Salicylic acid is an organic compound with the formula C7H6O3. A colorless (or white), bitter-tasting solid, it is a precursor to and a metabolite of acetylsalicylic acid (aspirin). It is a plant hormone, and the name is from Latin salix for willow tree, from which it was initially identified and derived. It is used as an ingredient in dermatologic cosmetics (e.g. anti-acne products). Salts and esters of salicylic acid are known as salicylates. In the USA, it has been listed by the EPA Toxic Substances Control Act (TSCA) Chemical Substance Inventory as an experimental teratogen.
Uses
Medicine
Salicylic acid as a medication is commonly used to remove the outermost layer of the skin. As such, it is used to treat warts, psoriasis, acne vulgaris, ringworm, dandruff, and ichthyosis. Similar to other hydroxy acids, salicylic acid is an ingredient in many skincare products for the treatment of seborrhoeic dermatitis, acne, psoriasis, calluses, corns, keratosis pilaris, acanthosis nigricans, ichthyosis, and warts. In the European Union, salicylic acid is regulated as a cosmetic ingredient under Annex III (entry 98) of the Cosmetics Regulation (EC) No 1223/2009. For uses other than as a preservative it is permitted at concentrations up to 3.0% in rinse-off hair products and 2.0% in other products, with a lower limit of 0.5% in certain leave-on products such as body lotion and eye and lip cosmetics; it is separately permitted as a preservative at up to 0.5%. Reflecting its classification as toxic to reproduction (CMR category 2), it may not be used in products intended for children under three years of age (other than shampoos), in oral products, or in applications that could be inhaled.
Uses in manufacturing Salicylic acid is used as a food preservative, a bactericide, and an antiseptic. Salicylic acid is used in the production of other pharmaceuticals, including 4-aminosalicylic acid, sandulpiride, and landetimide (via salethamide). It is also used in picric acid production. Salicylic acid has long been a key starting material for making acetylsalicylic acid (ASA or aspirin). ASA is prepared by the acetylation of salicylic acid with the acetyl group from acetic anhydride or acetyl chloride. ASA is the standard to which all the other non-steroidal anti-inflammatory drugs (NSAIDs) are compared. In veterinary medicine, this group of drugs is mainly used for treatment of inflammatory musculoskeletal disorders. Bismuth subsalicylate, a salt of bismuth and salicylic acid, "displays anti-inflammatory action (due to salicylic acid) and also acts as an antacid and mild antibiotic". It is an active ingredient in stomach-relief aids such as Pepto-Bismol and some formulations of Kaopectate. Other derivatives include methyl salicylate, used as a liniment to soothe joint and muscle pain, and choline salicylate, which is used topically to relieve the pain of mouth ulcers. Aminosalicylic acid is used to induce remission in ulcerative colitis, and has been used as an antitubercular agent often administered in association with isoniazid. Sodium salicylate is a useful phosphor in the vacuum ultraviolet spectral range, with nearly flat quantum efficiency for wavelengths between 10 and 100 nm. It fluoresces in the blue at 420 nm. It is easily prepared on a clean surface by spraying a saturated solution of the salt in methanol followed by evaporation.
Mechanism of action Salicylic acid modulates COX-1 enzymatic activity to decrease the formation of pro-inflammatory prostaglandins. Salicylate may competitively inhibit prostaglandin formation. Salicylic acid, when applied to the skin surface, works by causing the cells of the epidermis to slough off more readily, preventing pores from clogging up, and allowing room for new cell growth. Salicylic acid inhibits the oxidation of uridine-5-diphosphoglucose (UDPG) competitively with NADH and noncompetitively with UDPG. It also competitively inhibits the transferring of glucuronyl group of uridine-5-phosphoglucuronic acid to the phenolic acceptor. The wound-healing retardation action of salicylates is probably due mainly to its inhibitory action on mucopolysaccharide synthesis.
Safety
In excess, salicylates have toxic effects, which can be fatal. Toxicity is most often due to oral overdose. Cosmetic applications of the drug pose no significant risk. Even in a worst-case use scenario in which one was using multiple salicylic acid-containing topical products, the aggregate plasma concentration of salicylic acid was well below what was permissible for acetylsalicylic acid (aspirin). Since oral aspirin (which produces much higher salicylic acid plasma concentrations than dermal salicylic acid applications) poses no significant adverse pregnancy outcomes in terms of frequency of stillbirth, birth defects or developmental delay, use of salicylic acid containing cosmetics is safe for pregnant women. Salicylic acid is present in most fruits and vegetables as for example in greatest quantities in berries and in beverages like tea. In one documented case, a patient applied extreme levels of salicylate ointment topically (40% ointment, over 41% of the total skin surface), and subsequently received hemodialysis to reduce blood salicylate concentration.
Production and chemical reactions
Biosynthesis Salicylic acid is biosynthesized from the amino acid phenylalanine. In Arabidopsis thaliana, it can be synthesized via a phenylalanine-independent pathway.
Chemical synthesis Commercial vendors prepare sodium salicylate by treating sodium phenolate (the sodium salt of phenol) with carbon dioxide at high pressure (100 atm) and high temperature (115 °C) – a method known as the Kolbe-Schmitt reaction. Acidifying the product with sulfuric acid gives salicylic acid:
At the laboratory scale, it can also be prepared by the hydrolysis of aspirin (acetylsalicylic acid) or methyl salicylate (oil of wintergreen) with a strong acid or base; these reactions reverse those chemicals' commercial syntheses.
Reactions Upon heating, salicylic acid converts to phenyl salicylate:
Further heating gives xanthone. Salicylic acid as its conjugate base is a chelating agent, with an affinity for iron(III). Salicylic acid slowly degrades to phenol and carbon dioxide at 200–230 °C:
All isomers of chlorosalicylic acid and of dichlorosalicylic acid are known. 5-Chlorosalicylic acid is produced by direct chlorination of salicylic acid.
History
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