Steven 1. Murov and Miles Pickering1 SUNY a t Stony Brook Stony Brook, New York 11790
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The Odor of O~RCPII s o ~ ~ ~ s An experiment in organic chemistry
It is a truly remarkable result of recent work by Russell and Hills (I),and by Friedman a n d Miller (2) t h a t optical isomers can have different odors. This has been suspected for many years. Until recently it was not possible to eliminate t h e possibility t h a t some intensely odiferous trace component was responsible for t h e difference in odor. Such a species might be similar enough t o follow the isomers through the separation procedures, b u t be present in undetectably small amounts. Friedman and Miller have shown t h a t impurities are not responsible for t h e difference in odor by a clever synthetic interconversion of t h e two isomers. A previous article in THIS JOURNAL by Runquist (3) discussed the result of student work on separating and characterizing t h e components of esseutiai oils. work with natural products allows the students to master t h e basic techniques of organic chemistry such a s distillation and vapor phase chromatography. It also maintains student and faculty interest better than a d hoc technique experiments, since it seems more like "real" chemistry. Also the student must learn a t some point t h a t chemically pure substances occur only in chemistry stockroams, not in nature. Frequently purification is the prelude t o any sort of physical measurement. T h e experiment t o be described involves the separation and characterization of 1-carvone from spearmint oil and d-cawone Crom caraway seed oil. Spearmint oil is primarily composed of 1-carvone (about 60%).
from it with comparative ease. However, t h e substances I, 11, and III can b e separated from each other only with extreme difficulty, b u t under proper conditions give separate peaks in vapor phase chromatography. Caraway seed oil contains chiefly d-cawone and limonene (III) (5). Experimental Each student separates the carvone from one type of oi1,2 and compares his results with someone who worked on the other oil. The experiment requires minimally only three hours for the average student, hut depends on the number of different physical measurements to he made. The vapor phase chromatographs of the crude oils are shown in Figure 1, before and after distillation. Although a programmed temperature instrument was used, any relatively nonpolar, high temperature column should work. Temperature programming is necessary only if it is desired to indicate the number of low bailing components in spearmint oil. Since the boiling points of carvone (230°C) ( 6 ) and limonene (177'C) (7) are high, a vacuum distillation was used. With a pressure of less than 1 mm produced by a mechanical pump, the car"one was found in a fraction boiling over 60°C and the limonene in the fraction at 45-55°C. Using an aspirator vacuum (20 mm or so), the boiling point is 105-ll0'C for the earvone fraction and about 75°C for the low boiling fraction. No decomposition or discoloration was observed at this temperature. About thirty mls of oil will yield at least five mls of pure carvone, when the fractions have been cut. It is desirable to take 3 fractions (far 20 mm press&, 70-80, 80-105, 105-113). Cows, if available, are very useful for this and provide an excellent demonstration of fraction cutting under vacuum. On some occasions severe foaming was a problem. The problem was far more severe for caraway than far spearmint oil. In both eases the use of a trap, such as is used in a Kjeldahl distillation (Ace glass catalog number 6644, 5661, or 9169) is recommended. This trap also provides one more theoretical plate and increases the fractionation. At atmospheric pressure, the foaming problem was so severe that distillation was aimast impossible.
Less abundant components of spearmint oil are the terpenes a- and 8-phellandrene (structures I and II), and limonene ( I n ) (4), b u t it is the carvones which are responsible for the characteristic odors of the oil.
All of these latter compounds (I, 11, and III) have lower boiling points t h a n the carvone, and can h e separated 'Author to whom correspondenceshould be addressed. *Oils may be obtained quite inexpensively from George Luedem and Ca., 427 Washington Street, New York 10013 with affices in San Francisco and Chicago, and from Fritzche D and 0, 76 Ninth Avenue, New York, N. Y., 10011. In some cases minimum order requirements mean ordering a two-year supply at one time. The oils sell for less than $10/lb (about 500 ml). 74 1 Journalof Chemical Education
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Figure 1. Vapor phase chromatograms before and after distillation: (a) i-carvane from spearmint: (b) spearmint ail before distillation: (c) dcarvone from caraway; (d) caraway seed oil before distiilation. (In all cases the column used was 6-ft. X 'A in. 15% QF-1 on Chromasorb W 80/100 support. Programmed temperature 100-220°C at 2oo/rnin.1
WAVENUMBER (cm-')
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9 1 0 1 1 1 2 1 3 1 4 WAVELENGTH (microns) Figure 3. N M R spectra obtained on a Varian A 4 0 using student samples of: (a) I-carvone (from spearmint); (b) d-carvone (from caraway). 3
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Figure 2. IR spectra obtained an a Perkin Elmer 137 using student samples of carvone: (a) I-carvone (from spearmint): (b) d-carvone (framcaraway). Pure liquids between $all plates. The separation between components is not perfect, and redistillation is sometimes necessary for high purity. The purity should be checked by VPC, and retention times should be accurately compared. This test will give the student some idea of the efficiency of his distillation, and also demonstrates the vast superiority of vapor phase chromatography over distillation as a purification method for small amounts of material. The physical means used to characterize the compounds can be quite varied. The student should certainly verify the odor difference which is quite marked. Ahaut &lo% of the population has s specific anosmia, that is, they cannot detect the difference in odor, hut for most people, the differenceis easily discernable (2). IR spectra obtained from student products are shown in Figure 2. Same students also took refractive index measurements, and the nmr spectra shown in Figure 3. Optical rotations were obtained by the instructor on two samples prepared by students. The experiment is essentially open ended, and any pair of physical measurementson the two earvones should yield the same result. The experiment could be presented to students in several ways. With preparative vapor phase chromatography, one injection of crude oil should yield enough purified product to make a "smell test" and a reinjection to verify purity. This would eliminate the possibility that the differencesin odor are due to some trace component that is not abundant enough to appear in the spectra. Strictly speaking, the experiment as outlined presently does not eliminate this possihility. Another pedagogical variant would be to assign the problem of separating the carvane from spearmint and caraway seed oil, and allow the students to be surprised by the fact that the products have different odors. The toxicity of the carvone is similar to that of aspirin (a), and the authors found the taste to he so strong that accidental ineestion is almost imoossihle. The odor of the vapors " in the laboratory is not unpleasant. Discussion
It is clear from Figure 2 that the only differences between the two spectra are due to sample thickness effects changing the intensity of lines. Refractive indexes obtained by the students were 1.4970 for d-carvone, and 1.4989 for 1-carvone. Agreement is within experimental error. Figure 3 shows nmr spectra taken by the students. Again the identity of the two spectra is apparent. It is a useful exercise for t h e student to assign lines in the nmr spectrum and in this way verify the carvone structure.
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3An inexpensive polarimeter is described by Shaw (9)
Optical rotations measured by the instructor on two representative samples were +56.75" for d-carvone .and -54.71" for 1-carvone. These numbers were obtained on neat samples of purified compounds which are colorless liquids and therefore ideal for polarimetry. The corresponding specific rotations are [a]%= +58.8 for d-carvone (caraway) and [a]zDo = -56.7 for 1-carvone (spearmint). These agree within experimental error in absolute value, but are opposite in sign as expected. Our polarimeter is not rugged enough for mass student use, but the snecific rotations are so laree that students could easilv measure approximate values (9). We are planning to make the necessarv instrument modifications to nermit t h i s 3 In generai, then, it is true that all physical properties except optical rotations are identical for the two carvones. Yet there is a readily distinguishable difference in odor. The student finds that his senses are capable of a distinction that even the spectrometers cannot make. Presumably the effect arises because the odor receptors of the nose are chiral. This is not reallv surnrisine when one considen the specificity of most d i o ~ o & a ~&stems. For example. onlv the d form of LSD has hallucinogenic properti& There- are many other examples in theworld of enzyme chemistry. Summary
This experiment is a useful exercise for the beginning organic student, and familiarizes him with the phenomena of optical isomerism in a n unusual and dramatic way. The student also learns how to do a vacuum distillation and use a vapor phase chromatograph. Since the experiment is open ended, it can be used to begin the teaching of other instrumental methods. Acknowledgment
The assistance of Rudy Schlott, our glassblower, is gratefully acknowledged. The students, who first stumbled through this experiment, have also made several helpful suggestions. E. Roger Jones was kind enough to do the drafting for the illustrations. Literature Cited
p. 676. I51 Ref. 141, Vul. IV, p. 573. 161 -141, Vd.U, 0,414. (71 R c l ( 4 , Vo1.U. 25. (8) Jennes, P. M., Hagen. E.C.,T.ylor. J. M.,h k . E. L.,and Fitzhugh, 0. G.. Food Coarnet. Toxicol.. 213i.327 (19bl). 191 Shaw, W . H . R.. J. CHEM.EDUC.,32,1011955).
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Volume 50, Number 1, January 1973 / 75
