What is/are Camphor and Menthol Topical?
Menthol is an organic compound made synthetically or obtained from cornmint, 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. Menthol also acts as a weak kappa opioid receptor agonist.
As with many widely used natural products, the demand for menthol greatly exceeds the supply from natural sources. In the case of menthol it is also interesting to note that comparative analysis of the total life-cycle costs from a sustainability perspective, has shown that production from natural sources actually results in consumption of more fossil fuel, produces more carbon dioxide effluent and has more environmental impact than either of the main synthetic production routes.
Menthol is manufactured as a single enantiomer (94% ee) on the scale of 3,000 tons per year by Takasago International Corporation. The process involves an asymmetric synthesis developed by a team led by Ryōji Noyori, who won the 2001 Nobel Prize for Chemistry in recognition of his work on this process:
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. In both cases with further processing (crystallizative entrainment resolution of the menthyl benzoate conglomerate) it is possible to concentrate the L enantiomer, however this tends to be less efficient, although the higher processing costs may be offset by lower raw material costs. A further advantage of this process is that d-menthol becomes inexpensively available for use as a chiral auxiliary, along with the more usual l-antipode.
Menthol is included in many products for a variety of reasons. These include:
- In nonprescription products for short-term relief of minor sore throat and minor mouth or throat irritation
- Examples: lip balms and cough medicines
- As an antipruritic to reduce itching
- As a topical analgesic, it is used to relieve minor aches and pains, such as muscle cramps, sprains, headaches and similar conditions, alone or combined with chemicals such as camphor, eucalyptus oil or capsaicin. In Europe, it tends to appear as a gel or a cream, while in the U.S., patches and body sleeves are very frequently used.
- Examples: Tiger Balm, or IcyHot patches or knee/elbow sleeves
- In decongestants for chest and sinuses (cream, patch or nose inhaler)
- Examples: Vicks Vaporub, Mentholatum, vapoRem
- In certain medications used to treat sunburns, as it provides a cooling sensation (then often associated with aloe)
- In aftershave products to relieve razor burn
- As an additive in certain cigarette brands, for flavor, to reduce the throat and sinus irritation caused by smoking
- Commonly used in oral hygiene products and bad-breath remedies, such as mouthwash, toothpaste, mouth and tongue-spray, and more generally as a food flavor agent; e.g., in chewing gum, candy
- In a soda to be mixed with water it is used to obtain a very low alcohol drink or pure (brand Ricqlès which contains 80% alcohol in France); the alcohol is also used to alleviate nausea, in particular motion sickness, by pouring a few drops on a lump of sugar.
- As a pesticide against tracheal mites of honey bees
- In perfumery, menthol is used to prepare menthyl esters to emphasize floral notes (especially rose).
- In first aid products such as "mineral ice" to produce a cooling effect as a substitute for real ice in the absence of water or electricity (pouch, body patch/sleeve or cream)
- In various patches ranging from fever-reducing patches applied to children's foreheads to "foot patches" to relieve numerous ailments (the latter being much more frequent and elaborate in Asia, especially Japan: some varieties use "functional protrusions", or small bumps to massage ones feet as well as soothing them and cooling them down)
- In some beauty products such as hair conditioners, based on natural ingredients (e.g., St. Ives)
- As an antispasmodic and smooth muscle relaxant in upper gastrointestinal endoscopy
In organic chemistry, menthol is used as a chiral auxiliary in asymmetric synthesis. For example, sulfinate esters made from sulfinyl chlorides and menthol can be used to make enantiomerically pure sulfoxides by reaction with organolithium reagents or Grignard reagents. Menthol reacts with chiral carboxylic acids to give diastereomic menthyl esters, which are useful for chiral resolution.
This article uses material from the Wikipedia article Camphor and Menthol Topical, which is released under the Creative Commons Attribution-Share-Alike License 3.0.