Isolation:

• 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 recrystallisation 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 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.

Identification

1. A few drops of menthol mixed with 5ml nitric acid solution (prepared by adding 1ml nitric acid to 300ml glacial acetic acid) are heated on a water bath. Within 5 minutes, a blue colour develops which deepens on further heating and produces copper colour fluorescence that turns golden yellow after some time.