Isolation of Menthol: Menthol is a monoterpene alcohol found in mint oil or peppermint oil. The sources of mint oil are Mentha piperita Var. vulgaris, Mentha piperita Var. officinalis, Mentha arvensis, Mentha canadensis Var. piperascence belongs to the family Piperaceae. 1 to 3 percent volatile oil is present in peppermint. The two varieties Mentha piperita Var. vulgaris and Mentha piperita Var. officinalis contains not less than 45 percent of menthol whereas the varieties Mentha arvensis and Mentha Canadensis Var. piperascence contains up to 70 to 90 percent of menthol. The other constituents are (+) neomenthol, (+) isomenthol, menthofuran, menthone, menthyl acetate, and cineole. Menthol can be synthesized chemically by hydrogenation of thymol.
Isolation of Menthol
Mentha oil is obtained by hydrodistillation and steam distillation of fresh parts of the mentha plant. For the isolation of (-) menthol from peppermint oil, the oil is subjected to cooling. The menthol crystal crystallizes out and is separated by centrifugation.
The corn mint oil is obtained from steam distillation of Mentha arvensis flowering tops which contains about 70 to 80 percent of free (-) menthol. The oil is cooled and menthol crystals are separated by centrifugation. Pure (-) menthol is obtained from recrystallization from low boiling solvents. The Melting point of menthol is 41 to 44°C.
Thin-layer chromatography of Menthol
1 mg menthol is dissolved in 1 ml of methanol. Then apply a spot on silica gel-G plate and elute it in chloroform. The dried plates are sprayed with a 1 percent vanillin-sulphuric acid reagent. Heat the plate at 110°C for 10 minutes. The Rf value of menthol is 0.48 to 0.62.
- 10 mg menthol crystals are dissolved in 4 drops of concentrated sulphuric acid and added a few drops of vanillin sulphuric acid reagent. It shows orange-yellow colour which changes to violet upon the addition of a few ml of water.
- Crystals of menthol are dissolved in glacial acetic acid and add 3 drops of sulphuric acid and one drop of nitric acid. Menthol does not give green or blue-green colour (thymol gives green colour).
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