General · EdgepediaSeptember 17, 20266 min read

Salicylic acid

Salicylic acid (2-hydroxybenzoic acid) is a white crystalline solid, formula C₇H₆O₃, first isolated from the bark of willow trees (Salix spp.), from which it takes its name, and occurring as the free acid or its esters in many plant species.¹ It is the parent compound of aspirin (acetylsalicylic acid), a widely used topical keratolytic drug, a food preservative, and a major defense hormone in plants.¹^•²

Key fact	Detail
Identity	2-hydroxybenzoic acid, C₇H₆O₃, molecular weight 138.12 g/mol, CAS Registry Number 69-72-7³
Acidity	pKa1 = 2.98 at 20 °C (carboxylic acid); pKa2 = 13.6 (phenol)³
Physical data	Melting point 158 °C; solubility in water 0.2 g/100 mL at 20 °C⁴^•³
Natural sources	Willow bark (salicin), wintergreen leaves and sweet birch bark (methyl salicylate), which hydrolyze to salicylic acid³^•⁵
Industrial synthesis	Kolbe–Schmitt reaction: sodium phenoxide carboxylated with CO₂ under pressure, then acidified⁶
OTC drug uses	Acne treatments at 0.5 to 2 percent (FDA monograph M006); wart removers applied for up to 12 weeks⁷^•⁸
Plant role	Defense hormone mediating systemic acquired resistance, perceived through the NPR1 receptor family²

Chemistry and physical properties

Salicylic acid is benzoic acid bearing a hydroxy group at the ortho position, so a single benzene ring carries both a carboxylic acid and a phenol. The carboxyl group dissociates with pKa1 = 2.98 at 20 °C and the phenolic proton with pKa2 = 13.6, making it a diprotic acid that exists mainly as the anion near neutral pH.³ It melts at 158 °C and is only slightly soluble in water, 0.2 g per 100 mL at 20 °C, but dissolves far more readily in alcohols and acetone.⁴^•³

The adjacent hydroxyl and carboxyl groups are chemically consequential. Acetylation of the hydroxyl group gives acetylsalicylic acid, and the reverse reaction happens in storage: old aspirin decomposes back to salicylic acid and acetic acid, which is why a stale bottle smells of vinegar.⁵ Salicylic acid is also a metabolite of aspirin in the body, while aspirin is described as a prodrug to salicylic acid.¹

History

Salicylate-containing plants have been used as medicines for millennia. Ancient Sumerian clay tablets mention willow leaves for treating rheumatoid arthritis, and the Ebers papyrus, from about 1550 BC, recommends dried myrtle leaves for pain. Hippocrates wrote in the 5th century BC of willow bark's curative powers, and willow, poplar, and myrtle also appear in the works of Pliny the Elder and Galen.⁹^•¹⁰

Isolation and synthesis. In the mid-1700s the Reverend Edward Stone described the quantitative treatment of fever with powdered willow bark in a note to the Royal Society. Chemists isolated the active ingredient, salicin, from willow bark in the 1820s and 1830s; the bitter extract took its name from the Latin for white willow, Salix alba, and became known as salicylic acid because of its acidity in water.⁹ In the mid-1800s the German chemist Hermann Kolbe synthesized salicylic acid in the laboratory by heating phenol with carbon dioxide, and the Heyden Chemical Company became the first to mass-produce it commercially.⁵^•¹¹

The free acid was effective but harsh: it irritated the stomach so severely that many patients preferred their aches to the remedy.⁵ In 1897 Felix Hoffmann, a chemist at Bayer, acetylated the hydroxyl group of salicylic acid using acetic anhydride, improving on Charles Gerhardt's earlier 1853 synthesis that had used acetyl chloride, to create acetylsalicylic acid. Bayer marketed the drug in 1899 under the name aspirin, from "a" for acetyl, "spir" for the spiraea flower, another natural source of salicin, and "in", a common drug ending, and obtained a U.S. patent in 1900.⁹^•¹⁰

Industrial synthesis

Commercial salicylic acid is made by the Kolbe–Schmitt reaction, a carboxylation that adds a carboxyl group to the benzene ring of phenol. Phenol is treated with sodium hydroxide to form sodium phenoxide, which is heated with carbon dioxide under pressure to give sodium salicylate; acidification then releases salicylic acid.⁶^•¹² Kolbe's original 1860 procedure ran at atmospheric pressure and about 180 °C; Schmitt's 1885 modification, reacting dry sodium phenoxide with CO₂ in a closed vessel at high pressure and temperature, raised yields substantially and remains the standard industrial route.⁶ Under the modified conditions the reaction yields up to 79 percent salicylic acid after 8 hours at 125 °C and CO₂ pressures of 82 to 138 bar.⁶

Alternative routes exist. In an early biosynthetic process from 1966, researchers at Kerr-McGee Oil Industries prepared salicylic acid by microbial degradation of naphthalene, and it is now commercially biosynthesized from phenylalanine.¹

Medical and cosmetic uses

Topically, salicylic acid is a keratolytic agent used against hyperkeratotic and scaling conditions including acne, psoriasis, calluses, corns, keratosis pilaris, dandruff, and warts; it causes cells of the epidermis to slough off more readily, preventing pores from clogging.³ Mechanistic work has refined this description: rather than breaking down keratin itself, salicylic acid disrupts the corneodesmosomes that hold corneocytes together, so dermatologic reviews now classify it as a desmolytic agent.¹³

Regulated concentrations. In the United States, salicylic acid is a permitted active ingredient in OTC topical acne products at 0.5 to 2 percent under FDA monograph M006, alongside benzoyl peroxide and sulfur.⁷ Wart remover products run higher: the wart remover monograph M028 covers salicylic acid plasters and liquid formulations, with directions allowing repeat application for up to 12 weeks, and marketed liquid products contain as much as 17 percent salicylic acid.⁸^•¹⁴ In cosmetics it is the beta hydroxy acid most commonly used, listed under names including salicylate, sodium salicylate, and willow extract; the Cosmetic Ingredient Review panel concluded in 2000 that it is safe as used when formulated to avoid irritation and increased sun sensitivity.¹⁵

Salicylic acid and its esters also serve as food preservatives, and the acid itself directly and reversibly inhibits the COX-1 and COX-2 enzymes, decreasing conversion of arachidonic acid to prostaglandin and thromboxane precursors, the basis of salicylate analgesic and anti-inflammatory activity.¹^•³

Salicylic acid in plants

In plants, salicylic acid is a defense hormone central to immunity against a broad spectrum of pathogens and to systemic acquired resistance, a whole-plant defensive state induced after local infection.² Perception runs through the NPR protein family: NPR1 acts as an SA receptor and transcriptional co-activator that, on SA accumulation, promotes defense gene expression, while NPR3/NPR4 act as transcriptional repressors whose inhibition by SA releases repression of defense genes during infection.²^•¹⁶ SA signaling also crosstalks with other hormones, including jasmonic acid and ethylene, to balance plant growth against defense.¹⁶ A 2015 study at the University of North Carolina at Chapel Hill additionally found that native salicylic acid helps determine which microbes colonize the root microbiome of Arabidopsis thaliana.¹

Production and supply

Starting in the 2000s, low-cost Chinese competition drove production of salicylic acid, along with several other important chemicals, out of the United States. In 2024 the U.S. Department of Defense funded Lacamas Laboratories of Portland, Oregon, to produce seven reshored compounds, including salicylic acid, as part of an initiative covering 22 chemicals, with plans for 28 more.¹

References

Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026; Oct 21, 2026