Menthone is a chemical compound of the terpene class of natural product organic compounds found in a number of ,[ It is a specific pair of of the four possible such isomers for the chemical structure, 2-isopropyl-5-methylcyclohexanone. Of those, the stereoisoomer l-menthone—formally, the (2 S,5 R)- trans isomer of that structure, as shown at right—is the most abundant in nature.][ Menthone is structurally related to menthol, which has a secondary alcohol (>C-OH) in place of the carbon-oxygen double bond (carbonyl group) projecting from the cyclohexane ring.
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Menthone is obtained for commercial use after purifying essential oils pressed from Mentha species (peppermint and corn mint).[ It is used as a flavorant and in perfumes and cosmetics for its characteristic aromatic and minty aroma.
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Occurrence
Menthone is a constituent of the essential oils of Mentha pulegium, peppermint, Mentha arvensis, pelargonium geraniums, and other plant species.
Physical and sensory properties
Menthone is a liquid under standard conditions, and has a density of 0.895 g/cm3. Under the same conditions, the melting point is −6 °C, and its boiling point is 207 °C.
Menthone interacts cognitively with other components in food, drink, and other consumables, to present with what is termed a minty flavor.[
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Structure and stereochemistry
The structure of 2-isopropyl-5-methylcyclohexanone (menthones and isomenthones, see following) were established historically by establishing identity of natural and synthetic products after chemical synthesis of this structure from other chemical compounds of established structure; these inferential understandings have, in modern organic chemistry, been augmented by supporting mass spectrometric and spectroscopic evidence (e.g., from NMR and circular dichroism) to make the conclusions secure.
The structure 2-isopropyl-5-methylcyclohexanone has two asymmetric carbon centers, one at each attachment point of the two alkyl group substituents, the isopropyl in the 2-position and the methyl in the 5-position of the cyclohexane framework.
The (2 S,5 S) and (2 R,5 R) stereoisomers project the isopropyl and methyl groups from the same "side" of the cyclohexane ring, are the so-called cis isomers, and are termed isomenthone; the (2R,5S) and (2S,5R) stereoisomers project the two groups on the opposite side of the ring, are the so-called trans isomers, and are referred to as menthone.
Interconversion
Menthone and isomenthone interconvert easily, the equilibrium favoring menthone;[ Menthone can easily be converted to isomenthone and vice versa via a reversible Epimer reaction via an enol intermediate, which changes the direction of optical rotation, so that l-menthone becomes d-isomenthone, and d-menthone becomes l-isomenthone.][ Note, an earlier citation suggested appearance of this content on page 339, which cannot be confirmed with digital information accessible. Note, a further version of the book appears here, with some accessible content, but not the content on the epimerisation of menthone-isomenthone, see this link.]
Preparation and reactivity
Menthone is obtained commercially by fractional crystallization of the oils pressed from peppermint and cornmint, sp. Mentha.[
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In the experimental laboratory, l-menthone may be prepared by oxidation of menthol with acidified dichromate.
History
Menthone was first described by Moriya in 1881.[ It was later synthesized by heating menthol with chromic acid, and its structure was later confirmed by synthesizing it from 2-isopropyl-5-methylpimelic acid.]
Menthone was one of the original substrates reported in the discovery of the still widely used synthetic organic chemistry transformation, the Baeyer-Villiger (B-V) oxidation,[Now used substituting organic peracids—e.g., peracetic acid or m-chloroperbenzoic acid (m-CPBA), and regularly used in laboratory scale syntheses of "pharmaceutical intermediates, steroids, antibiotics and pheromones", see Chen & You, 2024, op. cit.] as reported by Adolf Von Baeyer and Victor Villiger in 1899; Baeyer and Villiger noted that menthone reacted with monopersulfuric acid to produce the corresponding oxacycloheptane (oxepane-type) lactone, with an oxygen atom inserted between the carbonyl carbon and the ring carbon attached to the isopropyl substituent.
In 1889, Ernst Beckmann discovered that dissolving menthone in concentrated sulfuric acid gave a new ketonic material which gave an equal but opposite optical rotation to the starting material.
See also
Further reading
