Salicylic acid

Plant Hormone

Salicylic acid (SA) is a pathogens^[1] 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. SA is the calorigenic substance that causes thermogenesis in Arum flowers.^[2]

Production

Salicylic acid is biosynthesized from the amino acid phenylalanine.

Sodium salicylate is commercially prepared by treating sodium Kolbe-Schmitt reaction. Acidification of the product solution gives salicylic acid:

It can be prepared by the methyl salicylate (Oil of Wintergreen) with a strong acid or base.

Analysis

Salicylic acid is an enol of an β-ketocarbonic acid and therefore forms purple complexes with iron(III) salts:

This tris(chelate) complex forms more readily in basic solution.

History

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 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 shampoos used to treat dandruff. Salicylic acid is also used as an active ingredient in gels which remove verrucas (plantar 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]

  esterification of the phenolic hydroxyl group of salicylic acid.

Subsalicylate in combination with antibiotic.

Other uses

• Although toxic in large quantities, salicylic acid is used as a food preservative and salicylate sensitivity even these small doses can be harmful.
• Sodium salicylate is a useful methanol followed by evaporation.

Safety

Salicylic acid has an ototoxic effect,^{[citation needed]} and 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.^{[citation needed]}

Salicylic acid is toxic in large amounts. Pregnant women are advised not to use products containing salicylic acid due to the danger of Reye's syndrome.^{[citation needed]}

Some people are hypersensitive to salicylic acid and related compounds.^{[citation needed]}

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]