Benzene

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12 atoms, 12 bonds.

IUPAC name: benzene
Molar mass: 78.114(13) g/mol
Atoms: 12
SMILES: C1=CC=CC=C1
InChIKey: UHOVQNZJYSORNB-UHFFFAOYSA-N
PubChem CID: 241

Made of

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

Michael Faraday isolated benzene in 1825 from a liquid that condensed out of compressed oil gas. It is a hydrocarbon, a compound of carbon and hydrogen alone, and OpenStax calls it the simplest member of the aromatic hydrocarbons. That large family is built on rings whose bonding cannot be written as fixed single and double bonds. Describing it takes either resonance or delocalisation, electrons spread over several atoms instead of held between two, from molecular orbital theory.

The difference shows in how benzene reacts. OpenStax notes that it does not behave like a typical alkene, a hydrocarbon with a carbon–carbon double bond. Aromatic compounds undergo substitution, the swap of a hydrogen atom for another group, more readily than addition. A substitution leaves the ring’s delocalised bonds intact. At room temperature benzene is a colourless to light-yellow liquid, with a smell the Hazardous Substances Data Bank (HSDB) likens to petrol, and it turns solid below about 5.5 °C.

Structure and bonding

The model is a flat hexagon of six carbon atoms, each holding one hydrogen pointing outwards. Turned edge-on, all twelve atoms line up in one plane. Measured in PubChem’s computed model, every angle around each carbon is 120°. Each carbon has three neighbours and no lone pairs, so OpenStax assigns it sp² hybridisation, a mixing of one s and two p orbitals. Stick shows only the bond pattern, Space-filling swells the atoms into a flat disc fringed with hydrogen, and Labels marks each atom C or H.

The viewer draws three double and three single bonds alternating round the ring. That is one of benzene’s two resonance forms, structures that differ only in where the electrons are placed. OpenStax says the six carbon–carbon bonds are equivalent, each between a single and a double bond.

Each carbon’s fourth valence electron sits in a p orbital upright to the ring. These electrons form π bonds, made by side-by-side overlap of p orbitals, spread round the whole ring. The model’s geometry agrees: every C–C distance measures 1.395 Å, drawn single or double. Chemists draw benzene as a hexagon with a circle inside to represent that bonding.

Where it turns up

Crude oil carries benzene naturally, and so do petrol and cigarette smoke; volcanoes and forest fires release it too. The US Environmental Protection Agency (EPA) traces the benzene in air to motor exhaust, petrol stations and burning coal and oil. In 2000 the European Union cut the most benzene allowed in petrol from 5 to 1 per cent by volume. A 2002 study cited by Haz-Map put North American petrol at about 1 per cent.

It is banned as an ingredient in household products, toys included, and the US Food and Drug Administration prohibits it in food. Manufacturers once added it to cosmetics; now it appears there only as a contaminant. Most people can detect its smell once air holds 1.5 to 4.7 parts per million. That smell warns of short, heavy exposures but not of long, low ones.

Why it matters

Benzene is a starting material far more than a finished product. Most of it is now used in closed processes to build other organic chemicals, which go on to make plastics, resins, nylon and other synthetic fibres. Rubbers, lubricants, dyes, detergents, drugs and pesticides draw on it as well. HSDB’s figures for uses other than fuel in 1981 give the split: 49.1 per cent went to ethylbenzene, 18.4 per cent to cumene and 15.1 per cent to cyclohexane.

On HSDB’s worldwide estimates, three routes supply most of it. Catalytic reforming turns hydrocarbons into aromatic rings, partly by stripping hydrogen from ring-shaped molecules and partly by first closing chains into rings, and gives about 30 per cent. Pyrolysis gasoline, a by-product of making ethylene, gives 30 to 35 per cent, and conversion of toluene 25 to 30 per cent. Until the 1940s the main source was light oil distilled from coal. Inks, rubber, lacquers and paint removers once used benzene as a solvent, a use now in the past.

Properties

Properties of benzene Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 241.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₆H₆
Molar mass	78.114(13) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	6
Hydrogen atoms	6
Bonds	12
Single bonds	9
Double bonds	3
PubChem molecular weight	78.11 g/mol	g/mol	PubChem

Safety

Benzene is a highly flammable liquid, and the EPA classes it as a known human carcinogen. Long or repeated exposure harms the bone marrow that makes blood cells, leading to anaemia, aplastic anaemia included, and to leukaemia. Short, heavy exposure can act as a narcotic, and the concern falls mostly on people who work with it. The authoritative sheets are the PubChem Laboratory Chemical Safety Summary and the benzene card of NIOSH, the US National Institute for Occupational Safety and Health.

Questions

Why does the benzene model show alternating double bonds?

PubChem's file stores one fixed bond pattern, so the viewer draws one of benzene's two resonance forms, with the double bonds on alternate sides of the ring. The second form simply swaps them round. OpenStax says neither one fully describes the π electrons, and the molecule never flips between the two: its real structure is always their average. Valence bond theory copes with that poorly, and molecular orbital theory describes such bonding better.

Is benzene a carcinogen?

Yes. The US Environmental Protection Agency classes it as a known human carcinogen for every route of exposure, and the International Agency for Research on Cancer (IARC) also counts it as causing cancer in people. It damages the DNA of the bone marrow stem cells that produce red and white blood cells, and IARC links it to acute myeloid leukaemia. People take it in mainly by breathing it or through the skin.

Who discovered benzene?

Michael Faraday isolated it in 1825 and called it bicarburet of hydrogen. In 1833 the chemist who made it again from benzoic acid proposed the name benzin. Coal tar entered the story in 1845, when benzene was found in its light oil, and C. Mansfield reported the first practical industrial recovery process in 1849. Benzine, spelt with an i, is a different thing: a low-boiling hydrocarbon mixture.

Does benzene dissolve in water?

Only slightly. The International Chemical Safety Card puts its solubility at 0.18 g in 100 mL of water at 25 °C. Benzene is also less dense than water, so a spill floats on top instead of mixing in. Organic liquids are another matter: the Hazardous Substances Data Bank lists it as miscible, blending in any proportion, with alcohol, chloroform, ether, carbon disulfide, acetone and oils.

Made of the same elements

C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.

All 18 compounds made of carbon and hydrogen

C₂H₄ Ethylene 6 atoms, 5 bonds.
C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₂H₂ Acetylene 4 atoms, 3 bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.