Menthone is a

monoterpene Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen func ...

with a minty flavor that occurs naturally in a number of

essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...

s. ''l''-Menthone (or (2''S'',5''R'')-''trans''-2-isopropyl-5-methylcyclohexanone), shown at right, is the most abundant in nature of the four possible

stereoisomer In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...

s. It is structurally related to

menthol Menthol is an organic compound, more specifically a monoterpenoid, made synthetically or obtained from the oils of corn mint, peppermint, or other mints. It is a waxy, clear or white crystalline substance, which is solid at room temperature ...

, which has a secondary

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

in place of the

carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...

. Menthone is used in flavoring, perfume and cosmetics for its characteristic aromatic and minty odor.

Occurrence

Menthone is a constituent of the essential oils of pennyroyal,

peppermint Peppermint (''Mentha'' × ''piperita'') is a hybrid species of mint, a cross between watermint and spearmint. Indigenous to Europe and the Middle East, the plant is now widely spread and cultivated in many regions of the world.Euro+Med Plantb ...

, '' Mentha arvensis'', ''

Pelargonium ''Pelargonium'' () is a genus of flowering plants that includes about 280 species of perennials, succulents, and shrubs, commonly called geraniums, pelargoniums, or storksbills. ''Geranium'' is also the botanical name and common name of a separa ...

'' geraniums, and others. In most essential oils, it is a minor compound; it was first synthesized by oxidation of menthol in 1881 before it was found in essential oils in 1891.

Structure and preparation

2-Isopropyl-5-methylcyclohexanone has two asymmetric carbon centers, meaning that it can have four different stereoisomers: (2''S'',5''S''), (2''R'',5''S''), (2''S'',5''R'') and (2''R'',5''R''). The ''S'',''S'' and ''R'',''R'' stereoisomers have the methyl and isopropyl groups on the same side of the cyclohexane ring: the so-called '' cis'' conformation. These stereoisomers are called isomenthone. The trans-isomers are called menthone. Because the (2''S'',5''R'') isomer has negative optical rotation, it is called ''l''-menthone or (−)-menthone. It is the enantiomeric partner of the (2''R'',5''S'') isomer: (+)- or ''d''-menthone. Menthone can easily be converted to isomenthone and vice versa via a reversible

epimerization In stereochemistry, an epimer is one of a pair of diastereomers. The two epimers have opposite configuration at only one stereogenic center out of at least two. All other stereogenic centers in the molecules are the same in each. Epimerization is t ...

reaction via an

enol In organic chemistry, alkenols (shortened to enols) are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene ( olefin) with a hydroxyl group attached to one end of the alkene double bond (). T ...

intermediate, which changes the direction of optical rotation, so that ''l''-menthone becomes ''d''-isomenthone, and ''d''-menthone becomes ''l''-isomenthone. In the laboratory, ''l''-menthone may be prepared by

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...

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 Herbert Charles Brown (May 22, 1912 – December 19, 2004) was an American chemist and recipient of the 1979 Nobel Prize in Chemistry for his work with organoboranes. Life and career Brown was born Herbert Brovarnik in London, to Ukrainian Jewis ...

, 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. By contrast, ''d''-isomenthone has a "green" note, increasing levels of which are perceived to detract from the odor quality of ''l''-menthone.

History

Menthone was first described by Moriya in 1881. It was then synthesized by heating menthol with chromic acid, and its structure was later confirmed by synthesizing it from 2-isopropyl-5-methylpimelic 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 Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...

gave a new ketonic material which gave an equal but opposite

optical rotation Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...

to the starting material.{{cite journal , title = Untersuchungen in der Campherreihe , author = Ernst Beckmann , journal = Liebigs Annalen , year = 1889 , volume = 250 , issue =3 , pages = 322–375 , doi = 10.1002/jlac.18892500306 , author-link = Ernst Beckmann , url = https://zenodo.org/record/1427429 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 an intermediate enol

tautomer Tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the relocation of a hy ...

in which the asymmetry of the carbon atom was removed when it changed from a tetrahedral to a trigonal (planar) 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

Flavors Cooling flavors Ketones Perfume ingredients Monoterpenes Isopropyl compounds Cyclohexanes