Aspirin

Ball-and-stick Stick Space-filling

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21 atoms, 21 bonds.

IUPAC name: 2-acetyloxybenzoic acid
Molar mass: 180.159(24) g/mol
Atoms: 21
SMILES: CC(=O)OC1=CC=CC=C1C(=O)O
InChIKey: BSYNRYMUTXBXSQ-UHFFFAOYSA-N
PubChem CID: 2244

Made of

Structural formula of aspirin. 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.

Two small groups sit side by side on one ring of carbon atoms, and the two behave differently. The acid group makes aspirin a weak acid, while the ester is the part that moist air and the body split apart. One is a carboxylic acid group, in which an oxygen carries a hydrogen atom. The other is an ester group, in which the second oxygen links to another carbon instead. Its name from IUPAC, the international union of chemists that sets naming rules, is 2-acetyloxybenzoic acid: a benzoic acid with an acetyloxy group on the neighbouring ring carbon.

Pure aspirin forms odourless white crystals with a slightly bitter taste. Industry makes it from salicylic acid and acetic anhydride. Only about 4.6 g dissolves in a litre of water at 25 °C. That solution is a weak acid: its acid group gives up some of its hydrogen ions, not all of them.

Structure and bonding

The model places 21 atoms, nine carbon, eight hydrogen and four oxygen, joined by 21 bonds. Its six-carbon ring is drawn with double and single bonds taking turns, three of each. OpenStax describes the real ring differently: all six carbon–carbon bonds are equivalent, each partway between a single and a double bond, and a substituted ring keeps that shared bonding. In PubChem’s computed conformer every ring bond measures 1.395 Å.

Turned edge-on in Ball-and-stick, the ring is a flat line, the acid group on one carbon and the ester on the next. Each group has a C=O double bond around a trigonal planar carbon. Oxygen has the higher electronegativity, the pull an atom exerts on shared electrons, so each C=O bond is polar.

In the Space-filling view the four red oxygen spheres bunch along one edge. Stick shows bonds alone, the doubled carbonyl sticks easy to pick out. With Labels on, every atom carries its element symbol, and the one H bonded to an O marks the acid group. The drawing is the neutral acid; in water a small share of molecules lose that hydrogen, as weak acids do.

Where it turns up

HSDB, the Hazardous Substances Data Bank quoted on PubChem, reported in 1981 that virtually all aspirin went into pharmaceutical products, alone or mixed with other ingredients. A 1973 split gave 54 per cent to combination painkillers and 46 per cent to plain aspirin tablets. The same database lists veterinary uses as a painkiller and anti-inflammatory, and California’s cosmetics programme counts 11 products that add it to calm irritation and inflammation.

An old bottle can give itself away by smell. Dry aspirin is stable, but moist air slowly hydrolyses it, splitting the ester into salicylic acid and acetic acid. Acetic acid is what makes vinegar smell of vinegar, so a tablet that has taken up damp develops that sharp odour. In water, aspirin is most stable between pH 2 and 3 and breaks down fastest below 2 or above 8.

Why it matters

Aspirin changes an enzyme permanently rather than blocking it for a while. Its acetyl group, the CH₃CO part of the ester, binds to a serine residue, one amino-acid unit, in cyclooxygenase-1 (COX-1). That enzyme leads to thromboxane and to prostaglandins, irritants that sharpen pain. The inhibition is irreversible.

In platelets the lost enzyme means less thromboxane A₂, the lipid that makes them clump into the start of a clot. Aggregation stays suppressed for about 7 to 10 days, the average platelet lifespan. That is why it is used to cut the risk of death or a further heart attack after a first one, or in unstable angina, a heart condition.

COX-2, the related enzyme, escapes more easily. DrugBank’s account is that its active site is slightly larger, so arachidonic acid, the molecule COX-2 works on, can slip past the bound aspirin. HSDB adds a second factor: the two enzymes recover their activity at very different rates. The effect is not one-sided either. Aspirin also lowers prostacyclin, which blocks clumping and widens vessels, the reverse of what thromboxane A₂ does.

Properties

Properties of aspirin Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 2244.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₉H₈O₄
Molar mass	180.159(24) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	9
Hydrogen atoms	8
Oxygen atoms	4
Bonds	21
Single bonds	16
Double bonds	5
PubChem molecular weight	180.16 g/mol	g/mol	PubChem

Safety

Though an everyday medicine, aspirin carries the Globally Harmonized System (GHS) signal word Warning in the EU’s classification notifications, above all as harmful if swallowed. About one report in five adds skin, eye or respiratory irritation. PubChem’s use records name a bleeding risk, a link to Reye syndrome in children and a caution in pregnancy. The PubChem Laboratory Chemical Safety Summary and the NIOSH card on acetylsalicylic acid are the authoritative sheets.

Questions

What shape is an aspirin molecule?

No single shape word fits, because aspirin has several central atoms and OpenStax says to consider each one independently. The six ring carbons form a flat hexagon. Each of the two carbonyl carbons, one in the acid group and one in the ester, is trigonal planar. The methyl carbon at the far end of the ester and the single oxygen that joins the ester to the ring each set their own local geometry.

How does aspirin relieve pain and fever?

Both effects are tied mainly to prostaglandins, irritating substances that make pain receptors more sensitive, made with the help of the COX enzymes. DrugBank says cutting their production may stop their action at pain receptors, and gives less prostaglandin E1 in the brain for fever. HSDB, writing of salicylates as a group, adds a central action on the hypothalamus that widens blood vessels in the skin. It also cites some evidence that fevers not driven by prostaglandins may respond too.

Is aspirin an acid?

Yes. Its carboxylic acid group gives up a hydrogen ion in water, and the solution is a weak acid, with a pKa of 3.49 at 25 °C on HSDB's figure. Weak means only partly ionised: OpenStax puts a typical carboxylic acid at about 1 per cent of dissolved molecules ionised at a time. A base takes that hydrogen outright, turning aspirin into an ionic salt, as it does any carboxylic acid.

What does aspirin turn into in the body?

The ester group does not survive long. HSDB reports that the stomach and the blood split acetylsalicylic acid into salicylic acid and acetic acid, and gives the intact molecule a plasma half-life, the time for half of it to disappear from the blood, of 15 to 20 minutes. The acetyl group it attaches to COX-1 in platelets stays attached, and DrugBank says the block on clumping lasts about 7 to 10 days, the average platelet lifespan.

Made of the same elements

C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
CH₂O Formaldehyde 4 atoms, 3 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.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.