Nature’s Treatment for Irritable Bowel Syndrome: Studies on the Isolation of (−)-Menthol from Peppermint Oil and Its Conversion to (−)-Menthyl Acetate

A simple, robust, and reproducible method was developed for the isolation of (−)-menthol from peppermint oil and to study the effect of different types of leaving groups, catalysts, solvents, and tertiary base on the extent of esterification of (−)-menthol to (−)-menthyl acetate. In this experiment, students compare leaving group properties of acetate and chloride ions from the acylating reagents acetic anhydride and acetyl chloride, respectively. The extent of conversion is compared when pyridine and 4-(dimethylamino)pyridine are used as catalysts, when N,N-diisopropylethylamine is used as tertiary base, and when the solvent is changed from dichloromethane to diethyl ether to N,N-dimethylformamide. Students are assessed on the chromatographic/spectroscopic purity and yield of (−)-menthol isolated and on their understanding of the factors that affect its extent of conversion to (−)-menthyl acetate. Full spectral characterization of both compounds is also conducted. They also complete a series of answers t...