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Salicylic Acid

C7H6O3 acid
Molar Mass
138.121 g/mol
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Properties

StateSolid (white crystalline needles)
ColorWhite
SolubilitySlightly soluble in water (2.2 g/L at 20 °C); soluble in ethanol, ether, acetone
Melting Point159 °C
Boiling Point211 °C (sublimes)

About Salicylic Acid

Salicylic acid (2-hydroxybenzoic acid, C7H6O3, 138.121 g/mol) is a white crystalline aromatic hydroxy acid in which an ortho-positioned phenolic OH and a carboxylic acid sit on the same benzene ring, and the intramolecular hydrogen bond between them is the reason the compound has its characteristic chemistry: a pKa of 2.97 (much more acidic than benzoic acid at 4.20), a sharp melting point at 159 C, and a tendency to sublime gently at 211 C without a true boiling point. The compound occurs naturally as the SAM-derived defense hormone in plants and is concentrated in the bark of Salix willows and the meadowsweet plant Spiraea ulmaria — the second of which gave the drug aspirin its name. Hippocrates documented willow-bark tea for fever and pain, and 19th-century chemists isolated salicin (1828, Leroux), reduced it to salicylic acid (1838, Piria), and finally industrialized synthesis through the Kolbe-Schmitt carboxylation of sodium phenoxide with CO2 at 125 C and 100 atm. Felix Hoffmann at Bayer acetylated the phenolic OH with acetic anhydride in 1897 to make acetylsalicylic acid (aspirin), reducing the gastric irritation that free salicylic acid causes at oral doses while preserving the COX-1 and COX-2 inhibition that delivers analgesia, anti-inflammation, and antiplatelet activity. The same Kolbe-Schmitt chemistry is still used to make megaton-scale quantities of salicylic acid for aspirin, methyl salicylate (oil of wintergreen), and the salicylate-based dyes and preservatives that came after.

Where you'll encounter it

If you have ever spot-treated a stubborn acne breakout with a 2% salicylic acid serum from CeraVe or Paula's Choice, you have seen the lipophilic beta-hydroxy acid penetrate sebum-clogged pores, dissolve the keratin glue holding dead corneocytes together (the keratolytic action), and unblock the comedone — the same reason podiatrists prescribe 17-40% salicylic acid plasters for plantar warts and corn removal. In undergraduate organic chemistry the aspirin synthesis lab (salicylic acid plus acetic anhydride with H3PO4 catalyst, recrystallized from ethanol-water) is one of the universal first-year experiments, complete with the FeCl3 spot test for unreacted phenol. In food science, methyl salicylate at trace levels is what makes wintergreen Lifesavers spark in the dark — triboluminescence from the methyl salicylate plus sucrose lattice when chewed in a dark room, a classic kitchen-physics demonstration. Salicylic acid is also the active substance behind the willow-tea remedies that show up in folk medicine across nearly every continent, because Salicaceae and Spiraea grow nearly everywhere.

Common Uses

  • Beta-hydroxy acid keratolytic for acne, comedones, and dandruff at 0.5-2% topical
  • Industrial precursor for aspirin via acetic anhydride acetylation
  • Wart, corn, and callus removal at 17-40% in plasters and gels
  • Plant signaling hormone studied in agricultural and pathology research
  • Preservative and antiseptic in cosmetics and food at trace levels

Safety Information

GHS: H302 harmful if swallowed, H318 causes serious eye damage. Skincare formulations at 0.5-2% are well tolerated and pregnancy-safe per the American Academy of Dermatology. Reye's syndrome risk in children with viral illness is the reason aspirin and salicylates are contraindicated under age 16. Chronic high-dose salicylate intake produces salicylism (tinnitus, dizziness, hyperventilation) and overdose can trigger respiratory alkalosis followed by metabolic acidosis. ACGIH TLV-TWA for the dust is 5 mg/m3. PPE for industrial handling: dust mask, splash goggles, nitrile gloves. The pure powder can sublime, so containers should be tightly sealed.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

C — Carbon H — Hydrogen O — Oxygen

Frequently Asked Questions

What is the molar mass of salicylic acid?
Salicylic acid (C7H6O3) has a molar mass of 138.121 g/mol, from 7 C (84.077) plus 6 H (6.048) plus 3 O (47.997). Acetylation by acetic anhydride to make aspirin adds a C2H2O group, increasing the molar mass to 180.158 g/mol — useful in the percent-yield calculation that always shows up in the undergraduate aspirin-synthesis lab.
How does salicylic acid become aspirin?
Aspirin is acetylsalicylic acid — the phenolic OH on salicylic acid esterified with acetic anhydride to give a phenyl acetate. The standard synthesis uses salicylic acid, excess acetic anhydride, and a catalytic amount of phosphoric acid or concentrated H2SO4, heated to about 80-90 C for 15 minutes, then quenched in ice water and recrystallized from hot ethanol-water. The acetyl group masks the phenolic OH that irritates gastric mucosa while preserving the carboxylic acid that the body partially hydrolyzes back to salicylate after absorption. The acetyl group is the part that does the irreversible covalent serine modification of cyclooxygenase that gives aspirin its days-long antiplatelet effect even though plasma half-life is only 15 minutes.
Why does salicylic acid work so well on acne?
Salicylic acid is lipophilic (LogP around 2.26) and the only common hydroxy acid that partitions into sebum, so it can travel through the oily plug filling a comedone in a way that water-soluble alpha-hydroxy acids like glycolic acid cannot. Once inside the pilosebaceous unit it dissolves the desmosome cement holding dead keratinocytes together (keratolysis) and clears the obstruction. It also has mild anti-inflammatory activity (downregulating prostaglandin synthesis at sufficient concentration) and a small antibacterial effect against Cutibacterium acnes. That triple action — penetration, exfoliation, anti-inflammatory — is why it remains the first-line OTC acne treatment for non-cystic comedonal acne, decades after its introduction.