Salicylic acid

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16 atoms, 16 bonds.

IUPAC name: 2-hydroxybenzoic acid
Molar mass: 138.122(18) g/mol
Atoms: 16
SMILES: C1=CC=C(C(=C1)C(=O)O)O
InChIKey: YGSDEFSMJLZEOE-UHFFFAOYSA-N
PubChem CID: 338

Made of

Structural formula of salicylic acid. Each unlabelled corner is a carbon atom, and the four hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

The willow, salix in Latin, gave salicylic acid its name, and Chem1 records that chewing willow served as a folk remedy for pain and fever from ancient times. The systematic name, 2-hydroxybenzoic acid, describes the molecule. ChEBI, a reference database of biological molecules, calls it benzoic acid carrying a hydroxy group, OH, at the ortho position, the ring carbon next door to the acid group.

It crystallises as white needles or a fluffy white powder, sometimes tinged light tan, and sunlight slowly discolours it. It can also sublime, passing from solid straight to vapour, as the International Chemical Safety Cards (ICSC) note. Water dissolves only 0.2 g per 100 mL at 20 °C. That fits a rule for carboxylic acids from CAMEO Chemicals, the US emergency-response database: those with more than six carbon atoms are only slightly soluble, and this one has seven.

Structure and bonding

The model holds 16 atoms, seven carbon, six hydrogen and three oxygen. Six carbons close a hexagon, four of them capped by hydrogen; of the other two, one holds the carboxyl group, COOH, and its neighbour the OH. With Labels on, only two H symbols stand on oxygen, and both can leave as ions. The Hazardous Substances Data Bank (HSDB) gives the carboxyl hydrogen a pK_a of 2.98 and the hydrogen on the ring’s OH 13.6. On that scale a lower number marks a hydrogen that leaves more easily.

Space-filling shows the carboxyl group and the OH as one crowded edge of the ring, all three oxygens together. Ball-and-stick and Stick draw the ring as alternating double and single bonds, three of each, yet in the model all six ring bonds measure 1.395 Å. In the real ring the double-bond electrons spread round all six carbons, as in benzene. Only the neutral acid is drawn; in water part of it gives up the carboxyl hydrogen and becomes salicylate ions, a part whose size depends on the pH.

Where it turns up

Plants make it. The NCI Thesaurus, the US National Cancer Institute’s terminology, describes it as occurring naturally in plants. ChEBI gives it the roles of plant hormone and of plant and algal metabolite, a substance made or used in a living thing’s metabolism. White willow bark is one source, according to ChEBI and MeSH, the US National Library of Medicine’s subject headings. The LOTUS natural-products database records it in Camellia sinensis and in Dischidia formosana. HSDB lists it among the free phenolic acids found in fractions of wheat.

Personal-care and pet-care products contain it, according to the US Consumer Product Information Database. NORMAN, a network of environmental laboratories, records two jobs for it in cosmetics: preservative and keratolytic, an agent that softens the skin’s horny outer layer until it flakes away. The International Fragrance Association (IFRA) includes it among fragrance ingredients.

Why it matters

Industry makes it by the Kolbe–Schmitt process from sodium phenoxide and carbon dioxide, HSDB records, and some is made from birch oil. Most becomes acetylsalicylic acid, aspirin: about 60 per cent of US output went that way in the late 1980s and early 1990s. The acid itself proved too harsh on the stomach lining, Chem1 records, which is why the German firm Bayer began selling the acetyl form as Aspirin.

Technical-grade acid also goes into agrochemicals, dyes and phenolic resins. It has been used as a food preservative, and formerly as a fungicide. The US Environmental Protection Agency (EPA) puts national volume for 2023 at between 1 and 5 million pounds.

Medicine uses it on the skin. The US Food and Drug Administration’s rules for over-the-counter medicines list it as an active ingredient in acne products, wart removers and products for dandruff, seborrhoeic dermatitis and psoriasis. It acts as a keratolytic. In HSDB’s account it raises the water held in the skin’s horny outer layer, probably by lowering the pH, and those cells swell, soften and peel away.

Properties

Properties of salicylic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 338.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₇H₆O₃
Molar mass	138.122(18) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	7
Hydrogen atoms	6
Oxygen atoms	3
Bonds	16
Single bonds	12
Double bonds	4
PubChem molecular weight	138.12 g/mol	g/mol	PubChem

Safety

Salicylic acid carries the signal word Danger in the EU’s harmonised classification: harmful if swallowed, it can cause serious eye damage and is suspected of damaging the unborn child. An acute overdose can bring on salicylism. Haz-Map, a US occupational health database, lists its signs as ringing in the ears, confusion, fast breathing, vomiting and, in severe cases, seizures and coma. The PubChem Laboratory Chemical Safety Summary for salicylic acid gathers the full classification.

Questions

What does a salicylic acid molecule look like?

Six carbon atoms form a ring, and two neighbouring ring carbons carry the two groups: a carboxyl group, COOH, and a hydroxy group, OH. The 3D model shows all 16 atoms joined by 16 bonds; four bonds are drawn double, three in the ring and the C=O of the carboxyl group. The structural formula draws the seven carbons as bare corners and leaves out the four hydrogens bonded to carbon.

Is salicylic acid a strong acid?

No. The International Chemical Safety Cards describe its solution in water as a weak acid, and the Hazardous Substances Data Bank gives a saturated solution a pH of 2.4. Only part of the dissolved acid lets go of a hydrogen ion, from the carboxyl group, leaving salicylate ions behind. Carboxylic acids give up hydrogen ions when a base is there to take them, CAMEO Chemicals notes.

Is salicylic acid absorbed through the skin?

Yes, and quickly: the Hazardous Substances Data Bank describes rapid absorption through intact skin. In guinea pigs with the outer skin layer removed, uptake rose as a larger share of the acid stayed un-ionised, in its neutral form rather than as salicylate ions. Once inside, the body joins it to glycine, giving salicyluric acid, or to glucuronic acid, giving glucuronides, and turns a little into gentisic acid by adding a further OH group.

What is salicylic acid used for in medicine?

The World Health Organization's Anatomical Therapeutic Chemical (ATC) classification places it among antifungals for the skin and among non-steroidal anti-inflammatory eye medicines. The WHO Model List of Essential Medicines includes it. MeSH, the National Library of Medicine's subject headings, credits it with bacteriostatic actions, which hold back bacterial growth, and fungicidal ones. It also has direct anti-inflammatory activity, the US National Cancer Institute states, and fights bacteria on skin by speeding the shedding of surface cells.

Made of the same elements

C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.