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Salicylates
Salicylates
General
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CAS number [?-?-?]
Properties
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Solubility in water  ? g/100 ml (?°C)
Melting point  ?°C (? K)
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Acidity (pKa)  ?
Basicity (pKb)  ?
Chiral rotation [α]D  ?°
Viscosity  ? cP at ?°C
Structure
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Dipole moment  ? D
Hazards
MSDS External MSDS
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NFPA 704
Flash point  ?°C
R/S statement R: ?
S: ?
RTECS number  ?
Supplementary data page
Structure and
properties
n, εr, etc.
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data
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Spectral data UV, IR, NMR, MS
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Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references


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. The salts and esters of salicylic acid are known as salicylates.


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. It is probably best known as a compound that is chemically similar to but not identical to the active component of aspirin (acetylsalicylic acid). In fact, salicylic acid is a metabolite of aspirin, the product of esterase hydrolysis in the liver. It is poorly soluble in water (0.2 g/100 ml H2O at 20°C).[1]


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:

Kolbe-Schmitt

It can be prepared by the hydrolysis of Aspirin (acetylsalicylic acid)[2] or methyl salicylate (Oil of Wintergreen) with a strong acid or base.

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 which 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.[3][4]

The medicinal properties of salicylate, mainly for fever relief, have been known since ancient times, and it was used as an anti-inflammatory drug.[5]

File:Salicylic acid pads.jpg

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 diarrhea, 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.[6] 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.
  • Methyl salicylate is used as a liniment, to soothe joint and muscle pain.

Safety

Salicylic acid has an ototoxic effect by inhibiting prestin.[7] 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.[8]

Some people are hypersensitive to salicylic acid and related compounds.

Salicylic acid overdose can lead to salicylate intoxication, which often presents clinically in a state of metabolic acidosis with compensatory respiratory alkalosis. In patients presenting with an acute overdose, a 16% morbidity rate and a 1% mortality rate are observed.[9]

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.[10]

There is data to support an association between exposure to salicylic acid and Reye's Syndrome. The National Reye's Syndrome Foundation cautions against the usage of these substances, and other substances similar to aspirin, on children and adolescents.

Footnotes

  1. Salicilyc acid. URL accessed on 2008-10-13.
  2. Hydrolysis of ASA to SA.
  3. Grimes P.E. (1999). The Safety and Efficacy of Salicylic Acid Chemical Peels in Darker Racial-ethnic Groups. Dermatologic Surgery 25: 18–22.
  4. Roberts W. E. (2004). Chemical peeling in ethnic/dark skin. Dermatologic Therapy 17 (2): 196.
  5. Philip A. Mackowiak (2000). Brief History of Antipyretic Therapy. Clinical Infectious Diseases, 31: 154–156.
  6. JAR Samson Techniques of Vacuum Ultraviolet Spectroscopy
  7. Wecker, H.; Laubert, A. (2004). Reversible hearing loss in acute salicylate intoxication. HNO 52 (4): 347–51.
  8. Acne and Pregnancy
  9. http://emedicine.medscape.com/article/818242-overview
  10. Beta Hydroxy Acids in Cosmetics. URL accessed on 2007-11-23.


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