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 cheaply available as a mixture of isomers; when enantiopure, it costs significantly more. In the laboratory, l-menthone may be prepared by oxidation of menthol with acidified dichromate. If the chromic acid oxidation is performed with stoichiometric oxidant in the presence of diethyl ether as co-solvent (a method introduced by H.C. Brown), the epimerization of l-menthone to d-isomenthone is largely avoided. If menthone and isomenthone are equilibrated at room temperature, the isomenthone content will reach 29%. Pure l-menthone has an intensely minty clean aroma.
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