Menthone is a naturally occurring organic compound

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 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

Terpene

Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...

 and a ketone

Ketone

In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

. It is structurally related to menthol

Menthol

Menthol is an organic compound made synthetically or obtained from peppermint or other mint oils. It is a waxy, crystalline substance, clear or white in color, which is solid at room temperature and melts slightly above. The main form of menthol occurring in nature is -menthol, which is assigned...

 which has a secondary alcohol

Alcohol

In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 in place of the carbonyl.

Menthone is used in perfumery and cosmetics for its characteristic aromatic and minty odor.

Occurrence

Menthone is a constituent of the essential oil

Essential oil

An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove...

s of pennyroyal

Pennyroyal

Pennyroyal refers to two plants in the mint family, Lamiaceae. For the American species, see American pennyroyal. The European pennyroyal, Mentha pulegium, , is a plant in the mint genus, within the family Lamiaceae. Crushed Pennyroyal leaves exhibit a very strong fragrance similar to spearmint...

, peppermint

Peppermint

Peppermint is a hybrid mint, a cross between the watermint and spearmint . The plant, indigenous to Europe, is now widespread in cultivation throughout all regions of the world...

, Pelargonium

Pelargonium

Pelargonium is a genus of flowering plants which includes about 200 species of perennials, succulents, and shrubs, commonly known as scented geraniums or storksbills. Confusingly, Geranium is the correct botanical name of a separate genus of related plants often called Cranesbills. Both Geranium...

geraniums, 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.

History

Menthone was first described by Moriya in 1881. It was then synthesized by heating menthol with chromic acid

Chromic acid

The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixture for glass. Chromic acid may also refer to the...

.

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

Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 gave a new ketonic material. Coincidentally, the product gave an equal but opposite optical rotation

Optical rotation

Optical rotation is the turning of the plane of linearly polarized light about the direction of motion as the light travels through certain materials. It occurs in solutions of chiral molecules such as sucrose , solids with rotated crystal planes such as quartz, and spin-polarized gases of atoms...

 to the starting material. The tetrahedral carbon having been recognized for only 15 years at that time, 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

Tautomer

Tautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization. This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond...

 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.

External links

• For a molecular modelling experiment involving menthone, see here.
• 1H and 13C NMR spectra at ChemSpider
  ChemSpider
  ChemsSpider is a free chemical database, owned by the Royal Society of Chemistry.-Database:The database contains more than 26 million unique molecules from over 400 data sources including those listed below.* A-L: EPA DSSTox, U.S...