 8 .0135-j ,k 128.5 137 -Cl ClOi. .. 95% Ethanol 59 95.51 96.0 -O M e ClOi. 95 % Ethanol 49 55.056.0 -H ClOi. 95% Ethanol 75 136.5135.0m 137.5 a L. Bradlow and C. A. Vanderwerf, J; Org. Chem. 1:i, 1143 (1951). .i:,. -..J

PAGE 59

Table 8. Continued. M. Liveris and J. Miller, Australian J. Chem., g, 297 (1958). cAnalysis: Calcd: C, 33,49; H, 4.01; N, 5,58. Found: C, 33.68; H, 3.97; N, 5.56. dN. F. Turitsyna and A. F. Vompe, DokZady Akad. Nauk S.S.S.R., z.!, 509 (1950); Chem. Abs tr,, ~' 3846h (1951). eK. Mecklenborg and M. Orchin, J. Org. Chem.,~' 1591 (1958). fRef. 51. gAnalysis: Calcd: C, 28.11; H, 3.37; N, 4,68. Found: C, 27.94; H, 3.45; N, 4.80. hAnalysis: Calcd: C, 28,21; H, 2.76; N, 5.48. Found: C, 28.00; H, 2.74; N, 5.38. iAnalysis: Calcd: C, 55.99; H, 5.87;.N, 8.16. Found: C, 55.69; H, 5.90; N, 8.14. j Ref. 19. kAnalysis: Calcd: C, 38.46; H, 3.23; N, 12.81. Found: C, 38,40; H, 3.23; N, 12.77. 1 Anaiysis: Calcd: C, 31.60; H, 3.09; N, 6.14. Found: C, 31.82; H, 3.04; N, 5.91. ms. Ukai and K. Hirose, Chem. Pharm. BuZZ. (Tokyo), 16, 195 (1968). n Extremely hygroscopic.

PAGE 60

49 solvent (Table 8). The final product was dried under vacuum over P20s in a drying pistol for 24 hours at room temperature. 3-Nitro-1-methylpyr:i.dinium ~odide.This salt was pre pared in 72 percent yield according to the method described by Pfleiderer, Sann, and Stock; 51 mp 217.0 (lit. 51 215). R_r_ridinium chl_oridcs. These compounds were prepared from the corresponding iodides by halide exchange. A solution of 45 mmoles of the 3-substituted-1-methylpyridinium iodide in the minimum volume of an appropriate solvent (Table 8) was refluxed with 22.6 g (158 mmoles) of silver chloride while stirring the mixture mechanically. The mixture was then cooled, the silver salts removed by filtration, and the solvent removed on a rotary evaporator. The pyridinium chloride was then recrystallized from an appropriate solvent and dried under vacuum over P20s in a drying pistol for 24 hours at room temperature. Pyridinium perchlorates.The general method used to prepare these compounds was adapted from that reported by Eisenthal and Katritzky 52 for the preparation of 1-methoxy pyridinium perchlorate. A mixture of 50 mmoles of the pyridine and 6.30 g (50 mmoles) of dimethyl sulfate was heated on the steam bath for 2 hours. After cooling to room temperature, 8 ml of absolute ethanol, 5 ml of 70 percent perchloric acid, and 36 ml of ethyl acetate were added, and the resulting solution cooled thoroughly in an ice bath. The crystalline perchlorate which separated was recrystallized twice from the minimum volume of ~ot 95 percent ethanol, washed with anhydrous ether, and dried in a pistol over

PAGE 61

50 P 2 0 5 under vacuum for 12 hours at room temperature. Jridinfum Betaines 1:.:_Methylpyridinium-3-sulfonate _be_ta_ine 29 The pro cedure for the preparation of this compound was adapted from that reported for the synthesis of the 2and 4isomers. 53 A mixture of 7.95 g (50 mmoles) of 3-pyridinesulfonic acid and 20 ml of dimethyl sulfate was heated at 150-160 for 2 hours with magnetic stirring. After cooling, 50 ml of anhydrous ether was added to the reaction mixture and the solid material thoroughly pulverized with a stirring rod. The solid was then removed by filtration and washed four more times with SO ml portions of ether. The crude product was recrystallized three times from the minimum volume of hot SO percent aqueous ethanol. The final crop was washed with ether, filtered, and dried overnight under vacuum in a pistol over P 2 0 5 at room temperature. The pure product weighed 3.60 g (42 percent yield); mp >300 (lit. 54 55 130). Analysis: Calcd for C 6 H 7 N0 3 S: C, 41.61; H, 4.07; N, 8.09. Found: C, 41.79; H, 4.17; N, 7.95. }.:.-Me_!]_iylpyridinium-3-carboxylate betaine. 5 6 This com pound was prepared in 83 percent yield from methyl nicotinate methiodide by employing the method of Kosower and Patton; 7 mp 23~.5-235.5 (lit. 57 230-233). f!~_12ara.tion of Solutions for Kinetic Runs Volumes of one to five milliliters of solution were prepared for each run. When solubility permitted, the

PAGE 62

51 solutions were app r oximately 0.SM in substrate. An appro priate amount of th e substrate (pyridine or pyridinium salt) was transferred to a tared volumetric flask and weighed accurately on an analytical balance. When hygroscopic sub strates were used, a dry bag, filled with nitrogen and holding an open container of P 2 O 5 was employed for these manipulations. Accurate volumes of stock acid, base, buffer, internal standard, or supporting electrolyte solutions were then delivered to the flasks by means of Hamilton Microliter syringes, and the solution finally diluted to mark with D 2 O or stock 3:17 (V:V) dioxan e -D 2 O solution. Deoxygenated solutions were prepared under a nitrogen atmosphere. Approximately one milliliter of the final solution was then transferred to an nmr tube which 1.vas sealed immediately. The ren1ainder of the solution was generally stored under nitrogen in a septu m -stoppered test tube for later compar:i~.ons or pD measurements. Kinetic Runs Methods Kinetics were obtained by heating the sample solutions in sealed nmr tubes. For kinetic runs at 75.0 + 0.5, the temp e rature 1-;as maintained by immersing the nmr tube in a Haake Model F constant-temp e rature circulating bath. The te~p e rature of this bath was checked periodically using a National Bureau of Standards Certi f ied th e rmometer. For runs at higher temp e ratures, the nmr tube was immersed in a re

PAGE 63

52 fluxing vapor bath ... Distilled water vtas used to maintain 100.0 + 0.5, mesitylene for 164.7 +; 0.5, ethylene glycol for 197.5 a.5, and naphthalene for 217.9 + a.5. From time to time, the temperatur e s of these baths were checked with an ordinary 76 mm immersion thermometer. In some of the earlier runs at elevated temperature, the variations in temperatures of the various vapor baths exceeded those re ported above. In no case, however, did i;1di v idua 1 variations exceed! 1.5, even over extended p e riods of time. Periodically, the nmr tube was removed from the bath, quenched, and the proton nmr spectrum of the solution re corded. Reactions were followed by measuring the change in the integrated area of the nmr signal of the proton of interest with respect to that of~ non-exthanging proton in the reaction mixture. The integrals of proton signals were measured in six successive sweeps, three in each direction, and the average value was take11. In favorable cases, a ring proton of the substrate served as the area reference. In others, an internal standard external to the substrate was used. The internal standards employed included methanol, !-butyl alcohol, TMAC, sodium isobutyrate, and diethyl ether. The choice of an internal standard for a particular sample solution was dictated largely by the position of its nmr signal(s) relative to those of the s~bstra t e or cosolvent, its stability under th..e conditions of the kinetic run, and its solubility in the sample solution.

PAGE 64

The dcuteration reactions studied may be expressed in the following general form: l'yrD + HOD 53 Experimentally, because the concentration of OD catalyst remains unchanged, pseudo-first order kinetics are observed. Furthermore, since deuteration of the intermediate hydrogen bGnded carbanion (ylid) occurs after the rate-determining step, 14 only the fraction of solvent molecules containing the isotope of interest appears in the rate expression. It is reasonable to assume that the kinetic isotope effect in these systems is negligible, due to extensive interna.l return, l 4 5 8 so that k1 = k l The rate of appearance of deuterated substrate is then: where [H] and [DJ represent the concentrations of the lighter and heavier isotopes, respectively, in the solvent pool. It should be noted that fH] represents the total concentration of the protium isotope, whatever its source(s); these include conta mination of the deuterium pool by H-D exchange at the position of interest, prior exchange at a more reactive position, use of a substrate possessing functional groups

PAGE 65

which undergo rapid exchange, the protium impurity (0. 3%) present in the commercially available D20, and the use of stock buffer solutions containing the lighter isotope. 54 Particularly unfavorable circumstances may be imagined. Consider exchange approaching com~letion at the 6-position of the 3-amino-1-methylpyridinium ion in a citric acid citrate (four exchangeable hydrogens) buffer. The deuterium pool becomes contaminated by 2 equivalents of protium from the amino group of the substrate, 2 more equivalents from exchange at the 6 and more reactive 2-positions, as well as from the buffer and normal solvent impurity. Even in this ~xtreme case, the fraction of deuterated solvent amounts to only 2-3 mole percent of the total at the substrate and buffer concentrations generally e~ployed. Therefore, the concentration of Hin the denominator of the first term as well as the entire second term of the above equation may be regarded as insignificant, and the rate expression simplifies to d[PyrD] dt = dfPyrH] -,rt= kl/I [PyrH] where kl/I, the pseudo-first order rate constant, is defined as Integrating, 1n IPyrHJ 0 [PyrHJt ;:;

PAGE 66

Practically, tr 1 e pseudo-first order rate constant k1/J was obtained by det e r m inin g the h a lf~life directly fro~ a plot of IPyrH] 0 /{PyrHJt ~ton semilogarithmic paper, and applying the relationship 0.693 Kinetics of Hydrogen-Deuterium Exchange in 3 Substituted1-Methyl pyri
PAGE 67

Table 9. Rates of H-D Exchange at the 2-Position of 3-Substituted-1-Methylpyridinium Ions in D20 at 75.0 + 0.5 and 1.0 Ionic Strength. 1 2 sec1 a kL l sec 1 b IDA]' Mc IA ] ]\l 3-G pDo pD M -N02 4.84 X 10-1+ e ,h 4.()42 4.937 1.87 X 10 5 0. 0 2 6j 0. 033j -CN 3.18 X 10-1+ f,i 6.017 6.005 1.05 X 101+ 1, 0.053"' 0.006 k -Cl 9.38 X 105 e,h 6.788 6.777 5.25 X 10 2 0.047k 0.012k e,h 0.035k -1 2.75 X 105 7.035 7.017 8. 8 5 X 10 1 0.024K -so; 11 2.88 X 10-1+ h 8.370 8.332 4.49 X 10 1 o.oso 1 0.009 1 7.70 X 105 h 7.939 7.774 4.32 X 10 1 0.055 1 0.004 1 8.02 106 h 6.854 6~840 3.90 X 10 1 0.035k 0.024k X Avg 4.24 X 10 1 -OMe 3.98 X 10-1+,i 9.054 9.035 1.23 X 10 1 0.029 1 0.030 1 -ND2n 2.50 X 10-sg,h 9.444 9.483 2.76 X 101 0.020 1 0.039 1 -1 -i 1.84 X 10-tif,i 10.498 10.490 1.99 X 101 O.OOlm 0.049m -co; n 1.20 X 10-tih 10.627 10.624 9.50 X 102 O.OOlm 0.049m 1.35 X 10-sh 9.738 9.641 1.03 X 101 0.012 1 0.048 1 Avg 9.90 X 102 V, OI

PAGE 68

Table 9. Continued. 3-G -Me 0 k2 1/J sec1 3.74 X 104 g,p 2.35 X 104 g,h 1.67 X 107 g 4.98 x 106 g,h pDO a 11.121 10.998 10.974 12.292 pD kL Ml sec l 11.116 9.54 X 102 10.981 8.19 X 102 Avg 8.87 X 102 1Q.527S 9.87 X 105 ll.021 5 3.67 X 104 Avg 2.33 X 104 a pD at 75.0 0.5 of unheated portion of original solution. b [DA] Mc [A l Mel J o.ooom 0.132m 0.004m 0.096m r o.ooom 0.154m r t t bCalculated from ki and a O D(based on the measured pD of the recovered solution). cBuffer acid. dBuffer base. eSubstrate counterion Cl-. Substrate counterion ClQ4 (J 0 Substrate counterion I h sL~pporting elect~olyte KCl.

PAGE 69

Table 9. Continued, 1 Supporting electrolyte NaCl. jDeuterocitric acid-deuterocitrate. kD2POi. --DPOi. 2 -. 1 D3B03-D2B03 m 2 DCO 3 -CO 3 nSolution becomes yellow during the course of the run. 0 Exchange appears to occur at the 2and 6-positions at approximately the same rate. PA small amount oi exchange occurred initially at room temperature. qMeOD internal standard. rCalculated from kJ and a 00 (based on the average of pD and pD 0 ). sSignificant etching occurred during the course of the run. tNo buffer system was employed. The run was carried out in O.lM KOD. u, 00

PAGE 70

Table 10. Rates of H-D Exchange at the 6-Position of 3-Suhstituted-l-Methylpyridinium Ions in D 2 0 at 75.0 0.5 and 1.0 Ionic Strength. 3 kG sec1 pDO a pD kL l sec1 b I DA] C [A ] Md .. 1' M M -S03 m 7.55 1 0 4 h 9.954 2.81 o.oooJ 0.059j X 3.59 1 0 4 h 9.522 9.594 3.07 O.Ollj 0. 04 3J X 1. 30 X 104 h,l 9.373 9.277 2.30 0.023j 0. 03 6j 6.60 X 10 5 h,l 8.977 8.873 2.96 0.035j 0.02 4 j Avg 2.79 -Cl m 1.79 J. o4 e,i 9.475 9.419 2.28 0.017j o. 04 zJ X 7.00 X 105 f,h 9.026 9.004 2.32 0.035j 0.024j Avg 2.30 -r 2.25 X 104 f,h 9.679 9.525 2 2 5 O.OSlj 0. 009j -CO2 m 3.98 104 h 10.627 10.624 3.16 10 1 k 0.049k X X 0.001 4.28 105 h 9.738 9.641 X 3.27 X 101 0.012j 0.048j Avg 3.22 X 101 -H 1. 84 X 104 e,i 10.498 10.490 1. 99 X 101 O.OOlk 0.049k -OMe m 3.87 1 0 i+ g,h 11.123 10.896 1.64 X 10 1 o.oook o .1 ook X V, ID

PAGE 71

Table 10. Continued. 1 a jvf Md 3-G k;' sec pDo pD kL M-1 sec i [DA] [A ] -Me n 3.74 10 4 g,o 11.121 11.116 9.54 102 o.oook 0.132k X X 2.35 X 1 0 4 g,h 10.998 10.981 8.19 X 102 0.004k 0.096k Avg 8.87 X 102 -ND2 m 4.00 105 g 11.214 X 11.037 1. 23 X 102 o.oook 0.159k -0 n,p 1. 67 107 g 10.974 10.527r 9.87 105 q o.oook 0.154k X X 4.98 10-~ g ,h 12.292 11. 021 r 3.67 10 11 q s s X X Avg 2.33 X 1 0 '+ -CN e,i t t t t 0.053j 0. 006j ______ .., ____ -----------NO2 g ,h, 1 t t t t o.oosj o. o s sj -------------. -------COMe f,h t t t t o. o s sj o. o o sj ---------------------SO2Me g,h t 8.20lu 6.104u t o. o s zj 0.007j --------------------apD at 75.0 0.5 of unheated portion of original solution. bCalculated from k: and a O D(based on the measured pD of the recovered solution). cBuffer acid.

PAGE 72

Table 10. Continued. Buffer base. eSubstrate counterion ClO~ Substrate counterion Cl-. gSubstrate counterion I hs 1 1 KCl upporting e ectro yte iSupporting electrolyte NaCl. jD3BQ3-D2B03-. kDCO 3 -CO 3 2 1 d 1 Deoxygenate so ution. mSolution becomes yellow during the course of the run. nExchange appears to occur at the 2and 6-positions at ~pproxim a tely ~he sam~ rate. 0 A small amount of exchange occurred initially a t room temperature. PMeOD internal standard. qCalculated from ki and a O D(based on the average of pD and pD 0 ). rSignificant etching occurred during the course of the run. sNo buffer system was employed. The run was carried ou t in 0.lM KOD.

PAGE 73

Table 10. Continued. tNo data were obtained, owing to the occurrence of side reactions involving the con sumption of base and substrate. u pD measurements made at 25,

PAGE 74

are generally much less hygroscopic than the corresponding chlorides). The ionic strength of the solutions was maintained at 1.0 by addition of a standard stock solution of KCl or NaCl in D 2 O. It was necessary to employ KCl for solutions with 63 pD >10 in order to minimize the sodium ion error encountered in making pD measurements. Sodium chloride was used when the substrate was a perchlorate because of the low solubility of potassium perchlorate. Buffered solutions were employed in order to insure constant pD; in several instances, the buffer ratio was varied for purposes of detecting the possible operation of general base catalysis. Except in the case of 3-cyano-1-methylpyridinium per chlorate, where the substrate concentration employed was approximately 0.3M because of low solubility, all substrate concentrations were approximately O.SM. The nmr spectra for most 3-substituted-1-methylpyridinium ions (G NO2, CN, SO 2 Me, SO 3 -, CO 2 -, Cl, I) exhibit the same general pattern for the annular protons. In the order of increasing field, they appear as a rather broad singlet for the 2-proton, a series of overlapped peaks for the 4and 6-protons, and a broad triplet for the 5-proton. As the 2-position undergoes exchange, its signal decreases in intensity; the appearance of the signals for the other annular protons is altered only slightly. As exchange approaches

PAGE 75

completion at the 6-position, the signals for the 4and 5-protons emerge as a sharp and broad doublet respectively. For exchange at the 2and 6-positions of these substrates, the 5-proton was used as the internal standard, so that and [H-6] 0 = = [H-S]t [H-2]t 64 The signals for the annular protons of the 1-methyl pyridinium ion itself appear as a broad multiplet representing the overlapped signals for the 2,6and 4protons and, at slightly higher field, a broad peak for the 3,5-protons. As exchange occurs at the 2,6-positions, the 4position emerges as a well-defined triplet slightly down field from the broad doublet for the 3,5-protons. In this case, the signal for the 3,5-protons was used as the area reference; the appropriate concentration r~tio, then, is defined by [H-2,6] 0 [H-2,6]t = [H-3,S]t As the electron-releasing ability of the 3-substituent is enhanced, the appearance of the nmr spectrum undergoes notable changes. The nmr spectrum of the 1,3-dimethyl pyridiniura ion exhibits itself as a set of overlapped peaks

PAGE 76

65 for the 2and 6-protons at lowest field, followed by a broad doublet for the 4-proton at slightly higher field, and finally a broad triplet for the 5-proton at highest field. In this case, the 5-position was used as the area reference. That exchange was occurring at the 2and 6-positions of this substrate at approximately equal rates was illustrated by the fact that a plot of log ([H-2,6] 0 /[H-2,6]t)= log (2[H S]t/[H-2,6]t) v~ t was linear up to 3.7 half-lives. A series of theoretical kinetic plots for two parallel first order reactions with rate cons tan ts k and k' were cons true ted for k=nk' (n= 1, 2, 3, 4) As expected, the curvature in these plots becomes more pronouoced and deviates more severely from that where k=k' with increasing n. It is apparent from these plots, however, that for n<3, the degree of curvature in the plot could conceivably escape detection. Hence, the uncertainty in the rate constants for the 2,6-positions of this substrate are larger than those for which well-defined reactivity differences exist between the 2and 6-positions. The nmr spectra of the 3-methoxyand 3-amino-1-methyl pyridinium ions appear as a downfield set of overlapped peaks for the 2and 6-positions and another set of overlapped signals for the 4and 5-positions at slightly higher field. As exchange approaches c0mpletion at the 2-position, the signal for the 6-proton reveals itself as a broad doublet. As the 6 position exchanges, a pair of sharp, partially ovexlapped doublets emerge as the signals for the 4and 5-protons. In these cases, the

PAGE 77

66 integral of the combined signals for the 4and 5-protons was used as the area reference, giving = and :: O.S([H-4]t + [H-S]t) [H-6]t In the case of l-methylpyridinium-3-oxide betaine, the signals for the 2and 6-protons appear as a set of overlapped peaks slightly downfield and partially overlapped with a iecond set of overlapped peaks representing the combined signals for the 4and 5-protons. In this case, the signal for the I-methylprotons was used as the area reference. That these protons did not undergo exchange was evidenced by the fact that the ratio of the area of their signal to that of the signal for the methyl protons of methanol, which was used as an internal standard, remained constant throughout the course of the run. A plot of log [[H 2) 0 + [H-6] 0 l which was defined by [H-2]t + [H-6]t [[H-Zt)+ 0 .667 [ 1-Cl-I 3] [1-CH,J, log [H-6] + [H-4] + [HS] 0.667 t t t vs twas linear for up to 3.2 half lives in an unbuffered run at 100.0.5 (see next section), indicating that, as in the

PAGE 78

67 case 0 tl1e 1,3-dimethylpyridinium ion, exchange was occurring at approximately equal rates at the 2and 6-positions. As mentioned previously, however, these rates may actually differ by as much as a factor of two or three, and this difference still remainsundetectable. Two kinetic runs were made on this substrate at 75.0.5. In both cases, since the material weighed was 3-hydroxy-1-methyl pyridiniurn iodide, the calculated amount of dilute KOD was added to completely neutralize the substrate. In one of the runs, a DC0 3 --C0 3 2 buffer system was employed. Because of the extreme sluggishness of the reaction (t1;2=69,300 minutes, approximately 1.5 months), exchange was only followed for 0.7 half-lives. At that time, the nmr tube was cracked open, pD measurements made, and t1'e solution acidified to pD 1-2 with dilute DCl. Neutralization of the 3-oxide group has the effect of separating the signals for the 2and 6-protons from those for the 4and 5-protons, and the final point in the kinetic plot, which fell on the line established by the previous points, was determined by using the 4and 5-positions as the area reference, so that [H-2] + [H-6] 0 0 [H-2] + [H-6] t t [H-4]t + [H-S]t [H-2]t + [H-6]t It should be pointed out that this plot exhibited a considerable degree of scatter because only a small fraction of exchange had occurred by the end of the run. Attempts to increase the rate of this reaction by increasing the buffer base strength met with failure. Amine

PAGE 79

68 buffers, generally the systems of choice at 25, are useless at 75 because the pronounced temperature dependence of their pK' s. 4 2 makes them little stronger than carbonate buffers at this temperature. The use of a DP01; 2 --Po1; 3 buffer resulted in severe etching of the glass walls of the nmr tube. A second run at 75.0.5 was carried out in 0.lM KOD. In this more strongly basic medium, exchange occurred at a much faster rate (t1;2 =2,320 minutes) than in the buffered run, and was followed for 1.5 half-lives. Curvature was evident in the 7-point kinetic plot, however, owing to a continuous decrease in pD due to etching. After the completion of each of the kinetic runs at 75.0, the pD of the solution recovered from the nmr tube as well as that of a portion of the original solution (pD 0 ) were determined at 75.0.5 (see secti6n on pD measurements). The activity of the deuteroxide ion was obtained from the relation KD O 75 2 where aD 3O + =antilog (-pD) and KD2.0' 75 0 = 2.985 x 1014 was calculated from recently reported data. 59 60 In most cases, pD and pD 0 agreed to within 0.02 pD units. The activity of deuteroxide ion so obtained and the pseudo first order rate constant were then used to calculate the bimolecular rate constant k for H-D exchange at a particular

PAGE 80

position frorr. the rate constant equation k= k\/i aOD69 ln all of the kinetic runs except those on the !-methylpyridinium-3-oxide betaine, a 030 + was obcained from the measured pD of the recovered solution. Because of the drastic pD changes (1.271 pD units during the run in 0.lM KOD, and 0.447 pD units during the buffered run) and curved kinetic plots encountered during kinetic runs on this substrate, it was arbitrarily decided to calculate a 00 in each case from the average of the pD measurements on original and recovered solutions. The two bimolecular rate constants obtained from these two runs differ by a factor of 3.7. This agreement appears remarkable in view of the large experimental uncert ainties (pD changes, limited number of points, curved kinetic plots) involved. Attempts to follow the rates of H-D exchange at the 6-positions of the 3-cyano-, 3-nitro-, 3-acetyl, and 3-methyl sulfonylpyridinium ions met with failure, owing to the occur rence of side reactions involving the consumption of base and substrate. Heating a solution of 3-cyano-1-methylpyridiniurn per chlorate in a D 3 B0 3 -D 2 B0 3 buffer at 75.0.5 for a total of 250 minutes resulted in partial exchange at the 6-position. Pronounced curvature in the kinetic plot began to become apparent after about 100 minutes, however, and new peaks were observed to arise in the aromatic region of the nrnr spectrum. The solution gradually became dark brown as heating progressed.

PAGE 81

70 T~e addition of basic buff~r to a solution of 3-nitro-1methylpyridinium iodide resulted in the _jmmediate formation of a deep red color in the solution. The initial nmr spectra revealed considerably broadened peaks in the aromatic region, which sharpened after several minutes of heating at 75.0.5. This first heating period was accompanied by considerable darkening of the solution; further heating produced little color change. The spectra after several minutes of heating were found to be superimposable on those for solutions of the substrate in neutral or acidic solution. Little, if any, exchange occurred at the 6-position, even after prolonged heating. Dissolving the substrate in a D 3 BO 3 -D 2 BO 3 buffer solution of pD-10 at room temperature resulted in a rapid decrease in pD to about 8, as evidenced by testing a portion of the resulting solution with Hydrion pH paper. Use of deoxy genated solutions did not eliminate the color or pD changes. A crude experiment was carried out in which an 0.5M solution of 3-nitro-1-methylpyridinium iodide in HCO 3 --CO 3 2 buffer was heated for 15 minutes at 75.0. The aqueous solution was extracted with ether, and the extract dried over anhydrous Na2SO~. Its nmr spectrum revealed four sets of peaks in a 1:1:1:3 ratio: a doublet, farthest downfield, with JAB~3Hz; a pair of doublets (JAB~3Hz; J 8 c~l0 Hz) at a position 34.5 Hz upfield from this peak; a doublet (J 8 c~l0Hz), appearing at a position 126 Hz upfield from the peak at lowest field; and a 3-proton singlet 300 Hz upfield from the sig11al at lowest field. Addition of basic buffer to a solution of 3-acetyl-1methylpyridinium chloride or i6diJe resulted in the intermediate

PAGE 82

71 formation of a dark red color. The nrnr spectrum of the initial solution revealed that exchange of the methyl protons of the acetyl group had occurred to a large extent at room temperature, as well as considerable decomposition of the substrate, as evidenced by the appearance of extraneous peaks in the aromatic and aliphatic regions, some of which were partially overlapped with the substrate and water peaks. Exchange did not occur at the 6-position, even after prolonged periods of heating. A solution of 3-methylsulfonyl 1-methylpyridinium iodide in D 3 B0 3 -D 2 B0 3 buffer was heated for a total of 1400 minutes at 75.0.5. H-D exchange of the methyl protons of the methyl sulfonyl group occurred at room temperature, and was essentially complete after 15 minutes' heating. A small amount of exchange was observed at the 6-position, but curvature in the kinetic plot became increasingly severe with time. The solution darkened slightly during the course of heating, but no new peaks were observed to arise in the nmr spectra. pD measure ments made at room temperature on portions of the original and recovered solutions indicated that the pD had dropped by more than 2 pD units during the course of heating. Kinetics of deuteratibn at the 2and 6-positions of l-~ethylpyridinium-3-oxide betaine at 100.0.5.The kinetics of H-D exchange at the 2and 6-positi6ns of l-methylpyridinium3-oxide betaine were determined at 100.0.5 in 0.06M and 0.30M KOD. The experimental data are given in Table 11. The ionic strengths of the two solutions were different. Methanol was added as an internal standard in both cases; the ratio of the

PAGE 83

Table 11. Kinetic Data for H-D Exchange at the 2and 6-Positions of 1-Methylpyridinium3-Oxide Betaine in KOD Solution at 100.0 + 0.5. [OD ] Ma k2,G sec1 pDO b pDC Md k2,G Ml sec1 e k2,s M-1 1/J aOD-, 2c 2a 0.30 a b 3.26 X 104 14.292 14.195 2.12 X 101 1.09 X 103 1. :;4 X 0.06 h 3.82 105 13.300 13.284 2.60 102 6.36 1 0 4 1. 47 X X X X aConcentration based on measured volume of stock KOD solution added by analytical syringe. bpD of portion of original unheated solution at 25 9 cpD of portion of recovered solution at 25. dActivity of deuteroxide ion at 25, calculated from pD of recovered solution. eSecond order rate constant, based on measured [OD-]. -F --second order rate constant, glonic strength 0.95. hI OnlC strength 0. 6 5. sec1 103 103 f '-J N

PAGE 84

73 area of the nmr signal for its methyl protons to that for the 1-methylprotons of the substrate remained constant throughout the course of each run. The 1-methylsignal was used as the area reference, and in both cases a plot oft vs log [H-2] 0 + [H-6] 0 [H-2]t + [H-6]t was linear up to as much as 3.2 half-lives, indicating that the rates of exchange at the 2and the 6-positions did not differ by more than a factor of two or three. An average of 15 poi11ts constituted each kinetic plot. After the completion of each run, pD measurements were made at 25 on portions of original and recovered solutions. The activity of deuteroxide ion at 25 was then determined fro1n the relation a = KD20, 25 OD aD30+ where aD 30 + =antilog (-pD) and KD 2 0, 25 o=l.351 x 10-Js was calculated from available literature data. 59 60 Second order rate constants k 2 6 and k 2 6 are reported in Tablell, and 2C 2a were calculated from the following expressions: k2,6 = 2C and where [OD-] represents the calculated concentration of deuteroxide ion (based on the volume of stock KOD solution added by syringe) and a 0 Dis the activity of deuteroxide ion as determined from the pD of the recovered solution. The apparent discrepancy between the values obtained for k 2 6 2C from the two runs is probably due to the error encountered in accurately syringing the rather large quantities of stock

PAGE 85

74 KOD solution necessary for neutralization of the hydroxy pyridinium ion, as the values obtained for k 2 6 are in excellent 2a agreement. The syringing error is expected to be more important in the preparation of the less basic solution, so that the value obtained for k 2 6 from the run in 0.30M KOD 2C should be regarded as the more accurate value. The nmr signals for all of the annular protons of l-methylpyridinium-3-oxide betaine overlap to a greater or lesser extent. It is therefore impossible to say with absolute c~rtainty that exchange occurs at the 2and 6-positions exclusively, simply from the appearance of the nmr spectra. After the pD measurement had been made on the recovered solution from the kinetic run in 0.30M KOD (presumably in 0.65M solution of l-methylpyridinium-2,6-d2-3-oxide betaine of 90 percent isotopic purity), the solution was neutralized (pD~7) with stock DCl solution. The neutral solution was stirred with 0.420 g (Srnmoles) of sodium bicarbonate and 570 (6 mrnoles) of dimethyl sulfate at room temperature for 8 hours, the mixture filtered, and the nmr spectrum of the filtrate recorded. The signals in the aromatic region of this spectrum possessed a different appearance and occurred at lower field (relative to the methyl protons of CH 3 0D) compared to the corresponding signals in the last spectrum of the kinetic run. That exchange had occurred exclusively at the 2and 6-positions of the 3-oxide was evidenced by the fact that the nmr spectrum of the rnethylation reaction mixture was identical to that of a solution of 3-meth9xy-1-methylpyridinium-2,6-~ 2 iodide of

PAGE 86

75 90 per~ent isotopic purity, obtained at the end of the kinetic run on this substrate at 75.0.5. Kinetics of deuteration at the 2and 6-~ositions of 1-methylpyridinium and 3-chloro-1-methylpyridinium ions in 0.030M DCl at 197.5.5.Kinetics of H-D exchange at the 2,6-positions of 1-methylpyridinium ion and at the 2and 6-positions of 3-chloro-1-methylpyridinium ion in 0.03M DCl were determined at 197.5.5. Pseudo-first order rate constants are reported in Table 12. The concentration of substrate was approximately O.SM 1n all runs. Acetic acid was found to be unsuitable as an internal standard because its methyl protons undergo exchange und e r these conditions on a time scale similar to that for exchange of the 6-proton of the 3-chloro-1-rnethyl,yridinium ion. The methyl protons of isobutyric acid were found to be suitable for this purpose. Weighed quantities of sodium acetate or sodium isobutyrate were employed in preparing solutions, and the volume of stock DCl solution necessary to completely neutralize the acetate or isobutyrate ions and to produce a D 3 0+ concentration of 0.030M was added by analytical syringe. The 5-position was used as the area reference.in the case of the 3-chloro-1methylpyridinium ion, and the 3,5-positions in the case of the 1-methylpyridinium ion. Appropriate area ratios were obtained as ~escribed previously. That these protons did not undergo exchange under the conditions of the run was evidenced by the fact that the ratio of the area of their signal to that of the methyl protons of the isobutyric acid

PAGE 87

Table 12. Rate Constants for H-D Exchange at the 2and 6-Positions of 1-Methvlpyridinium and 3-Chloro-1-Methylpyridinium Ions in 0.030M DCl at 197.5 + 0.5.c 3-G ki' -Cl c 1.70 X 1. 60 X 2.07 X Avg 1.79 X -H C 4.45 X sec 1 104 d,e 10 1+ f,g 104 g,h,i 104 107 g,h,i 1.82 x 106 d,e f,g 3.45 X l 6 g,h,i 4.45 X 107 g,h,i apD of portion of original unheated solution at 25. bpD of portion of recovered solution at 25. cSubstrate counterion Cl-. dTemperature maintained at 197.5 + 1.0. eMethanol formed by solvolysis of the substrate. D a p 0 1. 635 1.556 pD b 1. 937 1.915 Acetic acid, used as the internal standard, underwent exchange on a time scale similar to that for exchange at the 6-position of the substrate. gMethanol added to suppress solvolysis of the substrate. hDeoxygenated solutions employed. iisobutyric acid internal standard.

PAGE 88

77 internal standard remained constant. Reactions were followed for an average of 2.1 lilllf-lives at each position. An average of 14 points constituted each kinetic plot. An early run on the 3-chloro-1-methylpyridinium ion indi cated that an equilibrium concentration of methanol was formed by solvolysis of the substrate on a time scale comparable to that for exchange at the 6-position. Solutions for subsequent kinetic runs were 0.32M in methanol. The addition of methanol apparently had the effect of successfully suppressing this solvolysis reaction, as the ratio of the area of the signal for the methyl protons of methanol to that for the methyl pro tons of the isobutyric acid internal standard remained constant. The effect of the solvolysis reaction is to increase the acidity of the solution, so that the value for kt in Table 12, obtained from the run in the presence of added methanol, should be regarded as the more accurate value. Kinetics of H drogen-Deuterium Exchange in 3-Substituted yri ines Hydro g en-~euterium exchange experiments at 197.5.5. The kinetics of hydrogen-deuterium exchange at the 2and/or 6-positions of pyridine and several 3-substituted pyridines in D2O or 3:17 (V:V) dioxane-D 2 O solution were studied at 197.5.5. Pseudo-first order rate constants, and the con ditions under which they were obtained, are reported in Table 13. The presence of suitable internal standards verified the fact that, under these conditions, only the 2and 6positions of the various substrates undergo exchange. An average of 11 points constituted each kinetic plot, and each

PAGE 89

Table 13. Rates of H-D Exchange at the 2and 6-Positions of 3-Substituted Pyridines at 197.5 + 0.5. 3-G Solvent k, sec i ki' sec1 -H D2O 2.64 X 105 a 2.64 105 a X D2O b,c 4.11 X 105 e 4.11 105 e X Avg 3.38 X 105 Avg 3.38 X 105 D2O, = 1.00 (NaCl) 2.30 X 105 2.30 X 105 3:17 (V:V) Dioxane-D2O b,c 3.19 X 105 e 3.19 X 105 e 3:17 (V : V) Diox a ne-D2O 3.19 X 10 5 3.19 X 10 5 -Cl D2O b 5.80 104 a 7.00 106 a X X 3:17 (V:V) Dioxane-D2O b 2.26 X 104 e 3.69 X 106 e 3:17 (V:V) Dioxa-ne-D2O b,d 2.63 X 104 e 3. 8 7 X 106 e 3:17 (V:V) Dioxane-D 2 O b,d 3.30 X 104 4.92 X 106 Avg 2.73 X 104 Avg 4.16 X 106 DCl (35% Neutralized) b 3.44 X 104 a DCl (50 % Neutralized) b 2.21 X 104 a 2.87 X 106 a DCl (7 5 % Neutralized) b 1.30 X 104 a -...J 0:,

PAGE 90

Table 13. Continued. 3-G Solvent -Cl DCl (90% Neutralized) b -ND2 D2O C -OMe D2O f aRate constant based on three-point kinetic plot. bSolution deoxygenated. cTMAC internal standard. d Diethyl ether internal standard. e T emperature controlled to 197.5 + 1.0. f !-Butyl alcohol internal standard. 4.62 X 10-1+ a 2.41 X 10-1+ 4.36 X 10-1+ k 6 sec1 1/J

PAGE 91

80 run was followed for an average of 2.4 half-lives. Pseudo-first order rate constants for exchange at the 2and 6-positions of 3-chloropyridine were determined in 3:17 (V:V) dioxane-D 2 0 solution. The use of a cosolvent was necessary because of the low solubility of this substrate in D 2 0. The rate of exchange at the 2,6-positions of pyridine was also determined in this solvent system for purposes of comparison. The kinetics of H-D exchange at the 2,6-positions of pyridine were also studied in 1. OM NaCl (D 2 0) in order to deter mine the effect of ionic strength on the reaction rate. Preliminary experiments with 3-chloropyridine indicated that the presence of oxygen in the system resulted in serious decomposition on a time scale comparable to t~at for exchange at the 6-position of this substrate. Later runs were carried out in deoxygenatecl solutions. All sample solutions were approximately O.SM in substrate, with one exception. In order to study the rate of exchange at the 2and 6-positions of 3-chloropyridine in D 2 0, it was necessary to employ dilute (approximately 0.2M) solutions so that solubility of the substrate was effected. At this low concentration, however, the signal-to-noise ratio in the nmr spectrum diminishes to an extent which makes accurate integration an impossibility. To circumvent this problem, rate data were obtained by heating a portion of the dilute solution in a sealed tube for a certain period of time. After quenching, the tube was cracked open, the D 2 0 solution saturated with

PAGE 92

81 sodium chloride, and the substrate concentrated by extraction with a small volume of carbon tetrachloride. The extracts were dried over anhydrous sodium sulfate, the drying agent was removed, and the nmr spectrum of the carbon tetrachloride solutio11 was recorded. Appropriate area ratios were then obtained fro1n these spectra. Only two heating periods, generally covering about 60-70 percent of the reaction, were employed for each position. The positional rate constants obtained using this technique, then, result from kinetic plots consisting of only three points (the origin plus two experimental points), and are therefore to be regarded as only approximate. A series of experiments were carried out in which the rates of exchange at the 2and 6-positions 0 3-chloropyridine were studied as a function of the degree of neutralization of the substrate. An appropriate volume of deoxygenated stock DCl solution was added by analytical syringe to give the desired percent neutralization, and the solution divided into two equal portions. Since partial neutralization of this substrate results in considerable broadening and overlap of peaks in the nmr spectrum, rate data were obtained by employing the extraction technique described previously, after prior neutralization of the acid by addition of l.SM Na2CO3. Again, these rate constants are only approximate, since they result from kinetic plots consisting of only three points. The nmr spectrum of pyridine in D 2 O or 3:17 (V:V) dioxane-D 2 O appears, in the order of increasing field, as a

PAGE 93

82 series of complex multiplets representing signals for the 2,6-, 4-, and 3,5-protons. As excharige approaches completion at the 2,6-positions, the signals for the 4and 3,5-protons appear as a sharp triplet and doublet respectively. In this case, the signal for the 3,5-protons was used as the area reference, and the appropriate area ratio is then defined by [H-2,6] 0 [H-2,6]t = [H-3,S]t [H-2,6]t The nmr spectrum of 3-chloropyridine in 3:17 (V:V) dioxane D20 appears basically as a set of overlapped peaks for the 2and 6-protons at lowest field, followed at slightly higher field by a doublet for the 4-proton, and finally a quartet for the 5-proton at highest field, although a good deal of fine structure due to cross-ring coupling is aiso evident. As excha11ge occurs at the 2-position, the 6-position emerges as a qoublet which is further split by cross-ring coupling. As exchange at the 6-position approaches completion, the 4and 5-protons appear as a pair of doublets. The signal for the 5-proton was used as the area reference for this substrate, so that [H-2] 0 = ([H-2] + [H-6] ) [H-5] t t t [H-6] 0 and [H-6]t = The nmr spectra of 3-methoxyand 3-aminopyridine in D20 are very similar. The initial spectra both exhibit a complex multiplet representing overlapped signals for the 2and 6-protons at lo w est field followed hy another complex multiplet

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83 representing overlapped signals for the 4and 5-protons at higher field. As exchange approaches completion at the 2-position, the 6-position reveals itself as a triplet. The overlapped signals for the 4and 5-protons were used as the area references in these cases, with the appropriate ratio being given by [H-2] 0 ---[H-2]t = ([H-2] + t + [II-5] ) t Hydrogen-deuterium exchange experiments at 217.9.5. The kinetics of hydrogen-deuterium exchange at the 2and 6-positions of pyridine and several 3-substituted pyridines in D 2 0 solution Herc studied at 217. 9. 5. Pseudo-first order rate constants, and the conditio11s under which they were obtained, are reported in Table 14. The presence of suitable internal standards verified the fact that, under these conditions, cinly ~he 2and 6-positions of the various substrates undergo exchange. Pseudo-first order kinetic plots were linear for up to an average of 2.4 half-lives, and were composed of an average of 10 points. In order to determine the effect of ionic strength on the rate of H-D exchange reactions at this temperature, a run with pyridine as the substrate was carried out in l.0M NaCl Solid sodium carbonate was added to D 2 0 solutions of 3-pyridinesulfonic acid and nicotinic acid in order to insure that the substituents were in the anionic form. Kinetic runs with nicotinic acid were carried out at several different

PAGE 95

Table 14. Rates of H-D Exchange at the 2and 6-Positions 0 3-Substituted Pyridines at 217.9 + 0.5. -. -3-G Solvent ki, sec1 6 Ki/J' sec1 pD 0 pD ------------H D2O 1.58 1 0 ti a 1. 58 l b t+ a X x D2O 2 .10 X 1 oti b 2.lC X 10ti b Avg 1.84 X 10ti Avg 1. 84 X 1 0 ti D2O, = 1.00 (NaCl) 2.60 X 10 ti 2.60 X 10ti "'l"-CO2 D2O 7.50 X 105 2.30 X 10ti 7 d e .;,7 d,e D2O C 7.04 X 105 2.10 X 10ti 9.748 d,f 9.462 d,f D2O C 7.23 X 105 2.29 X 10ti 10.375 d,f 9.984 cl' f Avg 7.26 X 105 Avg 2.23 X 10 ti -S03 D2O C 3.36 10ti b 7.67 X X 105 b 10.084 d,f 9.897 d,f D2O 2.46 X 105 g 8.55 X 106 g -ND2 D2O h 1.40 103 5.72 105 11.108 f 11. 063 f X X -OMe D2O i 2.32 103 [8.15 105 ] j 8.914 f 7.887 f X X ac. L. Smith, Ph.D. Dissertation, Urdversity of Florida (1968). 00

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Table 14. Continued. Rate constant based on three-point kinetic plot. cMeOD internal standard. d pD adjusted by addition of solid sodium carbonate. eA pproximate pD at 25 as determined by testing a portion of the solution with Hydrion pH paper. fpD at 25. gRun in the absence of added base. hTMAC internal standard. i~-Butyl alcohol internal standard. jR~te constant unreliable because of the occurrence of side reactions and a substantial pD change during the course of the run. 00 Vl

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86 base concentrations. In these cases, pD measurements at 25 were made on portions of original and recovered solution. The kinetics of H-D exchange at the 2and 6-positions of 3-pyridinesulfonic acid were also studied in the absence of added base. TMAC, which was employed as the internal standard in the case of 3-aminopyridine, reacted under the conditions of the run, as evidenced by a continuous decrease in the area of its signal relative to those of the 4and 5-protons of the substrate. This loss of signal, which occurred on a time scale comparable to that for exchange at the 6-position, was accom panied by the appearance of a new signal about SO Hz upfield from that of the tetramethylammonium ion which increased in area with time, until, by the end of the run, both peaks were of equal area. The combined integral of these peaks did, howe~er, maintain a practically constant ratio to that of the 4and 5-protons. The strong, fishy odor characteristic of alkyl amines was apparent on opening the nmr tube at the end of the run; addition of a small amount of trimethylamine resulted in an increase in size of the peak farthest upfield. Acidification of the solution resulted in a downfield shift of both the tetramethylammonium and trimethylamine peaks; no other extraneous pe~ks were apparent. The nmr spectrum after addition of a small amount of 3-amino-1-methylpyridinium iodide indicated that alkylation of the annular nitrogen had not taken place. pD measurements on portions of original and recovered solution indicated that very little change had

PAGE 98

87 taken place during the course of the run. The rate constant reported in Table 14 was in good agreement with that obtained from a preliminary run with !-butyl alcohol as the internal standard. It seems likely that the loss of tetramethyl ammonium ion results from displacement of Me3N by water, deutcroxide, or chloride ion. The substitution products, methyl alcohol or methyl chloride, could conceivably escape detection. Oxidation, eventually resulting in the formation of formate ion, could occur in the former case. Because of the high volatility and insolubility of methyl chloride in D 2 0, it might also go undetected. In any event, this side reaction does not present a serious complication. The kinetics of hydrogen-deuterium exchange at the 6-position of 3-methoxypyridine was complicated by several side-reactions. Ether cleavage took place, as evidenced by a gradual decrease in the area of the signal for the methyl protons of the substituent relative to that of the internal standard. The nmr spectra also were characterized by the simultaneous appearance of a new peak of unknown origin about 12-15 Hz downfield from the -OCH 3 signal, as well as of a signal 28 Hz upfield from the -OCH 3 signal which was shown to be due to methanol (by addition of a small amount of the same). Furthermore, the combined area of these three peaks, relative to that of the internal standard, decreased with time, so that a 22 percent reduction in signal had occurred by the end of the run. Of the signal remaining, approximately 10 percent was due to the unidentified upfield peak, while 13 percent was d~ to methanol. In addition, the recovered solution was found to be a full pD

PAGE 99

88 unit more acidic than a portion of the original solution at 25. Although the combined area of the three peaks maintained a constant 3:2 ratio with that of the 4and 5-protons of the substrate, and the pseudo-first order kinetic plot was linear up to 2.5 half-lives, the reliability of the rate constant reported in Table 14 must be regarded with skepticism because of the complicating factors just discussed. The nmr spectra of pyridine and 3-aminoand 3-methoxy pyridine were discuss e d in the previous section. In the latter two cases, exchange at the 6-position was followed by measuring the area ratio given by [H-6] 0 O.S([H-4]t + [H-S]t) = [H-6]t [H-6]t The nmr spectra of the neutralized forms of 3-pyridine sulfonic acid and nicotinic acid are similar. Basically, a singlet for the 2-proton occurs at low e st field, followed by, in the order of increasing field, a pair of doublets and a quartet for the 6-, 4-, and 5-protons respectively, although, as in the case of most 3-substituted pyridines, significant cross-ring coupling is evident. As exchange approaches completion at the 2and 6-positions, the 4and 5-positions appear as a pair of sharp doublets. The 5-position was used as the area reference in both cases. The signals for the 4and 6-protons are overlapped in the nmr spectrum of 3-pyridinesulfonic acid in the ab~ence of added base. Exchange at the 6-position was followed by employing the area ratio defined by

PAGE 100

[H-6] 0 (H-6Jt = + [H-5] t [H-4] ) (H-5] t t Kinetics of hydrogen-deuterium exchange in quinazoline 89 in D 2 0 at 164.7.5. The kinetics of hydrogen-deuterium exchange at the 2and 4-positions of quinazoline in D 2 0 solution were studied at 164. 7. 5. Pseudo-first order rate constants are summarized in Table 15. Under these conditions, exchange occurred exclusively at these two positions. The 2and 4-protons appear as a pair of singlets in the nmr spectrum of quinazoline, with the signal for the 4-position appearing at lowest field~ 1 A complex multiplet representing overlapped signals for the remaining annular protons appears at highest field. The area of this multiplet for the benzo protons was used as the area reference, so tha~ [H] 0 0.25 ([H-5] + [H-6] + (H-7] + (H-8])t = [H]t (H]t represents the appropriate area ratio for the proton of interest. pD Measurements Measurements of pD were performed on many of the solutions employed in the various kinetic runs, both at 25 and at 75.0.5. NBS standard buffers were prepared as described by Bates. 62 For pD measurements at 25, the meter was standardized at pH 6.865 against the NBS phosphate buffer by adjusting the standardization control on the rneter: 62 When the pD of an acidic solution was being measured, the meter was linearized

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Table 15. Rate Constants for Hydrogen-Deuterium Exchange at the z .,., and 4-Positions of Quinazoline in D2O Solution at 164.7 + 0.5. Position 24kl/J, sec 1 1.3 X 10-t+ 5.9 X 105 90

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at pH 1.679 against an NBS potassium tetroxalate buffer by adjusting the temperature control on the meter. 62 When 91 the sample solution was alkaline, the meter was lineatized at pH 9.180 against the NBS borax buffer. 62 pD measurements at 75.0 0.5 were carried out in a similar manner, except that the electrode was allowed to equilibrate in distilled water at 75i0.5 for 15 minutes. Subseque11t standardization and pD measurem e nts were made without allowing tl1e electrode to cool significantly. This was accomplished by rinsing and storing the electrode in distilled water at 75.0.5 between actual measurements. Usi11g this technique, the equilibration period for a particular sample rarely exceeded 2 minutes. For pD measurements at 75.0.5, the meter was standardized at pH 6.852 against the NBS phosphate buffer; lin e arization was accomplished by employing either the NBS potassium tetroxalate (pH 1.754) or borax (pH 8.905) buffers, depending upon whether the sample solution was acidic or basic. 62 Reproducibility of individual pD readings was .03, except at the highest pD values. An average of seven determinations on a carbonate buffer gave 10.28 with a standard deviation of 0.13, for example. Since the pH meter was standardized and linearized against standard protea buffers, it was necessary to add a correction to the meter readings obtained on sample D20 s6lutions to convert these readings to pD values. For pD measurem e nts at 25, the pD value is reported to be

PAGE 103

92 obtained by adding 0.41 to the meter reading. 63 This difference was observed experimentally at 25 in the present work when 10~ 2 M HCl and 102 M DCl solutions of 1.0 ionic strength were employed. To determine the temperature dependence of the correction, new readings on both solutions were obtained at 75.0.S 0 and these were found consistently to differ by 0.35 units. Therefore, for measurements at 7S.0.S 0 the pD value was obtained by adding a correction of 0.3S to the meter reading. In order to determine whether or not this correction factor was valid at high pD values, the meter was standardized at pD 10.422 at 7S.0.S 0 against a standard deuterio carbonate buffer. 6 ~ pD measurements on several sample solutions at 7S.0.S 0 were then performed, the pD value being taken as the meter reading. These values were in good agreement with those obtained on the same sample solutions by first standardizing and linearizing the meter against NBS protea buffers and then adding 0.35 to the meter reading. Control Runs Control runs were not performed for any of the hydrogen deuterium exchange r~actions of pyridine or 3-substituted pyridines at 197.S.S 0 or 217.9.S 0 since, in these cases, the presence of an internal standard, pD measurements at room temperature on original and recovered solutions, and the linearity of the pseudo-first order kinetic plots indicated the absence of important complicating factors.

PAGE 104

93 A solution of 0.030M DCl approximately 0.SM in KCl and containing isobutyric acid and methanol internal standards was prepared to simulate the solutions of 1-methyland 3-chloro-1-methylpyridinium chloride employed in the kinetic runs at 197.5.5. This blank solution was heated for 7,300 minutes at 197.5.5, and then pD measurements at 25 were made on portions of original and recovered solution. The recovered solution was found to be 0.312 pD units more basic than the original solution. A series of control runs in protea buffers were performed for those 3-substituted-1-methylpyri
PAGE 105

BIBLIOGRAPHY 1. A. I. Shatenshtein, Advan Phys. Org. Chem., l, 156 (1963). 2. G. E. Hall, R. Piccolini, and J. D. Roberts, J. Amer. Chem. Soc., J_J_, 4540 (1955). 3. G. E. Hall, E. M. Libby, and E. L. James, J. Org. Chem., 2~, 311 (1963). 4. N. N. Zatsepina, I. F. Tupitsyn, A. V. Kirova, and A. J. Belashova, Reakts. Sposobnost Org. Soedin., ~' 257 (1969). 5. I. F. Tupitsyn, N. N. Zatsepina, A. V. Kirova, and Y. M. Kapustin, Reakts. Sposobnost Org. Soedin., ~' 601 (1968). 6. W.W. Paucller and S. A. Humphrey, J. Org. Chem., 35, 3467 (1970). 7. I. F. Tupitsyn, N. N. Zatsepina, Y. M. Kapustin, and A. V. K.irov a Reakts. Sposobnost Org. Soedin., ~ 613 (1968). 8. N. N. Zatsepina, Y. L. Kaminsky, and I. F. Tupitsyn, Reakts. Sposo b nost Org. Soedin., ~' 448 (1969). 9. A. Strcitweis er, Jr., and F. Mares, J. Amer. Chem. Soc., _?_Q_, 644 (196<:l). 10. J. A. Zoltewicz and C. L. Smith, J. Amer. Chem. Soc., ~, 3358 (1967). 11. R. A. Coburn, J.M. Landesberg, D. S. Kemp, and R. A. Olofson, Tetrahedron, ~' 685 (1970). 12. J. D. Vaughan, Z. Mughrabi, and E. C. Wu, J. Org. Chem., ~, 1141 (1970). 13. E. C. Wu and J. D. Vaughan, J. Org. Chem., 35, 1147 (1970). 14. J. A. Zoltewicz and L. S. Helmitk, J. Amer. Chem. Soc., ~, 7547 (1970). 15. R. A. Abramovitch, G. M. Singer, and A. R. Vinutha, Chem. Commun., 55 (1967). 94

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16. W. Brugel, Z. Eleatroahe~n ., ~, 159 (1962). 17. J.B. Merry and J. H. Goldstein, J. Amer. Chem. Soc ., ~, 5560 (1966). 18. R. E. Lyle, Chem. Eng. News, .!!, 72 (1965). 19. E. M. Kosoher and J. W. Patton, Tetrahedron., ~, 2081 (1966). 20. A. R. Katritzky and J.M. Lagowski, "Heterocyclic Chemistry," John Wiley and Sons, Inc. New York, 1960, pp. 61-62. 21. J. W. Iluff, J. Biol. Chem., 167_, 151 (1947). 22. A. SanPietro, J. Biol. Chem ., _0._2_, 589 (1955). 95 23. H. E. Dubb, M. Saunders, and J. H. Wang, J. Amer. Chem. Sea., ~' 1767 (1958). 24. R. B. Martin and J. G. Hull, J. Biol. Chem., 239, 123'7 (1964). 25. P. Beak and J. Bonham, J. Amer. Chem. Soc., _, 3365 (1965). 26. K. W. Ratts, R. K. Howe, and W. G. Phillips, J. Amer. Chem. Soc., 21_, 6115 (1969). 27. Y. Kawazoe, M. Ohnishi, and Y. Yoshioka, Chem. Pharm. Bull. (Tokyo), _!l, 1384 (1964). 28. J. A. Zoltewicz, G. M. Kauffmann, and C. L. Smith, J. Ame1 1 Chem. Soc., ~, 5939 Cl968). 29. Indexed in Chemical Abstracts under "Pyri
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35. P.R. Wells, "Linear Free Energy Relationships," Academic Press, New York, 1968 .. 96 36. R. T. C. Brownlee, F. E. Stary, and R. W. Taft, preprint. 37. C. D. Ritchie and W. F. Sager, Progr. Phys. Org. Chem., ~' 323 (1964). 38. M. Charton, J. Am e r. Chem Soc.,~, 2033 (1964). 39. C. G. Swain and E. C. Lupton, Jr., J. Amer. Chem. Soc., 2_Q_, 4328 (1968). 40. R. W. Taft, E. Price, I. R. Fox, I. C. Lewis, K. K. Andersen, and G. T. Davis, J. Amer. Ch em Soc., ~, 3146 (1963). 41. R. T. C. Brownlee, R. E. J. Hutchinson, A. R. Katritzky, T. T. Tidwell, and R. D. Topsom, J. Amer. Chem. Soc., 2-Q., 1757 (1968). 42. D. D. Perrin, "Dissociation Constants of Organic Bases in Aqueous Solution," Butterworths, London, 1965. 43. "Handbook of Chemistry and Physics," 45th Edition, R. C. Weast, ed., Chemical Rubber Co., Cleveland, Ohio, 1964, pp. C75-601. 44. E. C. Taylor and J. S. Driscoll, J. Org. Chem ., ~, 1716 (1960). 45. Sample supplied by C. L. Smith. 46. D. A. Prins, Rec. Trav. Chim., 76, 58 (1957); Chem. Abstr., g, 12088a (1957). 47. I. B. Chckmareva, E. S. Zhdanovich, T. S. Novopokroskaya, and N. A. Preobrazhenskii, Zh. PrikZ. Khim., 35, 1157 (1962); Chem. Abstr., ~' 8546a (1962). 48. A. J. Kirby and A. G. Varvoglis, J. Chem. Soc. B., 135 (1968). 49. G. B. Barlin and W. V. Brown, J. Chem. Soc. B, 648, (1967). SO. Sample supplied by C. Nisi. 51. G. Pfleiderer, E. Sann, and A. Stock, Chem. Ber., 2.l_, 3083 (1960). 52. R. Eisenthal and A. R. Katritzky, Tetrahedron, Q, 2205 (1965).

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97 53, H. Larive, P. Collet, and R. Dennilauler, Bull. Soc. Chim. France, 1443 (1956), 54. H. Meyer, B e r., ~' 616 (190;5). 55. It should be noted that the synthetic method described by Meyer 54 differed from the one employed in this study; his observed melting point of 130 appears abnormally low when compared to those. of 237-238 and 345 for the isomeric l-methylpyridinium-3and -4-sulfonate betaines respectively, reported by Larive, Collet, and Dennilauler. 53 56. Indexed in Chemical Abstracts under 11 Pyridinium com pounds, 3-carboxy-1-methyl-, hydroxide, inner salt." 57. E. M. Kosower and J. W. Patton, J. Org. Chem., ~' 1318 (1961). 58. N. N. Zatsepina, Y. L. Kaminsky, and I. F Tupitsyn, Reakts. Sposobnost Org. Soedin., !, 177 (1967). 59. A. K. Covington, R. A. Robinson, and R. G. Bates, J. Phys. Chem., J!}__, 3820 (1966). 60. G. s. Kell, J. Chem. Eng. Data, !l, 66 (1967). 61. A. R. Katritzky, R. E. Reav ill, and F. J. Swinbourne, J. Chem. Soc. B, 351 (1966). 62. R. Bates, "Determination of pH. Theory and Practice," John Wiley and Sons, Inc., New York, 1964. 63. A. K. Covington, M. Paabo, R. A. Robinson, and R. G. Bates An a Z Chem !Q_, 7 0 0 ( 19 6 8) 64. M. Paabo and R. G. Bates, Anal. Chem.,!!_, 283 (1.969).

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BIOGRAPHICAL SKETCH Robert E. Cross was born July 24, 1942, at Toledo, Ohio. In June, 1960, he was graduated from Cehtral High School. In August, 1965, he received the degree of Bachelor of Science with a major in Chemistry from the University of Toledo. In 1965 he enrolled in the Graduate School of the University of Toledo. He worked as a Graduate Assistant in the Department of Chemistry u11til September, 1967, when he received the degree of Master of Science with a major in Chemistry. In 1967 he enrolled in the Graduate School of the University of Florida. He was a Graduate Teaching Assistant from 1967-1968, an Interjrn Instructor from 1968-1969, and a Graduate School Fellow from 1969-1970, while pursuing his work toward the degree of Doctor of Philosophy. Robert E. Cross is married to the former Eileen Kay Baden, and is the father of a daughter, Laura Lynn. He is a member of Alpha Chi Sigma and the American Chemical Society. 98

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. uAAa ~ J~
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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequ a te, in scope and quality, as a dissertation for the degree of Doctor of Philosophy c ~:: C l 1 / \_; ~ ~ _1/ -=~ ~--Euge 4 G. Sander Asso i iate Professor of Biochemistry ~ / This dissertation was submitted to the Dean of the College of Arts and Sciences and to the Graduate Council, and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. June, 1971 ---------Dean, Graduate School