Menthone

Menthone is a naturally occurring organic compound with a molecular formula C10H18O. l-Menthone (or (2S, 5R)-trans-2-isopropyl-5-methylcyclohexanone), shown at right, is the most abundant in nature of the four possible stereoisomers. Menthone is a monoterpene and a ketone. It is structurally related to menthol which has a secondary alcohol in place of the carbonyl.

Menthone is a constituent of the essential oils of pennyroyal, peppermint, geranium, and others. In most esential oil it is minor compound. This is the cause for the fact that it was first synthesized by oxidation of menthol in 1881 before it was found in essential oils in 1891. It is used in perfumery and cosmetics for its characteristic aromatic and minty odor.

This molecule is at the heart of one of the great mechanistic discoveries in organic chemistry. Ernst Beckmann in 1889 discovered that dissolving menthone in concentrated sulfuric acid  gave a new ketonic material,  with a  (coincidentally) equal but opposite optical rotation to the starting material. With the recognition of tetrahedral carbon being only 15 years old (Jacobus Henricus van't Hoff and Joseph Achille Le Bel), Beckmann realized that this must result from an inversion of configuration at the asymmetric carbon atom next to the carbonyl group (at that time thought to be the methyl rather than the iso-propyl group), and he postulated this as happening through the intermediacy of an enol tautomer in which the asymmetric carbon atom was of trigonal (planar)  rather than of tetrahedral geometry. This was an early example of the inference of an (almost) undetectable intermediate in a reaction mechanism accounting for the outcome of the reaction.

References and footnotes

 * 1) Merck Index, 12th Edition, 5883.
 * 2) For a molecular modelling experiment involving menthone, see here.
 * 1) For a molecular modelling experiment involving menthone, see here.
 * 1) For a molecular modelling experiment involving menthone, see here.
 * 1) For a molecular modelling experiment involving menthone, see here.