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:
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.
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:
(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.
Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar
Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar
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