Information AboutMenthol |
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| alcohols | |
| cigarette additives | |
| terpenes and terpenoids | |
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Menthol is a Covalent Organic Compound Made Synthetically or obtained from Peppermint or other Mint oils. It is a waxy, crystalline substance, clear or white in color, which is solid at Room Temperature and melts slightly above. The main form of menthol occurring in nature is (-)-menthol, which is assigned the (1R,2S,5R) Configuration . Menthol has Local Anesthetic and counterirritant qualities, and it is widely used to relieve minor throat irritation. HISTORY AND OCCURRENCE There is evidenceJ. L. Simonsen, ''The Terpenes'' Volume I (2nd edition), Cambridge University Press, 1947, p230-249. that menthol has been known in Japan for more than 2000 years, but in the West it was not isolated until 1771, by Gambius.''Adversoriorum varii argumentii, Liber unus'', Leiden, 1771, p99. (-)-Menthol (also called ''l''-menthol or (''1R,2S,5R'')-menthol) occurs naturally in peppermint oil (along with a little Menthone , the ester menthyl acetate and other compounds), obtained from ''mentha x piperita''. Japanese menthol also contains a small percentage of the 1- Epimer , (+)-neomenthol. STRUCTURE Natural menthol exists as one pure Enantiomer , nearly always the (''1R,2S,5R'') form (bottom left of diagram below). The other seven Stereoisomers are: In the natural compound, the Isopropyl group is oriented '' Trans ''- orientation to both the Methyl and Alcohol groups. Thus it can be drawn in any of the ways shown: In the ground state all three bulky groups in the chair are Equatorial , making (-)-menthol and its enantiomer the most stable two isomers out of the eight. There are two crystal forms for Racemic menthol; these have melting points of 28 °C and 38 °C. Pure (-)-menthol has four crystal forms, of which the most stable is the α form, the familiar broad needles. APPLICATIONS Menthol is included in many products for a variety of reasons. These include:
Some supporters of : 3300 mg·kg−1; Skin (rabbit) LD50: 15800 mg·kg−1). In Organic Chemistry , menthol is used as a Chiral Auxiliary in Asymmetric Synthesis . For example, Sulfinate Ester s made from Sulfinyl Chlorides and menthol can be used to make Enantiomerically Pure Sulfoxides by reaction with Organolithium Reagent s or Grignard Reagents . Menthol is also used for Classical Resolution of chiral carboxylic acids, via the menthyl esters. SYNTHESIS As with many widely-used natural products, the demand for menthol greatly exceeds the supply from natural sources. Menthol is manufactured as a single Enantiomer (94% Ee ) by Takasago International Co. on a scale of 400 000 Tonne s per year. The process involves an Asymmetric Synthesis developed by a team led by Ryoji Noyori : The process begins by forming an Allylic amine from Myrcene , which undergoes Asymmetric Isomerisation in the presence of a BINAP Rhodium complex to give (after Hydrolysis ) enantiomerically pure ''R''- Citronellal . This is cyclised by a Carbonyl-ene-reaction initiated by Zinc Bromide to isopulegol which is then hydrogenated to give pure (''1R,2S,5R'')-menthol. Racemic menthol can be prepared simply by hydrogenation of Thymol , and menthol is also formed by hydrogenation of Pulegone . For preparation of other isomers such as neomenthol, see ref. 1. NATURAL MENTHOL '' Mentha Arvensis '' is the primary species of Mint used to make natural menthol Crystals and natural menthol flakes. This species is primarily grown in the Uttar Pradesh region in India . CHEMICAL PROPERTIES Menthol reacts in many ways like a normal secondary alcohol. It is oxidised to menthone by oxidising agents such as Chromic Acid , though under some conditions the oxidation can go further and break open the ring. Menthol is easily dehydrated to give mainly 3-menthene, by the action of 2% Sulfuric Acid . PCl5 gives menthyl chloride. BIOLOGICAL PROPERTIES Menthol's ability to chemically trigger cold-sensitive receptors in the skin is responsible for the well known cooling sensation that it provokes when inhaled, eaten, or applied to the skin. Menthol does not cause an actual drop in temperature.R. Eccles, Menthol and Related Cooling Compounds, Journal of Pharm. Pharmacol. 1994 46: 618-630 In this sense it is similar to Capsaicin , the chemical responsible for the spiciness of hot peppers (which stimulates heat sensors, also without causing actual temperature rise). NOTES REFERENCES
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