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Salicylic acid

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Salicylic acid
Names
IUPAC name
2-Hydroxybenzoic acid
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.000.648 Edit this at Wikidata
EC Number
  • 200-712-3
  • OC(=O)c1ccccc1O
Properties
C7H6O3
Molar mass 138.12 g/mol
Density 1.44 g/cm3
Melting point 159 °C
Boiling point 211 °C (2666 Pa)
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Salicylic acid (from the Latin word for the willow tree, Salix, from whose bark it can be obtained) is a beta hydroxy acid (BHA) with the formula C6H4(OH)COOH, where the OH group is adjacent to the carboxyl group. This colorless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is derived from the metabolism of salicin. In addition to being a compound that is chemically similar to but not identical to the active component of aspirin (acetylsalicylic acid), it is probably best known for its use in anti-acne treatments . It is poorly soluble in water (0.2 g/100 ml H2O at 20°C).[1]

Plant hormone

Salicylic acid (SA) is a phytohormone; and a phenol,is found everywhere in plants generating a significant impact on plant growth and development, photosynthesis, transpiration, ion uptake and transport and also induces specific changes in leaf anatomy and chloroplast structure. SA is recognized as an endogenous signal, mediating in plant defense, against pathogens[2] It plays a role in the resistance of pathogens by inducing the production of 'pathogenesis-related proteins'. It is involved in the systemic acquired resistance [SAR] in which a pathogenic attack on older leaves causes the development of resistance in younger leaves, though whether SA is the transmitted signal is debatable.

Production

Salicylic acid is an organic acid biosynthesized from the amino acid phenylalanine.

Sodium salicylate is commercially prepared by treating sodium phenolate (the sodium salt of phenol) with carbon dioxide at high pressure (100 atm) and high temperature (390K) - a method known as the Kolbe-Schmitt reaction. Acidification of the product with sulfuric acid gives salicylic acid:


It can be prepared by the hydrolysis of Aspirin (acetylsalicylic acid)[3] or methyl salicylate (Oil of Wintergreen) with a strong acid or base.

History

Main article: History of aspirin
White willow (Salix alba) is a natural source of salicylic acid

The Greek physician Hippocrates wrote in the 5th century BC about a bitter powder extracted from willow bark that could ease aches and pains and reduce fevers. This remedy was also mentioned in texts from ancient Sumer, Lebanon, and Assyria. The Cherokee and other Native Americans used an infusion of the bark for fever and other medicinal purposes for centuries.[4] The medicinal part of the plant is the inner bark and was used as a pain reliever for a variety of ailments. The Reverend Edward (Edmund) Stone, a vicar from Chipping Norton, Oxfordshire, England, noted in 1763 that the bark of the willow was effective in reducing a fever.[5]

The active extract of the bark, called salicin, after the Latin name for the white willow (Salix alba), was isolated in crystalline form in 1828 by Henri Leroux, a French pharmacist, and Raffaele Piria, an Italian chemist. Piria was able to convert the substance into a sugar and a second component, which on oxidation becomes salicylic acid.

Salicylic acid was also isolated from the herb meadowsweet (Filipendula ulmaria, formerly classified as Spiraea ulmaria) by German researchers in 1839. While their extract was somewhat effective, it also caused digestive problems such as gastric irritation, bleeding, diarrhea, and even death when consumed in high doses.

Medicinal and cosmetic uses

Also known as 2-hydroxybenzoic acid, one of several beta hydroxy acids (compare to AHA), salicylic acid is a key ingredient in many skin-care products for the treatment of acne, psoriasis, calluses, corns, keratosis pilaris, and warts. It works by causing the cells of the epidermis to shed more readily, preventing pores from clogging up, and allowing room for new cell growth. Because of its effect on skin cells, salicylic acid is used in several shampoos used to treat dandruff. Salicylic acid is also used as an active ingredient in gels that remove warts. Use of concentrated solutions of salicylic acid may cause hyperpigmentation on unpretreated skin for those with darker skin types (Fitzpatrick phototypes IV, V, VI), as well as with the lack of use of a broad spectrum sunblock.[6][7]

The medicinal properties of salicylate, mainly for fever relief, have been known since ancient times, and it was used as an anti-inflammatory drug.[8]

Cotton pads soaked in salicylic acid can be used to chemically exfoliate skin

Aspirin (acetylsalicylic acid or ASA) can be prepared by the esterification of the phenolic hydroxyl group of salicylic acid.

Subsalicylate in combination with bismuth form the popular stomach relief aid known commonly as Pepto-Bismol. When combined, the two key ingredients help control diarrhoea, nausea, heartburn, and gas. It is also a very mild antibiotic.

Choline salicylate is used topically to relieve the pain of aphthous ulcers.

Other uses

  • Although toxic in large quantities, salicylic acid is used as a food preservative and antiseptic in toothpaste. For some people with salicylate sensitivity even these small doses can be harmful.
  • Sodium salicylate is a useful phosphor in the vacuum ultraviolet with nearly flat quantum efficiency for wavelengths between 10 to 100 nm.[9] 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.

Safety

Salicylic acid has an ototoxic effect by inhibiting prestin.[10] It can induce transient hearing loss in zinc-deficient individuals.

This finding is based on clinical studies with rats. An injection of salicylic acid induced hearing loss in zinc-deficient rats, while a simultaneous injection of zinc reversed the hearing loss. An injection of magnesium in the zinc-deficient rats did not reverse the salicylic acid-induced hearing loss.

Salicylic acid is used to treat acne, warts and other dermatological problems. There are no studies specifically looking at topical salicylic acid in pregnancy. Oral salicylic acid (aspirin) has not been associated with an increase in malformations if used during the first trimester, but use in late pregnancy has been associated with bleeding, especially intracranial bleeding (Rumack et al., 1981). The risks of aspirin late in pregnancy are probably not relevant for a topical exposure to salicylic acid, even late in the pregnancy, because of its low systemic levels. Topical salicylic acid is common in many over-the-counter dermatological agents, and the lack of adverse reports suggests a low teratogenic potential.[11]

Some people are hypersensitive to salicylic acid and related compounds.

The United States Food and Drug Administration recommends the use of sun protection when using skincare products containing salicylic acid (or any other BHA) on sun-exposed skin areas.[12]

See also

Trolamine salicylate

Footnotes

  1. ^ "Salicilyc acid". Retrieved 2008-10-13.
  2. ^ S. Hayat, A. Ahmad (2007). Salicylic acid - A Plant Hormone. Springer. ISBN 1402051832.
  3. ^ "Hydrolysis of ASA to SA". {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  4. ^ Paul B. Hemel and Mary U. Chiltoskey, Cherokee Plants and Their Uses -- A 400 Year History, Sylva, NC: Herald Publishing Co. (1975); cited in Dan Moerman, A Database of Foods, Drugs, Dyes and Fibers of Native American Peoples, Derived from Plants.[1] A search of this database for "salix AND medicine" finds 63 entries.
  5. ^ Stone, E (1763). "An Account of the Success of the Bark of the Willow in the Cure of Agues". Philosophical Transactions. 53: 195–200. doi:10.1098/rstl.1763.0033.
  6. ^ Grimes P.E. (1999). "The Safety and Efficacy of Salicylic Acid Chemical Peels in Darker Racial-ethnic Groups". Dermatologic Surgery. 25: 18–22. doi:10.1046/j.1524-4725.1999.08145.x.
  7. ^ Roberts W. E. (2004). "Chemical peeling in ethnic/dark skin". Dermatologic Therapy. 17 (2): 196. doi:10.1111/j.1396-0296.2004.04020.x.
  8. ^ Philip A. Mackowiak (2000). "Brief History of Antipyretic Therapy". Clinical Infectious Diseases,. 31: 154–156. doi:10.1086/317510.{{cite journal}}: CS1 maint: extra punctuation (link)
  9. ^ JAR Samson Techniques of Vacuum Ultraviolet Spectroscopy
  10. ^ Wecker, H.; Laubert, A. (2004). "Reversible hearing loss in acute salicylate intoxication". HNO (in German). 52 (4): 347–51. doi:10.1007/s00106-004-1065-5. PMID 15143764.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ Acne and Pregnancy
  12. ^ "Beta Hydroxy Acids in Cosmetics". Retrieved 2007-11-23.
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