1,3-Butadiene

Ball-and-stick Stick Space-filling

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10 atoms, 9 bonds.

IUPAC name: buta-1,3-diene
Molar mass: 54.0920(92) g/mol
Atoms: 10
SMILES: C=CC=C
InChIKey: KAKZBPTYRLMSJV-UHFFFAOYSA-N
PubChem CID: 7845

Made of

Structural formula of 1,3-butadiene. Each unlabelled corner is a carbon atom, and the two hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Much of the rubber on a car’s tyres begins as 1,3-butadiene, and the car’s exhaust gives off the same gas. Its IUPAC name is buta-1,3-diene. Each molecule holds four carbon and six hydrogen atoms, C₄H₆, and a mole of it weighs 54.09 g. ATSDR, the US Agency for Toxic Substances and Disease Registry, describes a chemical made by processing petroleum, 36th by volume among those produced in the United States.

At room temperature it is a colourless gas. The National Cancer Institute’s thesaurus calls it practically insoluble in water but soluble in ethanol, ether, acetone and benzene. CAMEO Chemicals, a US hazard database, files it among the conjugated dienes, whose two double bonds are separated by one single bond, and among the polymerisable compounds. Because butadiene is liable to polymerise, linking its molecules into long chains, CAMEO says it must be shipped with an inhibitor added.

Structure and bonding

Butadiene’s four carbons run in a row, with six hydrogens around them. Each end carbon carries two H, each inner carbon one. In the Ball-and-stick view, double sticks join the outer pairs of carbons and a single stick links the pairs: C=C–C=C. The two double bonds point to opposite sides of that middle bond, and the chain zig-zags.

Turned edge-on, the model shows all ten atoms in one plane: the Space-filling view becomes a flat slab of overlapping spheres, and the Stick view a single straight line. PubChem’s computed geometry gives each double bond 1.338 Å and the central single bond 1.443 Å, with 123.4° at each C=C–C angle. These are the model’s values, not measurements.

The drawing fixes each double bond between one pair of carbons. Butadiene’s alternating chain is, in Chem1’s words, “a series of ethylene units”, each double bond built as in ethylene. Its pi electrons, those of each double bond’s second bond, are in that account delocalised: spread along the chain, not held between the pairs the double sticks join. The two double sticks are one simplified picture of that bonding.

Where it turns up

The US Environmental Protection Agency (EPA) calls the exhaust of motor vehicles a constant source of 1,3-butadiene. The gas is usually present at low levels in urban and suburban air. People breathe it in from car and truck exhaust, waste incineration and wood fires, according to ATSDR, and it is a component of tobacco smoke. Gasoline holds small amounts. Food stored in plastic or rubber containers can carry some, at levels ATSDR describes as generally very low or nil.

In the atmosphere it breaks down quickly, on EPA’s account, and ATSDR expects half of what enters the air to be broken down within 6 hours. It evaporates so easily that it is not expected to remain in water or soil. Nor is it expected to build up in fish.

Why it matters

Synthetic rubber takes about 75 per cent of manufactured 1,3-butadiene according to ATSDR’s substance page, and about 60 per cent according to its ToxFAQs sheet. The main rubbers made from it, as HSDB (the US Hazardous Substances Data Bank) lists them, are styrene-butadiene (SBR), polybutadiene (BR), acrylonitrile-butadiene (NBR) and chloroprene rubber (CR). In 1984, on HSDB’s figures, styrene-butadiene rubber alone took 45 per cent and polybutadiene rubber 20 per cent.

Polybutadiene, usually mixed with other rubbers, is the major component of tyres in Chem1’s account, and SBS, a styrene-butadiene-styrene rubber, is valued for its durability in tyre treads. Chloroprene rubber is neoprene, which butadiene reaches through chloroprene, according to HSDB. Invented in 1930, neoprene was the first synthetic rubber made in mass production, Chem1 notes; it now goes into roofing membranes and wet suits.

Outside rubber, butadiene goes into impact-resistant polystyrene, a two-phase blend of polystyrene and polybutadiene, and into ABS plastics. It is also a chemical intermediate, a starting material for other chemicals such as adiponitrile and 1,5,9-cyclododecatriene, on HSDB’s list.

Properties

Properties of 1,3-butadiene Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 7845.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₄H₆
Molar mass	54.0920(92) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	4
Hydrogen atoms	6
Bonds	9
Single bonds	7
Double bonds	2
PubChem molecular weight	54.09 g/mol	g/mol	PubChem

Safety

Under EU classification 1,3-butadiene carries the signal word Danger, as an extremely flammable gas that may cause genetic defects and cancer. IARC, the International Agency for Research on Cancer, and EPA are among the bodies that class it as a human carcinogen. Exposed workers may face more cancers of the blood, stomach and lymphatic system, ATSDR reports. The PubChem Laboratory Chemical Safety Summary and the Pocket Guide card from NIOSH, the US workplace-safety institute, set out the full hazards and the workplace limits.

Questions

What does the 1,3 in 1,3-butadiene mean?

The numbers mark where the two double bonds start along the four-carbon chain. ChEBI, a chemical database, places its unsaturation, the double bonds, at positions 1 and 3. The model draws one from the first carbon to the second and one from the third to the fourth. The other isomer, 1,2-butadiene, is a small by-product with no significant commercial use, according to the Hazardous Substances Data Bank.

Is 1,3-butadiene a gas at room temperature?

Yes. The Hazardous Substances Data Bank gives its boiling point as −4.5 °C, and ATSDR, the US toxic-substances agency, describes a colourless gas with a mild, gasoline-like odour. NIOSH, the US workplace-safety institute, notes that it becomes a liquid below 24 °F, about −4 °C, and travels as a liquefied compressed gas. CAMEO Chemicals adds that its vapour is heavier than air.

Where does industrial 1,3-butadiene come from?

In Japan, Western Europe and the United States, the Hazardous Substances Data Bank records it as mainly a coproduct of steam cracking, which is run to make ethylene. Some parts of the world still make it from ethanol. US makers reported at least 2.5 billion but under 4 billion pounds of it for 2023 to the US Environmental Protection Agency.

How does 1,3-butadiene cause cancer?

It acts through what the body turns it into, according to the Hazardous Substances Data Bank. Liver-cell fractions from mice, rats and humans oxidise it to epoxybutene and then diepoxybutane, compounds called epoxides that react with DNA. The genetic changes they cause appear to be how butadiene produces tumours. Mice make these epoxides more efficiently, while rats and humans detoxify them better, breaking them down with water.

Made of the same elements

C₃H₆ Propylene 9 atoms, 8 bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.

All 18 compounds made of carbon and hydrogen

C₇H₈ Toluene 15 atoms, 15 bonds.
C₂H₂ Acetylene 4 atoms, 3 bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 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.