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| Names | |
|---|---|
| IUPAC name (2S,5R)-2-Isopropyl-5-methylcyclohexanone | |
| Other names l-Menthone | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChEMBL | |
| ChemSpider |
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| UNII | |
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| Properties | |
| C10H18O | |
| Molar mass | 154.253 g·mol−1 |
| Density | 0.895 g/cm3 |
| Melting point | −6 °C (21 °F; 267 K) |
| Boiling point | 207 °C (405 °F; 480 K) |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Menthone is achemical compound of themonoterpene class ofnaturally occurringorganic compounds found in a number ofessential oils,[1] one that presents with minty flavor.[2] It is a specific pair ofstereoisomers of the four possible such isomers for the chemical structure, 2-isopropyl-5-methylcyclohexanone. Of those, the stereoisoomerl-menthone—formally, the (2S,5R)-trans isomer of that structure, as shown at right—is the most abundant in nature.[3] Menthone is structurally related tomenthol, which has a secondaryalcohol (>C-OH) in place of the carbon-oxygendouble bond (carbonyl group) projecting from thecyclohexane ring.
Menthone is obtained for commercial use after purifying essential oils pressed fromMentha species (peppermint andcorn mint).[4] It is used as aflavorant and in perfumes and cosmetics for its characteristic aromatic and minty aroma.[not verified in body]
Menthone is a constituent of the essential oils ofpennyroyal,peppermint,corn mint,pelargonium geraniums, and otherplant species.[1] In most essential oils, it is a minor component.[5] Menthone was first synthesized by oxidation of menthol in 1881,[6][needs update][7] before being found as a component in essential oils in 1891.[citation needed] Of the isomers possible for this chemical structure (see below), the one termedl-menthone—formally, the (2S,5R)-trans-2-isopropyl-5-methylcyclohexanone (see infobox and below)—is the most abundant in nature.[3]
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Menthone is a liquid under standard conditions, and has adensity of 0.895 g/cm3.[citation needed] Under the same conditions,[verification needed] themelting point is −6 °C, and itsboiling point is 207 °C.[citation needed]
Menthone interacts cognitively with other components in food, drink, and other consumables, to present with what is termed a minty flavor.[2] Purel-menthone has been described as having an intensely minty clean aroma;[according to whom?] in contrast,d-isomenthone has a "green" note,[This quote needs a citation] increasing levels of which are perceived to detract from the aroma quality ofl-menthone.[8][verification needed]
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 supportingmass spectrometric and spectroscopic evidence (e.g., fromNMR spectroscopy andcircular dichroism) to make the conclusions secure.[citation needed]
The structure 2-isopropyl-5-methylcyclohexanone has twoasymmetric carbon centers, one at each attachment point of the two alkyl group substituents, theisopropyl in the 2-position and themethyl in the 5-position of thecyclohexane framework.[9][better source needed] The spatial arrangement of atoms—theabsolute configuration—at these two points are designated by the descriptorsR (Latin,rectus, right) orS (L.,sinister, left) based on theCahn–Ingold–Prelog priority rules.[10] Hence, four unique stereoisomers are possible for this structure: (2S,5S), (2R,5S), (2S,5R) and (2R,5R).[9][better source needed]
The (2S,5S) and (2R,5R) stereoisomers project the isopropyl and methyl groups from the same "side" of the cyclohexane ring, are the so-calledcis 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-calledtrans isomers, and are referred to as menthone.[9][better source needed] Because the (2S,5R) isomer has an observed negative optical rotation, it is calledl-menthone or (−)-menthone. It is theenantiomeric partner of the (2R,5S) isomer: (+)- ord-menthone.[9][better source needed][verification needed]
Menthone and isomenthone interconvert easily, the equilibrium favoring menthone;[4][better source needed] if menthone and isomenthone are equilibrated at room temperature, the isomenthone content will reach 29%.[dubious –discuss][8][verification needed] Menthone can easily be converted to isomenthone and vice versa via a reversibleepimerization reaction via anenol intermediate, which changes the direction of optical rotation, so thatl-menthone becomesd-isomenthone, andd-menthone becomesl-isomenthone.[11]
Menthone is obtained commercially by fractional crystallization of the oils pressed frompeppermint and cornmint, sp. Mentha.[4]
In the experimental laboratory,l-menthone may be prepared byoxidation of menthol with acidifieddichromate.[12][needs update] If the chromic acid oxidation is performed with stoichiometric oxidant in the presence of diethyl ether as co-solvent, a method introduced byH.C. Brown and colleagues in 1971, the epimerization ofl-menthone tod-isomenthone is largely avoided.[8]
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Menthone was first described by Moriya in 1881.[6][7] It was later synthesized by heating menthol withchromic acid, and its structure was later confirmed by synthesizing it from 2-isopropyl-5-methylpimelic acid.[when?][9]
Menthone was one of the original substrates reported in the discovery of the still widely usedsynthetic organic chemistry transformation, theBaeyer-Villiger (B-V) oxidation,[13] as reported byAdolf Von Baeyer andVictor Villiger in 1899; Baeyer and Villiger noted that menthone reacted withmonopersulfuric acid to produce the corresponding oxacycloheptane (oxepane-type)lactone, with an oxygen atom inserted between thecarbonyl carbon and the ring carbon attached to the isopropyl substituent.[14]
In 1889,Ernst Beckmann discovered that dissolving menthone in concentratedsulfuric acid gave a new ketonic material which gave an equal but oppositeoptical rotation to the starting material.[15][non-primary source needed] Beckmann's inferences from his results situated menthone as a crucial player in a great mechanistic discovery in organic chemistry.[clarification needed][citation needed] Beckmann concluded that the change in structure underlying the observed oppositeoptical rotation was the result of an inversion of configuration at the asymmetric carbon atom next to the carbonyl group (which, at that time was believed to be the carbon atom attached to the methyl rather than the isopropyl group).[citation needed] He postulated that this occurred through an intermediateenol—atautomer of the ketone—such that the originalabsolute configuration of that carbon atom changed as its geometry went from terahedral totrigonal planar.[clarification needed][citation needed] This report is an early example of an inference that an otherwise undetectable intermediate was involved in a reaction mechanism, one that could account for the observed structural outcome of the reaction.[according to whom?]
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{{cite book}}: CS1 maint: multiple names: editors list (link)[page needed] 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, seethis link.{{cite book}}: CS1 maint: multiple names: authors list (link)