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 used in perfumery and cosmetics for its characteristic aromatic and minty odor.
Occurence
Menthone is a constituent of the essential oils of pennyroyal, peppermint, geranium, and others. In most essential oils, it is a 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.
Preparation
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.[1]
In this synthesis, menthol is slowly added to excess sodium dichromate, and heated to 55 °C. The white crystals turn black, then dissolve to give a deep green solution. The solution is allowed to react for 2 hours, before being extracted with diethyl ether and washed with 1 M sodium hydroxide solution. The yellow extracts are concentrated on a rotavap, and vacuum distilled to give a colorless oil smelling strongly of peppermint. Some menthol starting material may contaminate the product, which may be removed by redistillation to give a spectroscopically pure product (by NMR).
History
Menthone was first described by Moriya in 1881.[2][3] It was then synthesized by heating menthol with chromic acid.
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.[4] 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.
References
- Merck Index, 12th Edition, 5883.
- ↑ L. T. Sandborn l-Menthone. Org. Synth. 1941, <http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv1p0340>; Coll. Vol., 1, 340
- ↑ M. Moriya (1881). "Contributions from the Laboratory of the University of Tôkiô, Japan. No. IV. On menthol or peppermint camphor". Journal of the Chemical Society, Transactions 39: 77-83. doi:10.1039/CT8813900077.
- ↑ John Read (1930). "Recent Progress in the Menthone Chemistry.". Chemical Reviews 7 (1): 1-50. doi:10.1021/cr60025a001.
- ↑ Ernst Beckmann (1889). "Untersuchungen in der Campherreihe". Annalen 250: 322. doi:10.1002/jlac.18892500306.
External links
- For a molecular modelling experiment involving menthone, see here.
- 1H and 13C NMR spectra at ChemSpider
