Benzoic acid

Ball-and-stick Stick Space-filling

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

15 atoms, 15 bonds.

IUPAC name: benzoic acid
Molar mass: 122.123(17) g/mol
Atoms: 15
SMILES: C1=CC=C(C=C1)C(=O)O
InChIKey: WPYMKLBDIGXBTP-UHFFFAOYSA-N
PubChem CID: 243

Made of

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

A benzene ring with one carboxylic acid group, –COOH, bonded to it: that is benzoic acid as defined by ChEBI, a reference database of biological molecules. The pure acid forms white crystals or a powder. Plants and algae make it, and HSDB, the Hazardous Substances Data Bank, names benzoin resin as one source. In food it serves as a preservative, while in industry most of it goes on to become phenol.

HSDB puts its melting point at 122.35 °C, yet part of it sublimes, turning straight from solid to vapour, at 100 °C. Water takes up little of it, 3.5 grams per litre at 25 °C on HSDB’s figure. JECFA, the United Nations’ expert committee on food additives, finds that hardly any dissolves in cold water and a fair amount in hot. Oils and glycerol dissolve it too. An alkaline substance lets water hold more of it, HSDB notes.

Structure and bonding

Turned edge-on, the model is flat: in PubChem’s computed structure its 15 atoms, seven carbon, six hydrogen and two oxygen, lie within 0.002 Å of one plane. Six carbons close a hexagon, five of them capped by hydrogen. The sixth holds a seventh carbon carrying both oxygens, one double-bonded and one as an OH group. With Labels on, its hydrogen is the only H on an O, the one the acid gives up. Space-filling turns the ring into a flat disc with the oxygens bulging from one rim.

Ball-and-stick and Stick draw the ring as three double bonds alternating with three single ones, yet the model’s six ring bonds all measure 1.395 Å. The real ring has no fixed double bonds, and the benzene page explains why its six bonds come out alike.

Only the neutral acid is drawn. In water above a pH of about 4.2, most of it has lost that hydrogen and become benzoate ion. That ion is the form carried by the sodium and potassium benzoate added to drinks. In the solid, Chem1 notes, many carboxylic acids pair off as dimers held by hydrogen bonds.

Where it turns up

MeSH, the US National Library of Medicine’s subject headings, calls benzoic acid fungistatic, checking the growth of fungi, and widely used to preserve food. In drinks, sodium or potassium benzoate can be added to hold back bacteria, yeasts and moulds, according to the US Food and Drug Administration (FDA). Cranberries and their juice contain benzoate salts of their own.

In some drinks that combine benzoate salts with ascorbic acid, vitamin C, very low levels of benzene can form, FDA reports, and heat and light encourage it. Benzene in some soft drinks has been known to FDA since 1990, and reports of it in drinks with both ingredients reached the agency in November 2005. FDA calls benzene a human carcinogen; the benzene page sets out its classifications. The levels found so far pose no safety concern for consumers in FDA’s judgement, and manufacturers have reformulated products to keep benzene low or out.

Why it matters

Almost all benzoic acid comes from toluene, oxidised by air in the liquid phase over a cobalt catalyst, HSDB records. Material of US Pharmacopeia and Food Chemicals Codex grade is usually extracted and crystallised. Most of the acid never leaves the companies that make it: they use it captively to make phenol by the Dow process. HSDB’s breakdown of uses gives phenol 55 per cent, benzoate plasticisers 22 per cent and the sodium and potassium salts 8 per cent. The US national volume reported to the Environmental Protection Agency for 2023 lay between 100 million and 250 million pounds.

In the laboratory, HSDB names it the primary thermochemical standard for calibrating oxygen bomb calorimeters, instruments that measure the heat a sample gives out as it burns in oxygen. A mole of benzoic acid burnt in this way releases 3227 kJ, on HSDB’s figure. HSDB also records its use in calico printing and in curing tobacco.

Properties

Properties of benzoic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 243.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₇H₆O₂
Molar mass	122.123(17) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	7
Hydrogen atoms	6
Oxygen atoms	2
Bonds	15
Single bonds	11
Double bonds	4
PubChem molecular weight	122.12 g/mol	g/mol	PubChem

Safety

As a pure solid, benzoic acid is labelled Danger under the European Union’s harmonised classification. It irritates the skin, can seriously damage the eyes and harms organs through prolonged or repeated exposure. The International Chemical Safety Card adds that repeated exposure to its aerosol may affect the lungs. Workplace exposure limits exist, and the PubChem Laboratory Chemical Safety Summary for benzoic acid gathers them with the full classification.

Questions

Why is the formula of benzoic acid written in two parts?

Its condensed formula, C₆H₅COOH, splits it into two parts: a ring of six carbon atoms carrying five hydrogens, and the COOH acid group fixed to the sixth ring carbon. Added up, that is C₇H₆O₂, with a molar mass of 122.12 g/mol. The structural formula draws all seven carbons as unlabelled corners and leaves the five ring hydrogens undrawn, keeping the two oxygens and the hydrogen on one of them as the only letters.

Is benzoic acid a strong acid?

No. Like acetic acid it is a weak acid, giving up only part of its hydrogen in water, though it is the stronger of the two. Its pK_a, a measure of how readily an acid gives up that hydrogen, is 4.204 on the Hazardous Substances Data Bank's figure, against 4.76 for acetic acid; the scale is a negative logarithm, so the lower value marks the stronger acid. HSDB gives a saturated solution a pH of 2.8 at 25 °C.

What happens to benzoic acid in the body?

In its neutral form, before giving up its hydrogen, the acid is absorbed rapidly from the gut in animals and people, the Hazardous Substances Data Bank reports. How fast the body turns it into hippuric acid is limited by the supply of the amino acid glycine: when too little is available, benzoyl glucuronide forms instead. Rats showed no build-up of the acid or its sodium salt, and the Indian fruit bat passes almost all of it as the glucuronide.

Is benzoic acid safe in food?

The US Food and Drug Administration lists it as generally recognised as safe, and ACGIH, the American Conference of Governmental Industrial Hygienists, considers low-level exposure by mouth safe. JECFA, the United Nations' expert committee on food additives, evaluated it in 2021 and set a group acceptable daily intake covering it and the benzoate salts. On contact it can cause hives that do not involve the immune system, Haz-Map notes, and its alkali salts cause allergy-like, or pseudoallergic, reactions.

Made of the same elements

C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 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.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.