Citrus Experiment Station CES 68-8B
and Florida Citrus Commission,
Lake Alfred, Florida 450-10/12/67-WFN
CHEMURGY OF CITRUS STRIPPER OIL
William F. Newhall
University of Florida Citrus Experiment Station
Lake Alfred, Florida
The "chemurgy" or chemical utilization of citrus stripper oil (d-limonene)
has considerable appeal to the synthetic organic chemist. This is due pri-
marily to the high purity and low cost of this by-product which make it ideally
suited as a starting material for the synthesis of fine grade organic chemicals.
For this reason, State Project 817 was initiated on October 17, 1956 with
the object: "To synthesize (by the methods of organic chemistry) from citrus
limonene, chemical compounds which will extend the uses of this material and
bring a better monetary return to the citrus industry." Although this project
has been revised twice since its inception, the major objectives have not
changed. These are: 1) The preparation of new nitrogen containing derivatives
of limonene having useful biological activity. This category includes pharma-
ceutical compounds, fungicides, and agricultural chemicals including plant growth
retardants and nematocides. 2) The synthesis of new oxygenated derivatives of
limonene useful as odor and flavor compounds. Uses for limonene which relegate
it to the category of a cheap solvent are not considered practical because
cheaper chemicals that will perform the same function are available from the
Most of the investigations done to date at the Citrus Experiment Station
have been concentrated on the first objective for several reasons. Extensive
work has been done and reported in the chemical literature on the preparation
of odor and flavor compounds from limonene. In fact, the conversion of limonene
to carvone, the essence of natural oil of spearmint, is an economic process which
has been employed. Carvomenthone (tetrahydrocarvone) can also be prepared from
limonene and could be useful in food flavorings. However, the demand for these
materials is so variable that they are not the answer to the limonene utilization
At the 1961 Processors' Meeting, the preparation and testing of two new
fungicides was reported (1) (4). This work was done in cooperation with Dr.
Roger Patrick. Since that time, two additional chemically related fungicides
have been prepared and tested. These also have shown considerable promise as
antimildew agents for the control of mildew on cotton fabrics. They also have
proved to be very effective in the control of some fungus infections of the
skin, particularly ringworm.
At the Annual Citrus Processors' Meeting in 1963, the preparation of the
first pharmaceutical compounds from limonene was disclosed. The most active
derivative prepared was one fourth as active as neostigmine bromide which is
currently used in the treatment of atony of the intestinal tract and bladder as
well as to relieve muscle spasms in the treatment of myasthenia gravis and other
neuromuscular disorders. This work was done in cooperation with the Upjohn
Company which felt that this compound could be developed into a commercial drug.
However, after two years of animal testing, approximately one quarter of a
million dollars would have been required for clinical trials. This cost and the
sudden tightening of restrictions on new drugs by the Food and Drug Adminis-
tration, stopped further work on this compound.
The fungicides prepared from limonene and mentioned previously belong to a
class of chemicals known as quaternary ammonium compounds (quats). Certain
structural features of these compounds were considered sufficiently similar to
those of known plant growth retardants to warrant testing them for plant growth-
retarding activity. This work was done and is continuing in close cooperation
with Dr. A. P. Pieringer of the Citrus Experiment Station (2).
Plant growth retardants are chemicals which cause no formative changes in
plants and generally shorten the distance between plant nodes (leaves). This
abbreviates overall plant size and tends to produce more vigorous, prolific
fruiting. Growth regulators are relatively new to agriculture and Uniroyal
Chemicals "Alar" may be the first such compound available to farmers next
year. There is not sufficient time to review the possible benefits of plant
growth regulators, but the following quote from the August 7, 1967 edition of
Chemical and Engineering News, pertinent to "Alar", indicates the potential of
these chemicals: "Uniroyal found that "Alar" produces an essential height
reduction in apple trees but, of more importance to the U. S. fruit industry,
lowers bearing age (from seven or eight to four or five years), extends harvest
time, improves fruit firmness and color, cuts preharvest apple drop, and pro-
motes annual blooms in normally biennial trees. "Alar" also boosts apple's
resistance to drought, heat, frost and smog."
To date, twenty-three new quaternary ammonium derivatives of limonene have
been synthesized and Dr. Pieringer has tested them for growth retardant activity.
Much of this work has been published (2) (3). Primary tests were made on Black
Valentine bean plants which are the conventional test plant because of their
general response to plant growth regulators. Active growth retardants were then
tested on young 'Duncan' grapefruit seedling grown in 46-ounce containers under
greenhouse conditions. Typical test data obtained using Black Valentine bean
plants are included in Table 1. Test compounds are listed by code designation
to avoid lengthy chemical names. Results are expressed as percent activity --
(i.e.), average percent reduction in growth as compared to the untreated con-
trols. "Alar" is included as a reference.
Table 1. Growth retardant activity
of limonene quats. on Black
Extensive testing of these and related new quats. has shown that these
compounds are readily taken into grapefruit'seedlings when applied either as a
foliar spray or a soil drench (3). Compounds applied as a soil drench were
easily detected, by special chemical means, in the leaves after one hour.
The relationship between chemical structure and plant growth inhibition is
now under intensive study at the Citrus Experiment Station. It has been possi-
ble to more than treble the activity of a given limonene quat. by making slight
chemical changes in the growth retardant molecule. This work is being actively
supported by Amchem Products, Inc., a leader in the field of plant growth chemi-
cals, that is probably best known for its pioneering research on the herbicides
2,4-D and 2,4,5-T. Amchem is screening all of these limonene quats. on vege-
table and field crops as well as on woody plants and trees.
The preparation of the first fungicides and physiologically active drugs
from citrus limonene has been briefly reviewed. Current work on the prep-
aration of plant growth regulants from limonene has been discussed. These
regulants are considered to be the best potential outlet for limonene because
of the large volumes that could be absorbed for agricultural uses, perhaps
even on citrus.
(1) Newhall, W. F. J. Org. Chem. 24, 1673 (1959).
(2) Newhall, W. F. and
(3) Newhall, W. F. and
(4) Patrick, R. and W.
A. P. Pieringer, J. Agr. and Food Chem., 14, No. 1,
A. P. Pieringer, J. Agr. and Food Chem., 15, No. 3,
F. Newhall, J. Agr. and Food Chem., 8, No. 5, 397,
Florida Citrus Experiment Station
and Florida Citrus Commission,
Lake Alfred, Florida. 450-10/12/67-WFN