Salicylic acid


Salicylic acid is a lipophilic monohydroxybenzoic acid, a type of phenolic acid, and a beta hydroxy acid. It has the formula C7H6O3. 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 serving as an important active metabolite of aspirin, which acts in part as a prodrug to salicylic acid, it is probably best known for its use as a key ingredient in topical anti-acne products. The salts and esters of salicylic acid are known as salicylates.
It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system.

Uses

Medicine

Salicylic acid as a medication is used most commonly to help remove the outer layer of the skin. As such, it is used to treat warts, psoriasis, acne, ringworm, dandruff, and ichthyosis.
Similar to other hydroxy acids, salicylic acid is a key ingredient in many skincare products for the treatment of seborrhoeic dermatitis, acne, psoriasis, calluses, corns, keratosis pilaris, acanthosis nigricans, ichthyosis and warts.

Uses in manufacturing

Salicylic acid is used in the production of other pharmaceuticals, including 4-aminosalicylic acid, sandulpiride, and landetimide.
Salicylic acid was one of the original starting materials for making acetylsalicylic acid in 1897.
Bismuth subsalicylate, a salt of bismuth and salicylic acid, is the active ingredient in stomach relief aids such as Pepto-Bismol, is the main ingredient of Kaopectate and "displays anti-inflammatory action and also acts as an antacid and mild antibiotic".
Other derivatives include methyl salicylate used as a liniment to soothe joint and muscle pain and choline salicylate used topically to relieve the pain of mouth ulcers.

Other uses

Salicylic acid is used as a food preservative, a bactericidal and an antiseptic.
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.
Aspirin can be prepared by the esterification of the phenolic hydroxyl group of salicylic acid with the acetyl group from acetic anhydride or acetyl chloride.

Mechanism of action

Salicylic acid modulates COX2 gene expression to decrease the formation of pro-inflammatory prostaglandins. Salicylate may competitively inhibit prostaglandin formation. Salicylate's antirheumatic actions are a result of its analgesic and anti-inflammatory mechanisms.
Salicylic acid 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 competitively with nicotinamide adenosine dinucleotide 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

17% to 27% salicylic acid used in the form of a paint, and 20% to 50% in plaster form, which are sold for wart and corn removal should not be applied to the face and should not be used for acne treatment. Even for wart removal, such a solution should be applied once or twice a day – more frequent use may lead to an increase in side-effects without an increase in efficacy.
If high concentrations of salicylic ointment are applied to a large percentage of body surface, high levels of salicylic acid can enter the blood, requiring hemodialysis to avoid further complications.

Chemistry and production

Salicylic acid has the formula C6H4COOH, where the OH group is ortho to the carboxyl group. It is also known as 2-hydroxybenzoic acid. It is poorly soluble in water.
Salicylic acid is biosynthesized from the amino acid phenylalanine. In Arabidopsis thaliana it can be synthesized via a phenylalanine-independent pathway.
Sodium salicylate is commercially prepared by treating sodium phenolate with carbon dioxide at high pressure and high temperature – a method known as the Kolbe-Schmitt reaction. Acidification of the product with sulfuric acid gives salicylic acid:
It can also be prepared by the hydrolysis of aspirin or methyl salicylate with a strong acid or base.
Salicylic acid degrades to phenol and carbon dioxide at 200 - 230°C:

History

, Galen, Pliny the Elder and others knew that willow bark could ease pain and reduce fevers. It was used in Europe and China to treat these conditions. This remedy is mentioned in texts from ancient Egypt, Sumer and Assyria. The Cherokee and other Native Americans use an infusion of the bark for fever and other medicinal purposes.
In 2014, archaeologists identified traces of salicylic acid on 7th century pottery fragments found in east central Colorado. The Reverend Edward Stone, a vicar from Chipping Norton, Oxfordshire, England, noted in 1763 that the bark of the willow was effective in reducing a fever.
The active extract of the bark, called salicin, after the Latin name for the white willow, was isolated and named by the German chemist Johann Andreas Buchner in 1828. A larger amount of the substance was isolated in 1829 by Henri Leroux, a French pharmacist. Raffaele Piria, an Italian chemist, 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 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.

Dietary sources

Salicylic acid occurs in plants as free salicylic acid and its carboxylated esters and phenolic glycosides. Several studies suggest that humans metabolize salicylic acid in measurable quantities from these plants. High-salicylate beverages and foods include beer, coffee, tea, numerous fruits and vegetables, sweet potato, nuts, and olive oil, among others. Meat, poultry, fish, eggs, dairy products, sugar, and breads and cereals have low salicylate content.
Some people with sensitivity to dietary salicylates may have symptoms of allergic reaction, such as bronchial asthma, rhinitis, gastrointestinal disorders, or diarrhea, and so may need to adopt a low-salicylate diet.

Plant hormone

Salicylic acid is a phenolic phytohormone and is found in plants with roles in plant growth and development, photosynthesis, transpiration, ion uptake and transport. Salicylic acid is involved in endogenous signaling, mediating in plant defense against pathogens. It plays a role in the resistance to pathogens by inducing the production of pathogenesis-related proteins.
It is involved in the systemic acquired resistance in which a pathogenic attack on one part of the plant induces resistance in other parts. The signal can also move to nearby plants by salicylic acid being converted to the volatile ester methyl salicylate.Methyl salicylate is taken up by the stomata of the nearby plant, and once deep in the leaf, is converted back to salicylic acid to induce the immune response.