Menthol

History

The menthol was discovered with the Japan, there is more than 2000 years, but it was insulated only for the first time by Gambius in 1771. (-) - Menthol (also called L-menthol or (1R, 2S, 5R) - menthol) is naturally in the peppermint gasoline (Mentha ×piperita). The menthol Japanese also contains a light percentage of (+) - néomenthol.

Structure

The menthol exists in a native state in the shape of only one enantiomer: (1R, 2S, 5R), bellow on the left of the diagram.

In the natural compounds, the grouping isopropyl is directed trans. compared to the two methyl groups and alcohol in position cis. Thus it can be drawn one or the other in the ways below:

In the fundamental state, each of the three substituents are in equatorial position, making (-) - menthol and of its enantiomer (1S, 2R, 5S) the most stable isomers. There exist two forms crystalline for the racemic menthol . One at melting points of 28°C and the other of 28°C. Pure, menthol has four solid forms, of which most stable is the form α.

Applications

The menthol is included in many different products for several reasons like:

short-term Relief of the throat endolorie and the minor irritation of the Mouth or the throat. (baths of mouth for example)
Antipruritique to reduce itchings
Local anesthetic to relieve of the evils and minor pains such as muscular cramps, distorsions, migraines,… It can be used only or combined with Piment or Camphre. In Europe, it is rather used in gel or in cream
Relieving congestion for the respiratory tracts and the sines (in particular used in the product Vicks Vaporub)
Pesticides against the acarina
In certain drugs treating the minor burns, it produces a feeling of cold (often used in local application, in partnership with the Aloès)
Additif in some Cigarette S, like savor or to reduce irritations of the throat or the sines caused by the Tabagisme
It can also be used like Disinfecting for oral hygiene, or remedy against the bad breath, like collutoires, toothpastes, and more generally like agent of savor for the chewings-gum and the sugar refineries
In the sodas as well mixed with water to obtain a drink with very low alcoholic strength (as in the Ricqlès for example (Ricqlès is very alcoholic)). The syrup can be also used with an aim of reducing nausea by pouring some drops on a piece of sugar
In perfumery, to prepare esters menthylés to decorate floral notes
In patches to make fall the fever or obtain a feeling of cold (very developed with the Japan)
In certain beauty products (produced capping for example)

The defenders of the homeopathic pharmacy think that the menthol interferes with the homeopathic drug. Its use is strongly disadvised in partnership with homeopathic drug, at such point to proscribe the toothpaste containing menthol .

The menthol can be used as dietetic supplement in the form of oil of peppered Menthe (médicine natural), the indigestion, nausea, the evils of throat, the diarrhea, the headaches and coolings (Orientale medicine)

Menthol has a relatively low toxicity: (Rat) oral DL₅₀: 3300 mg/kg; Skin (rabbit) DL₅₀: 15800 mg/kg). In Organic chemistry, the menthol is used like chiral auxiliary in the synthesis of asymmetrical centers .

Synthesis

As much of natural products usually employed, the request for menthol exceeds the provisioning of the natural sources considerably. The menthol is manufactured like simple enantiomer by Takasago International Co. on a scale of 400.000 tons per annum. The process implies an asymmetrical synthesis developed by a team carried out by Ryoji Noyori:

The process starts by forming an allyl amine of the myrcene, which undergoes asymmetrical isomerization in the presence of a rhodium complex of BINAP to give (after Hydrolyze) R-citronellal enantiomeriquement pure. The result is treated with zinc bromide to obtain the Isopulegol which is then hydrogenated to give (1R, 2S, 5R) - pure menthol. The racemic menthol can be prepared simply by the hydrogenation of the Thymol, and the menthol is also consisted the hydrogenation of the pulegone.

Chemical properties

The menthol reacts often in the same way that a normal secondary alcohol. It is oxidized with the menthone by oxidizing agents such as the chromic acid, although under certain conditions oxidation can further go and break the cycle. The menthol is easily dehydrated to give mainly the 3-menthene, by the sulphuric action of 2% of acid. The treatment PCl5 gives menthyl chloride. PCl₅ gives menthyl chloride.

Biological properties

The capacity of the menthol to chemically start the cold sensitive receivers in the skin is responsible for the well-known feeling of cooling which it causes once absorptive. In this direction it is similar to the Capsaïcine, the chemical species responsible for the spiced feeling of the Poivre S.

Sources

E.E. Turner, Mr. Mr. Harris, Organic Chemistry , Longmans, Green & Co., London, 1952.
Handbook off Chemistry and Physics , 71st edition, CRC Close, Anarchist Arbor, Michigan, 1990.
The Merck Index , 7th edition, Merck & Co, Rahway, New Jersey, the USA, 1960.

External bonds

in. Colacot T.J. '' Platinum Metals Review '' ''' 2002 ''', '' 46 (2) '', 82-83.
in. Ryoji Noyori Nobel reading (2001)
in. Menthol Information
in. Cooler than Menthol

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