Chemistry and properties
Methyl salicylate has a molecular weight of 152.15 and a boiling point of 222-224 °C. Its vapor pressure is 0.0343 mm Hg at 25 °C. It is slightly soluble in water, soluble in chloroform and ether, and miscible with alcohol. The material is sensitive to light and heat.
Natural occurrence
Methyl salicylate is present in the leaves of Gaultheria procumbens and in the bark of Betula lenta. Gaultheria procumbens, also called eastern teaberry or American wintergreen, is a perennial native to northeastern North America from Newfoundland west to southeastern Manitoba and south to Alabama. Betula lenta, the sweet birch, also known as black birch, is native to eastern North America. The twigs of Betula lenta, when scraped, have a strong scent of wintergreen due to methyl salicylate, which is produced in the bark. Wintergreen oil contains about 98% methyl salicylate, together with α-pinene, myrcene, delta-3-carene, limonene, 3,7-guaiadiene and delta-cadinene. Spiraea plants also contain methyl salicylate in large amounts and are used similarly to wintergreen. Methyl salicylate can also be a component of floral scents, especially in plants dependent on nocturnal pollinators like moths. Natural wintergreen oil can be distinguished from artificial by gas chromatography (GC) and GC isotope ratio mass spectrometry analysis.
Natural supply
Black birch was once harvested extensively to produce oil of wintergreen. The tree was borderline endangered until the 1950s-60s, when synthetic oil of wintergreen appeared.
Uses
Methyl salicylate is used in high concentrations as a rubefacient and analgesic in deep heating liniments to treat joint and muscular pain. Randomised double blind trials report that evidence of its effectiveness is weak, but stronger for acute pain than chronic pain. In the body, methyl salicylate metabolizes into salicylates, including salicylic acid. Methyl salicylate is a common component of massage oils used for topical treatment of joint and muscular pains. Artificial wintergreen oil, which is pure methyl salicylate, is used in microscopy because of its high refractive index. Methyl salicylate also serves as a simulant for sulphur mustard in human decontamination studies, because it has a similar vapour pressure, water solubility and biological half-life but is low enough in toxicity to be applied safely to human skin.
Methyl salicylate is used in low concentrations (0.04% and under) as a flavoring agent in root beer, chewing gum, mints and medicine. Wintergreen flavoring is common in dental hygiene products such as mouthwash and toothpaste.
Safety and regulation
Most instances of human toxicity due to methyl salicylate are a result of overapplication of topical analgesics, as skin absorption occurs easily. Salicylic acid, the major metabolite of methyl salicylate, may accumulate in blood. Massage oils containing methyl salicylate may be toxic when taken orally. The ester is potentially the most toxic salicylate for infants and children, as 4 ml can be fatal. One milliliter (20 drops) of wintergreen oil is equivalent to about 1860 mg of aspirin, or almost six regular-strength adult aspirin tablets. Salicylate sensitivity is a common adverse reaction to the methyl salicylate in oil of wintergreen; it can produce allergy-like symptoms or asthma. The oral LD50 in rats is 887 mg/kg. The EU Cosmetics Regulation has an Annex III entry for methyl salicylate. In the EU, toothpaste containing methyl salicylate may have a maximum concentration of 2.52%, whereas the UK limits this to 2.5%. The EU does not permit the ingredient in products intended for children under 6 years of age except toothpaste.
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