Menthol-Synonyms 1-Menthol; 3-Menthanol; Menthan-3-ol; Peppermint camphor, Hexahydrothymol

2.1.3 Menthol

Synonyms 1-Menthol; 3-Menthanol; Menthan-3-ol; Peppermint camphor, Hexahydrothymol.
Biological Sources It is found in the peppermint oil obtained from the fresh flowering tops of the plants commonly known as Mentha piperita Linn., or other allied species of Mentha, belonging to family Labiatae.
Geographical Source Various mentha species are duly cultivated in various parts of the world. It grows both abundantly and widely in Europe, while it is cultivated in Japan , Great Britain, Italy,France, United States, CIS countries, Bulgaria and India.
Preparation It is normally prepared from Japanese Peppermint Oil, from the flowering tops of Mentha avensis Linne’ var piperascens, by subjecting it to refrigeration below –22oC whereby the menthol crystallizes out distinctly. The crystals of menthol are separated by filteration and squeezes between layers of filter papers to remove the adhering oil and finally purified by recrystallization.
Synthetic racemic menthol is prepared by the hydrogenation of either pulegone or thymol as shown below:

Synthetic racemic menthol
It may also be prepared from pinene.
Description
Colour : Colourless
Odour : Pleasant peppermint like odour
Taste : Characteristic, aromatic and cooling taste
Shape : Hexagonal cyrstals usually needle like, prisms; crystalline powder; fused masses.
Chemical Structure Menthol has three chiral centres (*), hence it would give rise to eight (23) optically active isomers and four racemic forms. Menthol on oxidation gives menthone (a ketone), by the sacrifice of one chiral centre; therefore, the resulting menthone must exist in four (22) optically active isomers and two recemic forms and all, these have been actually prepared.

Chemical Structure Menthol
Special Features following are the special features of menthol, namely:
(a) Dehydrogenation: Menthol first on dehydration yields two isomeric forms of p-menthane, which on subsequent dehydration gives rise to p-cymene as follows:

Dehydrogenation: Menthol
(b) Reduction: Menthol on reduction with hydroiodic acid yields p-menthane as under:

Chemical Tests
1. When 10 mg crystals menthol are first dissolved in 4 drops of concentrated sulphuric acid and then a few drops of vanillin sulphuric acid reagent are added it shows an orange yellow colouration that ultimately changes to violet on the addition of a few drops of water.
2. A few crystals of menthol are dissolved in glacial acetic acid and to this solution a mixture of 3
drops of H2SO4 and 1 drop of HNO3 are added. It fails to produce either green or bluish green colouration (Thymol gives a green colouration).
3. Menthol provides a plethora of compounds of diagnostic value for differential identification, for instance: menthoxy acetate; p-nitrobenzoate; d-camphor sulphonate; acid phthalate; phosphoric acid-complex; and 3,5-dinitrobenzoate.
Uses
1. It is used profusely in various types of mouth washes, toothpastes and similar oral formulations.
2. It finds its enormous use as a flavouring agent for chewing gums, candies, throat lozenges and also certain mentholated cigarettes.

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