An Engaging Illustration of the Physical Differences among Menthol

In the Laboratory

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An Engaging Illustration of the Physical Differences among Menthol Stereoisomers Edward M. Treadwell* and T. Howard Black Department of Chemistry, Eastern Illinois University, Charleston, IL 61920-3099; *[email protected]

One of the important concepts introduced early in organic chemistry courses is stereochemistry, which is often difficult for students to grasp. Students struggle with the mechanics of assigning absolute stereochemistry and determining the relationship between two stereoisomers, as well as the broader implications of stereoisomerism in terms of chemical and physical behavior. Demonstrations and laboratory exercises can be of great assistance in reinforcing these ideas. Classic classroom demonstrations of the difference in behavior of enantiomers include overhead polarimetric demonstrations using either carvone or limonene, and the “sniffing” of carvone enantiomers (1). Most laboratory experiments incorporating stereochemical concepts involve completion of a reaction and comparison of the differing physical properties of either the enantiomeric or the diastereomeric products. Some of these reaction-based experiments involve optical resolutions of a racemic starting material where the separated enantiomers are compared to each other (2). Alternatively, stereospecific reactions to produce a single diastereomeric product, or stereoselective reactions to give a mixture of major and minor diastereomeric products are employed (3). In these stereospecific and stereoselective experiments, the difference in physical properties of diastereomers is clearly evident given the melting point and spectral data. We wished to devise an experiment illustrating stereochemical principles that did not require a reaction, principally to allow implementation concurrent with discussion of stereoisomers in the lecture class without having to introduce reactions that had yet to be covered. Previously, we had been using an experiment employing cis- and trans-1,2-cyclohexanediol, where the diastereomers are differentiated by TLC analysis (4); unfortunately, the difference in Rf values is very small, and the chemicals are quite expensive.1 The expense would be exacerbated significantly if the experiment was expanded to include the use of enantiomers, as the nonracemic trans-1,2-cyclohexanediol stereoisomers cost over $150
g in the most recent Aldrich catalog. We sought a compound where several stereoisomers were commercially available, relatively inexpensive, preferably nontoxic, and (for the diastereomers) possessing significantly different physical properties. An attractive candidate was found in a series of menthol stereoisomers (5). (+)-Menthol, (−)menthol, (+)-neomenthol, and (+)-isomenthol are all commercially available, with the most expensive of the four isomers being (+)-neomenthol at five grams for $55, or $11 per gram. This set of four compounds (Figure 1) is composed of a pair of menthol enantiomers, along with two different diastereomers [(+)-neomenthol and (+)-isomenthol]. Menthol is also attractive from a pedagogical standpoint since it is a compound with numerous everyday uses, including as an ingredient in cough drops, lip balms, cigarettes, perfumes, and liqueurs (6). We then devised an experiment that was intended to have the students learn: (i) that diastereomers have different physi1046

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CH3

CH3

OH CH3

CH3

OH CH3

CH3

(ⴚ)-menthol mp = 43 – 45 °C

(ⴙ)-menthol mp = 43– 45 °C

CH3

CH3

OH CH3

CH3

(ⴙ)-isomenthol mp = 77–83 °C

OH CH3

CH3

(ⴙ)-neomenthol mp = ⴚ22 °C

Figure 1. Structure of commercially-available menthol (2-isopropyl5-methylcyclohexanol) stereoisomers.

cal properties using TLC and melting point analysis; (ii) that enantiomers have the same physical properties in achiral environments (TLC analysis) but different properties in chiral environments (polarimetry and mixed melting points); (iii) how to assign absolute stereochemistry; and (iv) how to relate the stereoisomeric relationships to differences in absolute stereochemistry. Experimental Overview An easily observed difference between the diastereomers is their melting points. In the laboratory, the melting points are sufficiently distinct to allow clear identification, as each menthol enantiomer melts at 43–45 ⬚C while (+)-isomenthol melts at 77–83 ⬚C and (+)-neomenthol is a liquid at room temperature.2 Thin-layer chromatographic analysis was also performed on all the menthol isomers, using a vanillin solution for visualization. First, the students preformed a TLC analysis on the samples of (+)-menthol and (−)-menthol to find that the enantiomers traveled an identical distance, exhibiting an identical Rf value of 0.46.3 Then, the students preformed a TLC analysis of (+)-menthol, (+)-neomenthol, and (+)-isomenthol to determine that the diastereomers displayed different Rf values (0.59 for neomenthol and 0.51 for isomenthol) not only from the (+)-menthol, but also from each other. To demonstrate the differences between the enantiomers, the optical rotations, [α]25D, of (+)- and (−)-menthol in 95% ethanol were obtained at lower concentrations than previ-

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In the Laboratory

ously reported (c 10, EtOH).4 A sample of (+)-menthol at a concentration of 0.0027 g
mL gave a reproducible specific rotation of +50⬚, while a sample of (−)-menthol at a concentration of 0.0030 g
mL gave a reproducible specific rotation of ᎑47⬚. An intriguing demonstration of how enantiomers have the same physical properties until placed in chiral environments (thus forming diastereomeric associations) involved preparing a 1:1 by weight mixed sample of (+)-menthol and (−)-menthol and observing the mixed melting point. The melting point was indeed depressed in the mixed sample (mp = 28.3 − 35.0 ⬚C), and this can be remarkably demonstrated without the use of a melting point apparatus. If small, wellground samples of (+)-menthol and (−)-menthol are placed apart on a student’s gloved hand, the enantiomers remain a solid. Upon mixing, however, the samples quickly begin to melt and soon no solid material is left. Alternatively, this can easily be demonstrated to a large group.5 A glass crystallizing dish is preheated by holding it an inch from the exhaust port of a typical overhead projector for ca. 30 seconds. The warm dish is then placed on the writing surface of the projector, and two separate piles of finely ground (−)-menthol and (+)-menthol are added. After a minute, the piles are mixed using a spatula and the material begins to clump and then melt literally in front of the audience’s eyes. Lastly, we had the students identify the chiral centers and assign the absolute stereochemistry for each compound. This exercise is particularly rewarding in that the assigning the priorities at the C-5 carbon it is necessary to proceed through several bonds to find the first point of difference, as well as the structures including a methine that is not a stereocenter (the central carbon of the isopropyl group). The concept that enantiomers have all the stereocenters inverted while diastereomers have some but not all of the stereocenters inverted is also reinforced in this exercise as the students attempt to assign the stereochemistry of all four compounds.

of the stereoisomers being far enough apart to give unequivocal results. In the reports, a large majority of the students clearly stated that diastereomers have different physical properties, while enantiomers only had different physical properties in chiral environments. Having the students assign the stereochemistry gave them practice at assigning priorities to groups where the first point of difference is not directly at the chiral center. All the students were able to assign the C-1 stereocenter, and about 80% correctly assigned the C-2 and C-5 stereocenters. Most students also quickly realized that after assigning the stereochemistry of the first compound, they only need to focus on the chiral centers that have changed in the remaining structures. They then realized that if the stereochemistry has switched, the assignment is simply reversed [(R) to (S ) or (S ) to (R)]. Another nice facet of these compounds is that there are two different diastereomers used, dispelling the mistaken conceptions that (i) a compound can only have one enantiomer and one pair of diastereomers and (ii) that all diastereomers of a compound behave identically – that since both (+)-neomenthol and (+)-isomenthol are diastereomers of (+)-menthol they should be no difference between the behavior of isomenthol and neomenthol. The four commercially-available menthol stereoisomers are ideally suited for the illustration of the differences in physical behavior between diastereomers and enantiomers in either a first-semester organic laboratory experiment, or as demonstrations in an organic lecture course. Additionally, the mixed melting behavior of the (+)- and (−)-menthol makes for a fun and interesting demonstration of a principle that is often neglected in the discussion of the properties of enantiomers. W

Supplemental Material

Instructions for the students and notes for the instructor are available in this issue of JCE Online. Notes

Hazards The visualization solution contains dilute sulfuric acid and can cause burns. The components of the developing solvent are flammable and irritants, and prolonged exposure to hexanes can affect the central nervous system. Results and Conclusions We have been using this experiment for the past four semesters and have found that the students not only enjoy it but also easily learn the conceptual information we wished to impart. The effectiveness of the experiment was assessed by the degree of understanding shown in the student’s written lab reports as well as comments made by the students in the laboratory. From the TLC analysis, they successfully observe that diastereomers have different physical properties, as isomenthol, neomenthol, and menthol are clearly resolved on the plate. In some cases, the menthol and isomenthol spots were not clearly resolved owing to students applying too much sample to the plate; usually after a second attempt, a clear TLC plate can be obtained. The melting point analyses were unequivocal for most of the students, with the melting points www.JCE.DivCHED.org

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1. The cis isomer is $19.10
g in the 2004–2005 Aldrich Chemical Company, Inc. catalog. 2. An additional exercise would be to have the students obtain the melting point of the individual enantiomers and observe their similarity. 3. Alternatively, as suggested by one of the reviewers, (+)-menthol, (−)-menthol, and a co-spot of the two enantiomers could be chromatographed on this plate to more emphatically demonstrate that the enantiomers behave identically on silica gel. 4. c given in units of grams per mL. The literature rotation of (+)-menthol was obtained from the Aldrich catalog. 5. Such a demonstration was used for the organic chemistry I lecture, which typically numbers around 40–50 students.

Literature Cited 1. (a) Knauer, B. J. Chem. Educ. 1989, 66, 1033–1034. (b) Solomon, S. J. Chem. Educ. 1989, 66, 436–437. (c) Murov, S. L.; Pickering, M. J. Chem. Educ. 1973, 50, 74–75. 2. (a) α-phenylethylamine: Ault, A. J. Chem. Educ. 1965, 42, 269. Durieu, V.; Martiat, G.; Vandergeten, M. Ch.; Pirsoul, F.; Toubeau, F.; Van Camp, A. J. Chem. Educ. 2000, 77, 752–

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In the Laboratory 753. (b) ibuprofen: Sen, S. E.; Anliker, K. S. J. Chem. Educ. 1996, 73, 569–572. (c) phenylsuccinic acid: Stephani, R.; Cesare, V. J. Chem. Educ. 1997, 74, 1226. (d) α-methylbenzyl acetate (enzymatic): Steca, D.; Arends, I. W. C. E.; Hanefeld, U. J. Chem. Educ. 2002, 79, 1351–1352. (e) γ-methyl-γbutyrolactone synthesis (enzymatic): Lee, M. J. Chem. Educ. 1998, 75, 217–219. (f ) β-hydroxyester synthesis (enzymatic): North, M. J. Chem. Educ. 1998, 75, 630–631. 3. (a) bromination of fumaric and maleic acids: Tomsho, J.; McKee, J.; Zanger, M. J. Chem. Educ. 1999, 76, 73–74. (b) reduction of 1,3-diphenyl-1,3-propanedione: Deprés, J.-P.; Morat, C. J. Chem. Educ. 1992, 69, A232–A239. (c) reduc-

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tion of benzoin: Rowland, A. T. J. Chem. Educ. 1983, 60, 1084–1085. 4. Gilbert, J. C.; Martin, S. F. Experimental Organic Chemistry: A Miniscale and Microscale Approach, 2nd ed.; Saunders College Publishing: Fort Worth, TX, 1998; pp 178–181. 5. To the best of our knowledge, there is only one other publication that employs menthol in an introductory stereochemistry experiment; in this instance, (−)-menthone is reduced to give (−)-menthol and (+)-neomenthol (Barry, J. J. Chem. Educ. 1973, 50, 292). 6. The Merck Index, 13th ed.; Budavari, Susan, Ed.; Merck Research Laboratories: Whitehouse Station, NJ, 2001; p 5862.

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