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| IUPAC name (2S,5R)-2-Isopropyl-5-methylcyclohexanone | |
| Other names l-Menthone | |
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| Properties | |
| Chemical formula | C10H18O |
| Molar mass | 154.253 g·mol |
| Density | 0.895 g/cm |
| 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 their standard state (at 25 °C , 100 kPa).
 N verify (what is ?)
Infobox references
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Menthone is a chemical compound of the monoterpene class of naturally occurring organic compounds found in a number of essential oils, one that presents with minty flavor. It is a specific pair of stereoisomers of the four possible such isomers for the chemical structure, 2-isopropyl-5-methylcyclohexanone. Of those, the stereoisoomer l-menthone—formally, the (2S,5R)-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.
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.
Occurrence
Menthone is a constituent of the essential oils of pennyroyal, peppermint, corn mint, pelargonium geraniums, and other plant species. In most essential oils, it is a minor component. Menthone was first synthesized by oxidation of menthol in 1881, before being found as a component in essential oils in 1891. Of the isomers possible for this chemical structure (see below), the one termed l-menthone—formally, the (2S,5R)-trans-2-isopropyl-5-methylcyclohexanone (see infobox and below)—is the most abundant in nature.
Physical and sensory properties
 | This section needs expansion with: full secondary and tertiary sourcing, of what appears and further in the usual array of properties covered by this section. You can help by adding to it. (December 2024) |
Menthone is a liquid under standard conditions, and has a density of 0.895 g/cm. 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. Pure l-menthone has been described as having an intensely minty clean aroma; in contrast, d-isomenthone has a "green" note, increasing levels of which are perceived to detract from the aroma quality of l-menthone.
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 spectroscopy 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 spatial arrangement of atoms—the absolute configuration—at these two points are designated by the descriptors R (Latin, rectus, right) or S (L., sinister, left) based on the Cahn–Ingold–Prelog priority rules. Hence, four unique stereoisomers are possible for this structure: (2S,5S), (2R,5S), (2S,5R) and (2R,5R).
The (2S,5S) and (2R,5R) 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. Because the (2S,5R) isomer has an observed negative optical rotation, it is called l-menthone or (−)-menthone. It is the enantiomeric partner of the (2R,5S) isomer: (+)- or d-menthone.
Interconversion
Menthone and isomenthone interconvert easily, the equilibrium favoring menthone; if menthone and isomenthone are equilibrated at room temperature, the isomenthone content will reach 29%. Menthone can easily be converted to isomenthone and vice versa via a reversible epimerization 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.
Preparation and reactivity
Menthone is obtained commercially by fractional crystallization of the oils pressed from peppermint and cornmint, sp. Mentha.
In the experimental 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 and colleagues in 1971, the epimerization of l-menthone to d-isomenthone is largely avoided.
History
| This section needs expansion with: a secondary source-derived history covering key discoveries regarding the compound and its role in the development of organic chemistry (through to the modern). You can help by adding to it. (December 2024) |
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, 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. Beckmann's inferences from his results situated menthone as a crucial player in a great mechanistic discovery in organic chemistry. Beckmann concluded that the change in structure underlying the observed opposite optical 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). He postulated that this occurred through an intermediate enol—a tautomer of the ketone—such that the original absolute configuration of that carbon atom changed as its geometry went from terahedral to trigonal planar. 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.
See also
Further reading
| This section needs expansion with: secondary and tertiary sources that include substantive content on menthone, its structure, history, chemistry, and applications. You can help by adding to it. (December 2024) |
References
- ^ Hirsch, Alan R. (March 18, 2015). Nutrition and Sensation. Boca Raton, FL: CRC Press. p. 276ff. ISBN 9781466569089. Retrieved December 3, 2015.
- ^ Ager, David (2005). Handbook of Chiral Chemicals (2nd ed.). Boca Raton, FL: CRC Press. p. 64. ISBN 9781420027303. Retrieved December 3, 2024.
- ^ Sell, Charles S. (2006). "Terpenoids". Kirk-Othmer Encyclopedia of Chemical Technology. doi:10.1002/0471238961.2005181602120504.a01.pub2. ISBN 0471238961.
- ^ Read, John (1930). "Recent Progress in the Menthone Chemistry". Chemical Reviews. 7 (1): 1–50. doi:10.1021/cr60025a001. Retrieved December 3, 2024.
- ^ Moriya, M. (1881). "XV.—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. Retrieved December 3, 2024 – via Zenodo.org.
- ^ Brown, H.C.; Garg, C.P.; Liu, K.-T. (1971). "The Oxidation of Secondary Alcohols in Diethyl Ether With Aqueous Chromic Acid. A Convenient Procedure for the Preparation of Ketones in High Epimeric Purity". J. Org. Chem. 36 (3): 387–390. doi:10.1021/jo00802a005. Retrieved December 3, 2024.
- ^ Singh, G. (2007). Chemistry of Terpenoids and Carotenoids. New Delhi, India: Discovery Publishing House. p. 41. ISBN 9788183562799. Retrieved December 3, 2024.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "absolute configuration". doi:10.1351/goldbook.A00020
- Seidel, Arza; Bickford, Michalina & Chu, Kelsee, ed. (2012). Kirk-Othmer Chemical Technology of Cosmetics. New York, NY: John Wiley & Sons. ISBN 9781118518908. Retrieved December 3, 2024.
{{cite book}}: CS1 maint: multiple names: editors list (link) 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. - Sandborn, L. T. (1929). "l-Menthone". Organic Syntheses. 9: 59; Collected Volumes, vol. 1, p. 340.
- 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.
- Chen, Fen-Er & You, Hengzhi (2024). "Ch. 7.04—Asymmetric Baeyer-Villiger Oxidation". Comprehensive Chirality (2nd ed.). New York, NY: Academic Press. pp. 78–121. ISBN 9780323906456. Retrieved December 3, 2024.
{{cite book}}: CS1 maint: multiple names: authors list (link) - Beckmann, Ernst (1889). "Untersuchungen in der Campherreihe" [Investigations in the Camphor-series]. Liebigs Annalen. 250 (3): 322–375. doi:10.1002/jlac.18892500306. Retrieved December 3, 2024 – via Zenodo.org.