An Engaging Illustration of the Physical Differences among Menthol Stereoisomers

The differences and similarities in the physical behavior of enantiomers and diastereomers can easily be demonstrated using the commercial stereoisomers (?)-menthol, (+)-menthol, (+)-isomenthol, and (+)-neomenthol. Thin-layer chromatography and melting point determinations clearly show that diastereomers have different physical properties from enantiomers and each other, but that enantiomers have identical physical properties in achiral environments. By obtaining a mixed melting point and optical rotations the difference in enantiomers can be observed. The exercise serves to reinforce the consequences of stereochemistry in terms of physical properties, as well as relating the differences in structure that lead to different types of stereoisomers. An attention-getting demonstration, suitable for the classroom or laboratory, of the melting point depression of a mixture of two enantiomers is also featured.