STUDIES OF THE ABSORPTION SPECTRA OF SOME POLYSUBSTITUTED BENZENES
By io>^^5 PATRICIA ANN, FISH
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 April, 1963
The author wishes to express her deepest appreciation to her research director, Dr. Armin H. Gropp, for his advice and encouragement throughout the course of this study. She is also grateful to the other members of her committee, Drs. J. D. Winefordner, A. P. Black, C. E. Reid, and T. D. Carr.
Special thanks are due Dr. W. S. Brey, Jr. for the contribution of his organic compounds to this investigation.
TABLE OF CONTENTS
LIST OF TABLES ........................ iv
LIST OF FIGURES....................... v
I. INTRODUCTION ................... 1
II. EXPERIMENTAL PROCEDURE .,............. 4
III. RESULTS AND DISCUSSION.............. 7
IV. SUMMARY..................... 21
I. TABLES...................... 23
II. FIGURES..................... 35
BIOGRAPHICAL SKETCH..................... 44
LIST OF TABLES
I. INTENSITIES OF THE 1500 AND 1600 cm."1 BANDS
RELATIVE TO TOLUENE.................. 24
II. INTENSITIES OF THE 1500 cm."1 BANDS OF 1,4-DISUBSTI-
TUTED COMPOUNDS RELATIVE TO 1,4-DEffiTHYLBENZENE ... 31
III. INTENSITIES OF THE 1500 AND 1600 cm."1 BANDS OF
1,2,4-TRISUBSTITUTED BENZENES RELATIVE TO 1,2,4-TRI-METHYLBENZENE.................... 33
IV. INTENSITIES OF THE 1500 cm."1 BANDS OF 1,2,4,5-TETRA-. SUBSTITUTED BENZENES RELATIVE TO 1,2,4,5-TETRAMETHYL-BENZENE....................... 34
V. INTENSITIES OF THE 1500 cm."1 BANDS OF 1,2,3,4,5-PENTASUBSTITUTED BENZENES RELATIVE TO 1,2,3,4,5-PENTAMETHYLBENZENE.................. 34
LIST OF FIGURES
1. The Direction of the Spectroscopic Moments in Poly-substituted Benzenes.................. 36
2. Illustration of the Vectorial Addition of Spectroscopic Moments for Disubstituted Benzenes ....... 36
3. Absorption Spectrum of 0.2634 M Benzene in Tetra-chloroethylene in the 0,21k mm. Cell.......... 37
4. Absorption Spectrum of 0.2501 M Methylbenzene in Tetrachloroethylene in the 0.214 mm. Cell....... 38
5. Absorption Spectrum of 0.2486 M 1,4-Dimethylbenzene
in Tetrachloroethylene in the 0.214 mm. Cell ...... 39
6. Absorption Spectrum of 0.2542 M 1,2-Dimethylbenzene
in Tetrachloroethylene in the 0.214 mm. Cell...... 40
7. Absorption Spectrum of 0.2500 M 1,3-Dimethylbenzene
in Tetrachloroethylene in the 0.214 mm. Cell ..... 41
8. Absorption Spectrum of 0.2500 M 1,3,5-Trimethylbenzene
in Tetrachloroethylene in the 0.214 mm. Cell...... 42
Every absorption band in the infrared region of the spectrum is characterized by the frequency at which maximum absorption occurs and by the absorption intensity. The terra intensity refers, not to the absorption coefficient at the band maximum, but to the integrated absorption coefficient for the entire band envelope. This integrated intensity is fundamentally related to the electronic properties of the molecule (1).
According to classical electrodynamics any motion of an atomic system that produces a change in dipole moment, leads to emission or absorption of radiation. A vibration is considered infrared active if the periodic change in the charge distribution during a normal vibration results in a change in the molecular dipole moment.
The observed molecular dipole may be represented as the sum of
four individual bond components: primary + overlap + hybridization + core (2). The primary moment arises from the difference in the fractional parts of the bonding electron cloud in the atomic orbitals. If the overlap cloud is divided equally between the two bonded atoms, the primary moment arises from the difference in electronegativities, which gives the bond its ionic character. The overlap moment is due to the fractional parts of the electron cloud occupying the orbital overlap region. The hybridization moment is due to the displacement
of the centers of gravity of the electronic clouds in the atomic orbitals from the nuclei of the two atoms. The core moment arises from polarization of the core by the primary moment.
The integrated intensity of an infrared band, 1^, corres-oonding to the i th fundamental vibrational transition can be expressed as (3):
where N is a number depending on the units in which I. is expressed,
c is the velocity of light, M- is the molecular dipole moment, and
is the normal coordinate for the i th vibration. When 1^ is expressed
in units of 10 liter/mole/cm. N in equation 1 is Avogadro's number.
The observed intensity, I, is related to the absorption coefficient, a^, by (4)
where b is the path length, c is a unit of concentration, and A is the absorbance at any frequency,
(d'jilLj3CL^j represents the rate of change of the vector dipole moment when the molecule is deformed in the manner specified by the normal coordinate, Q. The quantity, can be deter-
mined directly from intensity measurements; however, these measurements give only the magnitude of the change in moment and not the direction.
To reduce the measured intensities to quantities characteristic of individual bonds several assumptions are made: (1) When a bond is
- 3 _^
stretched by an amount dr, a moment, ^^~)dr, is produced in the
direction of the bond. (2) When a bond is bent through an angle, d0,
a moment, ft d0 where jT is the effective bond moment, is produced o o
In the plane of bending and perpendicular to the bond direction. (3) When any one bond is bent or stretched, no moments are produced in other bonds.
Studies on benzene vibrations have shown that the characteristic skeletal stretching modes of the semiunsaturated carbon-carbon bonds
lead to the appearance of a group of four bands between 1650 and 1450
-1 -1 -1
cm. (J2). Of these four bands, those near 1600 cm. and 1500 cm.
are highly characteristic of the aromatic ring itself. There is some uncertainty in the literature as to the occurrence and significance of the 1600-1560 cm."1 band. Some investigators (6) regard this band as an indication of conjugation of a double bond with the aromatic ring. The fourth band, appearing in the range 1470-1439 cm."1 is frequently obscured by intense CH^ deformations.
For these reasons, this investigation was confined to a comparison of the relative integrated intensities of the 1600 and 1500 cm."1 absorption bands of some substituted benzenes.
II. EXPERIMENTAL PROCEDURE
Experimentally, the measurement of absorption intensities for molecules in solution is simpler than gas-phase measurements and more reliable than solid-phase measurements because of the scattering and reflection effects. Because rotational motion is diminished for almost all molecules in the liquid-phase, only a single band occurs for a vibrational transition. These single bands are relatively wide because of collisional broadening and, generally, are represented by the Lorentz distribution ()
e i )2 + b2
where is the frequency of the maximum absorption, and a and b
are constants. In curves such as these the absorption approaches zero very slowly, and the integrated intensity must be determined at frequencies far removed from 1s,' .
The procedure used in this study to determine the intensities of the absorption bands was to plot the values on a graph which is linear in both frequency and absorbance. The area under these curves was then determined by means of an Ott planiraeter. Since the spectrophotometer is incapable of measuring small absorptions accurately, a base line was constructed, and the measurements were made within a small interval on either side of 3^. This leads to negligible error in the measured
intensities, since absolute values were not desired. Rather, bands were studied which are common to all the molecules in the series and a comparison made of their relative values.
A Perkin-Elmer Model 21 double-beam infrared recording spectrophotometer, which graphs linear wavelength and transmittance, was used throughout this study. The instrument was equipped with a calcium fluoride prism, which gives best resolution in the range 3335-H75 cm."1 (8).
Wavelength calibration was made using the water bands occurring in the region from I785 to 1390 cm."1. Slit control, response and gain were kept constant, and the scanning speed was approximately seven minutes per micron.
Four sets of matched sodium chloride fixed cells were used. Measurement of the cell thickness was made by observing the interference fringes resulting from an empty cell in the beam. The relation between the thickness of the cell, 1, and the number of interference fringes, n, between any two wavelengths, A^, and is given by ().
A Mettler H-15 Analytical Balance was used to make up all solutions. Weighing was accurate to 0.5 mg.
Most compounds investigated were reagent grade chemicals obtained from various chemical companies. The remainder of the reagents were prepared and purified.
Solutions of the compounds studied were prepared in spectral grade tetrachloroethylene, selected as a solvent because: (1) It is highly transparent in the spectral region investigated. (2) It is non-polar to such an extent that the greatest majority of the substituted benzene derivatives studied were soluble in it. The non-polarity of the solvent should contribute very little in the way of any dipole-dipole interaction with the solute molecules. The only absorption by the solvent in the region under investigation is a weak C-Cl overtone at 1572 cm.""1. Matched cells compensated for any solvent absorption.
Because absorption decreases at with increased temperature as a result of collisional processes, all solutions were prepared and studied in an air-conditioned laboratory. Also, the refractive index of the solutions changes with a change in temperature, so that the integrated intensity is probably temperature dependent. To keep the indices of refraction as constant as possible, all solutions were made up to approximately 0.25 M.
III. RESULTS AND DISCUSSION
The intensity of a weak band, whose weakness is due to geometrical symmetry, is enhanced by substituents which destroy the symmetry CIO). From the spectrum of 0.2634 M benzene in tetrachloroethylene (Figure 3) measured in a 0.214 mm. cell, it can be seen that the skeletal vibrations at 1500 and 1600 cm. cannot be detected. This can be attributed to the high degree of symmetry in the benzene molecule.
For the benzene ring, the direction of |A is that of a vector
lying in the plane of the ring perpendicular to the ring substituent
axis. The value of JA is conventionally assigned a positive magnitude for ortho-para directing substituents, which increase the electron density in the ring, and a negative magnitude for meta directing or electron-withdrawing substituent, (-10)..
In the case of polysubstitution, if the direction rules are applied to any substituent in position 1, the same rules hold for positions 3 and 5 The direction of the vectors is reversed in the remaining positions, 2, 4, and 6 as shown in Figure 1 (11).
It is also assumed that the spectroscopic moments of weakly resonating groups are vectorially additive. Figure 2 illustrates the vectorial addition of two different substituents in the three types of disubstituted benzenes. The resultant moment in each case is designated
by vector r. If two substituents, having moments a and b, are of the same sign, r cannot exceed the magnitude of the larger of the two vectors in ortho and meta substitution, but r may be larger than either a or b in ortho and meta substitution, but less than the larger of the two moments in para substitution.
The intensity of the 1500 and 1600 cm."1 bands in each substituted benzene was measured and listed in Table I relative to the intensity of each of these peaks in toluene. This method has the advantage of not only being dimensionless, but also giving the factor by which these bands have been intensified.
Monosubstituted Benzenes. 1500 cm."1 Band
In accordance with other studies (11) the methoxy group appears to be the strongest resonator of the substituents examined, the intensity of the 1500 cm."1 band in anisole being 4.24 times the intensity of that band in toluene.
The fluorine atom shows the next largest effect upon intensification of the 1500 cm."1 band, fluorobenzene having a peak 3.62 times more intense than toluene. The fact that the intensification of this band decreases in the order F)> CI ^ BrJ> I in the ratio of 3.62:2.92: 2.00:1.03 suggests that the inductive effect of the halogens is more responsible for enhancement than the mesomeric effect. This effect is also in agreement with the shift in frequency of the maximum of the 1500 cm."1 peak. It can be seen from Table I that the shift is F, 1490 cm."1; CI, 1476 cm."1; Br, 1&73 cm."1; I, 1470 cm."1. The maximum shift of 20 cm."1 between F and I is much too small to account for
the difference in the integrated intensities of these two peaks.
The OH group is found to be a weaker resonator than the OCH^ group, intensifying the 1500 cm."1 band 2.59 times as much as the CH^ group.
The alkyl radicals intensify the band in the order of t-Bu } i-Pr > C12H2^> CH^ ) n-Pr } 1
would enhance the ring vibration to a larger extent than a straight chain radical because of the hyperconjugation effect, i.e.,
That ^22^25 aPPears ^ ^e out ^ orcier ^e accounted for in that the inductive effect of this group may offset the effect of hyperconjugation .
Benzonitrile has a 1500 cm."1 band that is only 0.65 times as intense as the toluene band. A value of this magnitude would be expected, since the CfN group is known to be a meta orienting substituent. This property of the group decreases the electron density on the ring, thereby decreasing the intensity of the band. The position of the band (1490 cm."1) indicates that the inductive effect is also rather weak.
The halogenated alkyl radicals decrease the band intensity in the order CH^Cl } CHBrCH > CHC12 > CFgC^CF indicating that the
0CH 0CH OCH,
In almost every benzene derivative examined, in which hyperconjugation appeared probable, a relatively weak band occurred in the neighborhood of 1600-1560 cm."1. This peak is present in the spectra of the para halogenated anisoles, with the sole exception of the F substituted compound. This seems to indicate that no conjugation with the
inductive effect of the halogens decreases the resonating ability of the alkyl group.
Para Disubstituted Benzenes. 1500 cm."1 Band
According to the theory of vectorial additivity of moments, the total intensity of the 1500 cm."1 band of 1,4-dimethoxybenzene should be approximately 8.47 times the intensity of this band in toluene. From Table I, though, it can be seen that this band is 9.59 times as intense as the toluene peak. It may also be seen that there is added enhancement of the 1500 cm."1 band in every example of a para substituted OH or OCH^ group, with only one exception: p-fluoromethoxybenzene .
A possible explanation of the increased moment of the para substituted phenols and anisoles is their ability to conjugate with the ring. For example:
ring is occurring in this instance. It may be noted that fluorine is
the only member of the series that does not have d-orbitals available
for halogen conjugation with the benzene,ring.
That the intensity of the 1500 cm. band is less than predicted for p-fluoroanisole, may be accounted for on the basis of mutual repression of the electron-releasing ability of these two very strong resonators.
It seems reasonable from the preceding arguments to expect the intensity of the 1500 cm."1 band of p-fluorpphenol to also be less than predicted by the theory of the additivity of vectorial moments. Such is not the case, however, and the intensity of this peak is actually larger than predicted. Upon examination of Table I, it may be seen that the fluorine atom intensifies the 1500 cm."1 peak in fluoro-benzene to a much larger extent than the OH group in phenol; whereas F and OCH^ are comparable in resonating ability. Probably the mutual repression seen in p-fluoroanisole is not experienced by p-fluoro-phenol. It should be noted that the band assigned to hyperconjugation of a double bond with the ring is absent in p-fluorophenol.
The intensities of the 1500 cm."1 bands for the series p-difluorobenzene, p-dichlorobenzene and p-dibromobenzene are also less than predicted by theory. Of the three members of the series the Br substituted benzene comes closest to the predicted value. This, again, may be attributed to the close-lying d-orbitals in the Br atom, which may participate in conjugation. However, even in p-dibromobenzene, the repression of the resonating ability of each atom appears to have a greater effect on the intensity of the peak than the ability to use the
low-lying d-orbitals for conjugation. The absence of the 1600-1560 cm."1 band substantiates this hypothesis. The fourth member of the series, p-diiodobenzene was insoluble in the chosen solvent.
The para substituted alkyl benzenes display the mutual repression of their resonating power to such an extent that the 1500 cm."1 peak in p-xylene actually is less intense than the same peak in toluene.
Few compounds were examined which contained meta directing substituents. The series of nitrobenzenes was omitted because of the very strong absorption of the N02 group in the region of interest. This band generally obscured the weaker absorption of the benzene skeletal vibrations. Since the CsN group is an electron-withdrawing substituent, it will conventionally have a moment vectorially negative. The intensification of the 1500 cm."1 band in p-tolunitrile should, therefore, be approximately equal to the difference in intensifications due to toluene and benzonitrile. The band is actually about twice as intense as predicted, indicating that the mesomeric effect of the CH^ group coupled with the inductive effect of the CN group increases the total moment of the phenyl ring.
In Table II of the 1500 cm."1 band of p-xylene is assigned an intensity value of 1.00 and the intensity of this band in all the para substituted benzenes is calculated relative to p-xylene. It can, in general, be noted that the same order of intensification occurs in para substitution as was found in the mono substituted benzenes.
Within the series of para halogenated phenols the order of intensification occurs as expected: F } CI > Br. The order of
intensification within the series of p-alkylated phenols is just the
reverse of that expected: cj2^25^ Pr ^ CH3 ^ i~Pr* This Indicates that in the dodecyl and n-Pr groups the inductive effect is becoming increasingly important, while the mesomeric effect predominates in the methyl and isopropyl substituted phenols. All four of these compounds display the 1600-1560 cm.-1 band, indicative of hyperconjugation.
Meta Disubstituted Benzenes. 1500 cm."1 Band
According to the theory of vectorially additive moments, the resultant moment of two substituents of like sign in a meta position cannot be greater than the larger of the two vectors. This, however, was not found to be the case in the greatest number of meta substituted benzenes studied. This enhanced intensity of the 1500 cm."1 band was, in general, observed in compounds containing resonators of nearly comparable strength, such as OCH and OH or NH and the halogens. Apparently, the mutual repression is such as to actually invert the moment of one of the substituents.
In m-iodoanisole the peak intensity was actually larger than the sum of the intensities of these bands in toluene and iodobenzene. A possible explanation of this effect is that the inductive effect of I produces additional asymmetry of charge distribution by concentration of the electron density by the electronegative atom.
All of the meta substituted phenols and anisoles show evidence of conjugation with the ring. Indications of conjugation also appear in the spectra of the three halogenated anilines, substantiating the theory that the permanent mesomeric effect is enhanced by the electric
effect of the halogens.
Since the CH^ group is ortho-para directing and the C=N group is meta directing, the resultant moment of m-tolunitrile should be theoretically as large as the moment of the larger of the two substituents (Figure 2). This was not the case; but the actual intensification of the 1500 cm."1 band was less than that of toluene. It Is thought that the electron-withdrawing ability of the C=N group, must increase the electron density of the meta positions to such an extent that the mesomeric effect of the CH^ group is repressed.
Ortho Disubstituted Benzenes. 1500 cm.""1 Band
From Figure 2 it can be predicted that the intensification of the 1500 cm."1 band in the benzene derivatives having like-sign substituents positioned ortho to each other, should not be greater than the intensification due to the most effective of the two substituents. Contrary to this, each ortho substituted anisole, phenol and aniline was much more enhanced than the theory would predict, with the exceptions of o-tertiarybutylphenol and o-isopropylphenol. For example, o-hydroxyanisole has an intensity more than 11 times the intensity of toluene, while theory indicates an intensification of only about a factor of four. This increase in the absorption intensity of the band might very well be due to intramolecular hydrogen bonding. If the hydrogen atom on the functional group is pulled away from the central atom by some electronegative atom in an ortho position, the mesomeric effect is increased, allowing a greater release of electronic charge to migrate into the ring. This Increases the effective spectroscopic
moment of the functional group. At the same time the electron density about the electronegative atom or group of atoms is increased, tightening the bond, decreasing this effective moment. From the vector addition rules in Figure 2, it can be shown that these two effects tend to increase the resultant spectroscopic moment.
This effect is not a function of electronegativity only, since the largest increase in intensification above that predicted would have been expected to appear in o-fluorophenol rather than o-hydroxy-anisole or o-dimethoxybenzene, as was the case.
The order of deviation from the predicted intensification for the substituted phenols is OCH^^ I ^ CI} F, which indicates that, the effectiveness of intramolecular hydrogen bonding is influenced somewhat by the size of the electronegative atom or group of atoms.
That this general enhancement of the ortho substituted phenols was not experienced by o-isopropylphenol and o-tertiarybutylphenol, may be the result of the inductive properties of the i-Pr and t-Bu groups. Their attraction for electrons would be transmitted through the ring, thereby decreasing any intramolecular hydrogen bonding. Also, steric effects may be partially responsible. If the benzene ring is oriented,
and if substituent X has a pair of non-bonding P electrons, they may
migrate into the ring (12). Thus, if this orbital becomes non-parallel to the P orbital on the ring, the resonance effect may be z
These two substituted phenols have spectra indicative of conjugation with the ring. The mesomeric effect of C-H is due to bound electrons and is, therefore, very weak. The C-C bond, such as in i-Pr or t-Bu groups, has no P^ component. Enhanced intensification by these groups must be an inductive effect. More evidence leading to this conclusion is that o-tertiarybutylphenol is more intense than o-isopropyl-phenol in agreement with the order due to induction.
1,2,4-Trisubstituted Benzenes. 1500 cm."1 Band
Only six compounds were studied having 1,2,4-trisubstitution, but the order of intensification follows the same pattern already observed. Table III contains these compounds with their intensities relative to 1,2,4-trimethylbenzene.
The three 2,4-disubstituted phenols had intensities larger than expected, while 1,2,4-trichloro-, 1,2,4-tribromo- and 1,2,4-trimethyl-benzene were within the range predicted. This may be explained on the basis of intramolecular hydrogen bonding in 2,4-dichlorophenol, which shows the largest degree of enhancement, and probably a combination of the mesomeric effect and inductive effect in 2,4-diisopropylphenol and 2,4-dimethylphenol.
1,2.4,5-Tetrasubstituted Benzenes. 1500 cm."1 Band
Three 1,2,4,5-tetrasubstituted compounds were studied. The largest intensification of the 1500 cm."1 band was observed in 1,2,4 5-tetrachlorobenzene, an increase of 8.75 times the peak in toluene. This intensification is much larger than expected on the basis of additive moments. Because the band appears at 1441 cm."1 rather than 1500 cm."1, this indicates an extremely large mesomeric effect which results in an increase in the polarity of the ring.
The intensification of 2,4,5-trichloroaniline was 8.35 times that due to toluene. The mobility of the non-bonding electron pair in NH2 slightly offsets the strong mesomeric effect of the three CI atoms, which is indicated by a shift in the band maximum to 1475 cm."1. The intensification is much larger than predicted, however.
l2,4,5-tetramethylbenzene comes closest to having the predicted intensification value. Because the CH^ group is a very weak resonator and because the molecule has a moderate degree of symmetry, no serious deviations from the additive moments theory would be anticipated. This is substantiated by the normal position of the band, maximum at I5O7 cm."1 (Table IV).
1,2,34,5-Pentasubstituted Benzenes. 1500 cm."1 Band
Table V shows that an exchange of an OH group for a CH^ in position 3 of pentamethylbenzene only enhances the intensification I.O3 times above that of the 1500 cm."1 peak. The decrease in band frequency indicates that this increase in intensification of the peak arises from the mesomeric ability of OH. Further exchange of the
methyl groups to 2,4-dichloro-3,5-dimethylphenol enhances the band only 1.23 times that of pentamethylbenzene. A further decrease in band maximum frequency suggests the CI atom's electron-releasing ability is more important than its electronegativity. Indications of hyperconjugation appear in both of the tetrasubstituted phenols, but not in pentamethylbenzene .
1.3.5-Trisubstituted Benzenes. 1.500 cm.""1 Band
An interesting phenomenon which occurred is that the 1500 cm."1 band was totally absent in each of the eight 1,3,5-trisubstituted benzenes, regardless of the type of functional group or groups. This suggests that this vibration may be primarily due to asymmetry and corroborates the hypothesis of additivity of vector moments, which should be theoretically zero or very nearly zero in any 1,3,5-trisubstituted benzene.
Because the 1500 cm."1 band was present in each of the 1,2,3-trisubstituted benzenes examined, it is thought that this is further evidence that effects other than simple vectorial addition of moments are responsible for the intensity of this vibration. The calculated total moment of this type of molecule should also be zero.
1,2,6-Trisubstituted Benzenes. 1500 cm."1 Band.
Within the series of 2,6-disubstituted phenols the normal pattern is exhibited, OCH^ and CI causing the largest amount of intensification. The 2,6-dialkylated phenols are intensified in the order: CH^ y Pr y i-Pr y t-Bu, again suggesting the importance of mesomerism rather than induction in the change of dipole moment of the ring.
1600 cm."1 Band
The intensity of this band was generally weak with the exception of the m-disubstituted benzenes, which exhibited a peak of medium intensity, and the series of 1,3,5-trisubstituted benzenes, in which the band was extremely intense. The largest intensification of either band observed within the several series of compounds, which were examined, occurred for the 1600 cm."1 band in 1,3,5-trichlorobenzene.
This band was generally absent in the p-disubstituted benzenes. In some of the compounds in this series a weak band appeared in the region of 1590 cm."1, but occurred in those compounds for which resonance forms can be drawn, which indicated conjugation with the ring. A band appeared at 1610 cm."1 in p-methylbenzonitrile, but even the general structure of this compound should have hyperconjugation with the ring. In accordance with the electron-withdrawing properties of the C=N group, and subsequent tightening of the phenyl ring, an increase in the band maximum frequency is to be anticipated.
From Table I it can be observed that the 1600 cm."1 band occurs haphazardly and is frequently absent in the polysubstituted benzenes. This suggests that the band may not be as characteristic of the benzene ring as the 1500 cm."1 band, and depends strongly on the position as well as the type of substitution.
Another difficulty arises from use of the 1600 cm."1 band in that no obvious pattern of intensification emerges, suggesting conflicting mechanisms. The same types of functional groups that intensified the 1500 cm."1 band are responsible for intensification of the 1600 cm. band, but to a lesser degree and with much more random order. For
example, 0C H-^ OCH and the halogens are all moderate to strong resonators in the order F y I / Br y CI. Alkyl groups are generally x*eak resonators, as was observed upon examination of the 1500 cm."1 band, but again no particular pattern is obtained.
An attempt has been made to extend the theory of the vectorial additivity of moments to the infrared active vibrations of the phenyl ring, which occur at 1500 and 1600 cm."1, respectively. These vibrations are assumed to be insensitive to mass and coupling effects upon substitution.
The vibrations were studied by measuring the integrated intensities of the peaks of interest and examining the changes in intensity of the appropriate band in toluene, since the bands are absent in dilute solutions of benzene.
Ideally, it should be possible to predict the direction and extent of an integrated intensity shift from quantitative studies of the electrical effects, which are predominately mesomeric and inductive.
This investigation attempts to explain deviations from the estimated intensification factors on the basis of electrical effects primarily. In a few cases, steric hindrance, which leads to intensity changes due to the loss of coplanarity of the substituent with the ring, seems to be the cause of the deviations.
In general, mesomerism is the principal cause of distortions within the ring, since a shift to lower frequencies often accompanied an increase in intensity of a band. In addition, large positive deviations from anticipated intensities occurred in compounds, such as the
phenols, anisoles and anilines, in which resonance structures were possible.
Because the problem is so complex, much more investigation seems to be necessary in this area. For example, the effects of solvents on changes of intensification should be studied, particularly because many of the compounds of interest were insoluble in the solvent chosen for this phase of the investigation.
It might be possible, in the future, to correlate reactivities and physical properties to the electrical effects which may be responsible for changes in intensification.
This investigation has aided in the identification of substituted benzenes as to type and position of the substituent. It has been shown that the 1500 cm.'1 band is the more reliable of the skeletal vibrations occurring in the 1650-1450 cm,-1 region. It has been observed that the 1600 cm."1 band occurs haphazardly and is frequently absent in the polysubstituted benzenes. This indicates that this band is not as characteristic of the phenyl ring as the 1500 cm."1 band and is strongly dependent on the position as well as the type of substitution. It has been shown, also, by this investigation that no pattern of intensification, such as that which occurred for the 1500 cm."1 band, is immediately obvious. This suggests that conflicting mechanisms are responsible for the ring distortions which give rise to activity in the infrared region.
APPENDIX I TABLES
INTENSITIES OF THE 1500 AND 1600 cm. BANDS RELATIVE TO TOLUENE
Band ^ Band 3
Frequency I x 10"J Relative fe^enf x | Relative
Compound in cm.--1- 1/cm.''-mole in cm. l/cm.^-mole
0CH3(1)0H(4) 1508 9.20 12.96 - -
0CH3(1)0H(2) 1499 8.07 11.37 1597 2.68 12.01
01(1,2,3,4) 1427 6.97 9.82 - -
00^(1,4) 1507 6.81 9.59 -
00^(1)01(4) 3490 6.67 9.39 1582 I.38 6.17
0H(1)CH3(2,4,6) 1486 6.34 8.93 - -
01(1,2,4,5) 1441 6.21 8.75 - -
NH2(1)C1(2,4,5) 1475 5.93 8.35 - -
0H(1)C1(2,4) 1477 5.91 8.32 - -
0H(1)F(4) 1506 5.66 7.97 - -
0CH3(1,2) 1502 5.45 7.67 1593 1.71 7.62
0CH3(l)Br(4) 1486 4.95 6.98 1590 2.03 9.06
Br(l)CH3(2,4,6) 1464 4.89 6.89 - -
0H(1)C1(4) 1491 4.84 6.82 1591 1.61 7.20
C1(1)NH2(4) 1493 4.78 6.74 - -
0H(I)F(2) 1500 4.74 6.68 - -
0H(1)C1(2,6) 1460 4.68 6.59 1580 1.14 5.08
Frequency I x 10"-^ I .. Frequency I x 10"-* I ,.
_ ___, x i t / 2 t relative H i" / o -i relative
Compound in cm.--1- l/cm.^-mole in cm.--1- l/cm.^-mole
0CH3(1)1(4) 1484 4.62 6.51 1590 1.95 8.69
NH2(1)C1(3,4) 1476 4.59 6.47 1599 2.03 9.06
0CH3(1)F(4) 1505 4.55 6.40 - -
OCH3(l)OH(3) 1490 4.31 6.07 I613 3.25 1.45
Br(l)F(4) 1485 4.29 6.05 1585 0.39 1.74
F(l,4) 1502 4.22 5.95 -
0H(1)C1(2) 1479 4.09 5.75 1595 1.41 6.31
0H(1)0CH (2,6) 1505 4.04 5.69 1595 0.48 2.16
0H(l)Pr(2,6) 1497 4.04 5.69 1591 O.50 2.24
0CH3(1,3) 1489 3.99 5.63 1612 3.63 16.19
och3(i)i(3) 1473 3.83 5-39 1585 4.97 22.18
0H(1)CH3(2,6) 1474 3.78 5.33 1590 0.73 3.27
0H(l)i-Pr(2,6) 1458 3-74 5.27 1589 0.16 0.73
0H(1)C32H25(4) 1512 3-74 5.27 1594 I.38 6.17
0H(l)Br(4) 1486 3.66 5.16 1589 1.87 8.36
0H(l)Pr(4) 1511 3.58 5.04 1595 0.89 3.99
0H(1)CH3(4) 1511 3-58 5.04 1595 0.73 3.27
NH2(1)C1(2)CH3(6) 1473 3.50 4.93 1590 0.86 3.83
Cl(l,2,4) 1459 3.49 4.92 1570 I.38 6.17
Compound Band Frequency in cm.-l I x 10~3 l/cm.2-mole ^relative Band Frequency in cm.-l I x 10"3 l/cm.2-mole "^relative
l(l)NH2(4) 1484 3.41 4.80 - -
0H(l)i-Pr(2,4) 1490 3-37 4.75 - -
0H(1)C1(2,4)CH3(3,5) 1460 3-33 4.69 - -
0H(1)I(2) 1466 3.28 4.62 - -
0H(l)i-Pr(4) 1512 3.24 4.57 1596 0.97 4.34
NH2(1)CH(4) 1516 3.09 4.35 - -
NH2(1)(2") 1477 3.01 4.24 -
OCH (1) 1495 3.01 4.24 1600 1.88 8.39
Br(l)NH2(3) 1479 3.01 4.23 1592 3.25 14.51
Br(l)Cl(3) 1460 2.93 4.12 1570 3.41 15.24
0H(1)CH3(2,4) 1506 2.91 4.09 1595 O.32 1.44
0H(1)CH3(2,3,5,6) 1465 2.80 3-95 - -
NH2(1)I(3) 1477 2.76 3-89 1597 2.19 9.80
0^(1,2,3.4,5) 1472 2.71 3.81 - -
0H(1)CH3(2,3) 3468 2.70 3.80 1588 2.04 9.32
Br(l,2,4) 1444 2.68 3.77 1551 1.14 5.07
Br(l,4) 1469 2.64 3.72 - mm
Cl(l,4) 1475 2.62 3-69 -
C1(1)NH2(3) 3482 2.60 3.66 1597 2.99 13.35
Band ~ Band
Frequency I x 10"*-5 3^-1-+.:, Frequency I x 10~3 I ..
Compound in cm.-l l/cm.2-mole relative m cm.-l i/Cm.2-mole relative
F(l) 1490 2.57
0H(1)CH (3,4) 1501 2.52
HH2(1)CH3(3,4) 1507 2.48
C1(1)CH (4) 1490 2.40
0C2H5(1) 1495 2.39
01(1,2,3) 1434 2.36
Cl(l,3) 1460 2.28
Br(l)CH (4) 1485 2.25
Cl(l) 1476 2.07
0H(1)CH2CH=CH2(2) 1486 1.96
OCH3(l)CH3(3) 1490 1.96
CH3(1,2,3) 3473 1.89
Br(l,2) 3448 1.84
0H(1) 1496 1.84
Cl(l)Br(2) 1452 1.68
0H(1)C1(3) 1485 1.64
01(1,2) 1455 I.63
01^(1,2,3,5) 1483 1.48
C1(1)C2H5(2) 1473 1.47
3.62 1593 2.08 9.29
3.55 1604 I.83 8.17
3.50 1585 1.04 4.65
3.39 -' -
3.36 1601 2.00 8.94
3.32 1563 1.95 8.71
3.22 1576 3.30 14.74
3.16 - -
2.92 1582 0.77 3.45
2.76 1610 0.29 1.28
2.75 I6O3 3.34 14.91
2.66 1585 0.22 0.96
2.59 1551 0.29 1.28
2.59 1596 1.99 8.90
2.36 1568 0.37 1.64
2.31 I605 2.03 9.07
2.30 1571 0.25 1.09
2.08 1615 0.30 1.34
Band _~ Band _
Frequency I x 10~J it,0ta+4tr
Br(l,3) 1453 1.43 2.01 1564 3.91 13.35
Br(l) 1473 1.42 2.00 1578 0.89 3.99
OH(l)t-Bu(2,6) 1478 I.38 1.94 1582 0.15 0.66
OH(l)CH3(3) 148? 1.25 1.77 1612 1.33 5.94
0H(l)t-Bu(2) 1500 1.19 1.67 1608 0.45 2.01
OH(l)Br(3) 1481 1.14 1.61 1601 2.53 11.28
CH3(1)NH2(3) 1490 1.13 1.59 1590 I.62 7.22
(2^(1,2,4,5) 1507 1.12 1.58 - -
i-Pr(l,4) 1504 1.06 1.49 - -
CH (1,2,4) 1504 I.05 1.47 1620 O.34 1.50
0H(l)i-Pr(2) 1499 1.02 1.44 1610 0.53 2.37
C2H5(1,2) KDCH (3) 1487 0.97 1.37 1605 0.11 0.50
1476 0.94 1.32 1596 0.69 3.09
t-Bu(l) 1497 0.90 1.26 1602 0.26 1.17
Br(l)CH3(3) 1482 0.80 1.13 1601 0.54 2.42
CH3(1)C2H5(4) 1512 0.79 1.12 - -
OH(l)CH3(2,5) 1509 O.78 1.10 1585 1.26 5.63
CH3(1,3) 1492 0.77 1.08 1617 0.67 3.00
KD 1470 0.73 1.03 1572 1.05 4.72
Band I x 10~3 l/cm.2-mole Band
Compound Frequency in cm.-l ''"relative Frequency in cm.-l I x 10-Ji l/cm.2_mole "'"relative
i-Pr(l) 1490 0.73 1.03 1606 0.20 0.92
C]L2H25(1) CH3(1) 1491 0.71 1.00 1600 O.34 1.50
1495 0.71 1.00 1608 0.22 1.00
CH3(1,4) CH3(1)C=N(3) 1515 0.70 0.98 - -
0484 0.64 0.90 1602 O.38 1.68
Pr(l) 1490 O.63 0.89 1606 0.17 0.75
CH3(1,2) 1493 O.63 0.88 1601 0.13 0.57
c2H5(i,3) C2H5(1) CH3(1)C=N(4) CH3(1)C=N(2) 1485 0.62 0.87 1608 0.58 2.58
1495 0.60 0.84 1608 0.34 1.50
1508 0.56 0.79 1610 0.82 3.66
1483 O.54 O.76 1601 0.22 1.00
C=N(1) 1490 0.46 0.65 1598 0.06 0.25
CH2CH2C1(1) 1490 0.42 0.59 1600 0.07 O.32
CF3(1,4) CHBrCH3(l) C2H50H(1) 1521 0.39 0.55 - -
1495 0.39 0.55 I603 0.12 0.56 .
1490 O.38 0.53 1599 0.14 0.64
CHC12(1) 1495 0.22 0.3I - -
CF2CF2CF3(1) 1454 0.11 0.16 I607 0.13 0.57
Cl(l,3,5) - 1566 9.33 41.67
Frequency I x 10"J Irelative Frequency I x 10"J Compound in cm."-1- l/cm.^-mole in cm."x 1/cm-mole
NH2(1)C1(3,5) - 1596 6.89 30.75
C2H (1,3,5) I603 5.98 26.70
Br(l,3,5) 1554 5.62 25.11
OH(l)CH3(3)C2H5(5) - 1595 4.49 20.03
OH(l)CH3(3,5) ... 1595 2.83 12.62
i-Pr(l,3,5) - 1600 1.46 6.53
CHL(1,3,5) - 1609 1.31 5-84
INTENSITIES OF THE 1500 cm. BANDS OF 1,4-DISUBSTITUTED COMPOUNDS RELATIVE TO 1,4-DIMETHYLBENZENE
Band 0 Band _
Frequency I x 10*v Irelative Frecluen?y x J
Compound in cm.1 l/cm. -mole in cm."-1- l/cm.z-mole
0CH3(1)0H(4) 1508 9.20 13.23 -
OCH (1,4) 1507 6.81 9.79 -
00^(1)01(4) 1490 6.67 9.59 1592 I.38
0H(l)F(4) 1506 5.66 8.13 -
0CH3(l)Br(4) I486 4.95 7.12 1590 2.03
0H(1)C1(4) 1491 4.84 6.96 1591 1.61
NH2(1)C1(4) 1493 4.78 6.87 -
00^(1)1(4) 1484 4.62 6.65 1590 1.95
0CH3(1)F(4) 1505 4.55 6.53 -
Br(l)F(4) 1485 4.30 6.18 1585 O.39
F(l,4) 1502 4.22 6.07 -
0H(1)C12H25(4) 1512 3.74 5.38 1594 I.38
0H(l)Br(4) 1486 3-66 5.27 1589 I.87
0H(l)Pr(4) 1511 3.58 5.14 1595 0.89
0H(1)CH3(4) 1511 3-58 5.14 1595 0.73
NH2(1)I(4) 1484 3.41 4.90 -
0H(l)i-Pr(4) 1512 3.24 4.66 1596 0.97
Band Band 0
Compound Frequency in cm,"-'- I x 10~J l/cm.2-mole "^relative Frequency I x 10"v in cm." l/cm.2_mole
NH2(1)CH3(4) Br(l,4) 1516 3.09 4.44
1469 2.64 3.79 -
Cl(l,4) 1475 2.62 3-77 -
01(1)0^(4) 1490 2.40 3.46 _
Br(l)CH3(4) 1485 2.25 3.23 -
i-Pr(l,4) 1504 1.06 1.53
CH3(1)C2H5(4) CH (1,4) 1512 0.79 1.14 -
1515 0.70 1.00 -
CH3(1)CSN(4) 1508 O.56 0.80 1610 0.82
CF (1,4) 1521 0.39 0.56 -
INTENSITIES OF THE 1500 AND 1600 cm. BANDS OF 1,2,4-TRISUBSTITUTED BENZENES RELATIVE TO 1,2,4-TRIMETHYLBENZENE
Compound Band Frequency in cm.--1- I x 10"3 l/cm.2-mole "^relative Band Frequency in cm."-1- I x 10"3 l/cm.2-raole ''"relative
0H(1)C1(2,4) 1477 5.91 5.64 - -
Cl(l,2,4) 1459 3.50 3.34 1570 1.38 4.11
.OH(1)i-Pr(2,4) 1490 3.37 3.22 - -
0H( 1)0^(2,4) 1506 2.91 2.78 1595 0.32 0.96
Br(l,2,4) 1444 2.68 2.56 1551 1.14 3.38
0^(1,2,4) 1504 1.05 1.00 1620 O.34 1.00
INTENSITIES OF THE 1500 cm."1 BANDS OF 1,2,4,5-TETRASUBSTITUTED BENZENES RELATIVE TO 1,2,4,5-TETRAMETHYLBENZENE
Compound Band Frequency in cm.-l I x 10"3 l/cm.2-mole '''relative Band Frequency in cm.-l I x 10"3 I n .. relative l/cm.2-mole
Cl(l,2,4,5) 1441 6.21 5.54 -
NH2(1)C1(2,4,5) 1475 5.93 5.29 -
CH^l.2,4,5) 1507 1.12 1.00 -
INTENSITIES OF THE 1500 cm."1 BANDS OF 1,2,3,4,5-PENTASUBSTITUTED BENZENES RELATIVE TO 1,2,3,4,5-PENTAMETHYLBEMZENE
Compound Band Frequency in cm.-l I x 10"3 l/cm.2_mole '''relative Band Frequency in cm.-l I x 10"3 I .. .. / 0 relative 1/cm-mole
0H(1)C1(2,4)CH3(3,5) 1460 3.33 1.23 -
CH3(l,2,4,5)OH(3) 1465 2.80 I.03 -
CH3(1,2,3.4,5) 1472 2.71 1.00
APPENDIX II FIGURES
Fig. 1. The Direction of the Spectroscopic Moments in Poly-substituted Benzenes,
Fig. 2. Illustration of the Vectorial Addition of Spectroscopic Moments for Disubstituted Benzenes.
-* *+ -> - A. ------ -^-i
-* 2, ---->
fig. 3. Absorption Spectrum of O.2634 M Benzene in Tetrachloroethylene in the 0.214 mm. Cell.
1667 1612 1562 1515. 1470 1428
Frequency in cm.
Fig. 4. Absorption Spectrum of 0.2501 M Methylbenzene in Tetrachloroethylene in the 0.214 mm. Cell.
Fig. 6. Absorption Spectrum of 0.2542 M 1,2-Dimethylbenzene in Tetrachloroethylene in the 0.214 mm. Cell.
Fig. 7 Absorption Spectrum of 0.2500 M 1,3-Dimethylbenzene in Tetrachloroethylene in the 0.214 mm. Cell.
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Patricia Ann Fish was born August 17, 1931, at Miami, Florida. After attending public schools in that city, she was graduated from Miami Edison Senior High School in June, 1949. In February, 1958, she received the degree of Bachelor of Science in Chemistry from the University of Florida. She was employed by the Department of Biochemistry of the University of Florida Teaching Hospital for two and one-half years before becoming a Teaching Assistant in the Department of Chemistry in September, I960.
She received the degree of Master of Science from the University of Florida in January, 1961. Since then she has been a Teaching Assist, and and Interim Instructor in the Department of Chemistry.
The author is married to Dr. Lloyd A. Fish, an intern in the Department of Pediatrics of the University of Florida Teaching Hospital and has four children.
This dissertation was prepared under the direction of the chairman of the candidate's supervisory committee and has been approved by all members of that committee. It was submitted to the Dean of the College of Arts and Sciences and to the Graduate Council, and was approved as partial fulfillment of the requirements for the degree of Doctor of Philosophy.
April 20, 1963
Dean, College of Arts and Sciences
Dean, Graduate School
In reference to the following dissertation: AUTHOR: Fish, Patricia
TITLE: Studies of the absorption spectra of some polysubstituted benzenes.
(record number: 421859) PUBLICATION DATE: 1963
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