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 used in perfumery and cosmetics for its characteristic aromatic and minty odor.
Menthone was crucial to one of the great mechanistic discoveries in organic chemistry. In 1889, Ernst Beckmann discovered that dissolving menthone in concentrated sulfuric acid gave a new ketonic material. Coincidentally, the product gave an equal but opposite optical rotation to the starting material. The tetrahedral carbon having been recognized for only 15 years old, 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 carbon attached to the methyl, rather than the isopropyl 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.