Styrene

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16 atoms, 16 bonds.

IUPAC name: styrene
Molar mass: 104.152(18) g/mol
Atoms: 16
SMILES: C=CC1=CC=CC=C1
InChIKey: PPBRXRYQALVLMV-UHFFFAOYSA-N
PubChem CID: 7501

Made of

Structural formula of styrene. 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.

Pure styrene smells sweet and pleasant. The commercial liquid usually carries other chemicals as well, which give it a sharp, penetrating odour, according to the Hazardous Substances Data Bank (HSDB). NIOSH, the US National Institute for Occupational Safety and Health, describes an oily liquid, colourless to yellow. ChEBI, a chemical dictionary of molecules of biological interest, calls styrene benzene carrying a vinyl group. One hydrogen of a benzene ring gives way to CH=CH₂, two carbons joined by a double bond, and the whole molecule is carbon and hydrogen alone.

In light and air the liquid slowly polymerises, joining into long chains, and oxidises, so inhibitors are added to keep it stable. Commercial styrene usually contains one, such as tert-butylcatechol, according to NIOSH. Warmth speeds the change: it runs slowly at room temperature and readily above 65 °C.

Structure and bonding

Styrene’s sixteen atoms, eight carbon and eight hydrogen, fall into two parts. Six carbons close a flat hexagon, and five of them hold one hydrogen each. The sixth carries the side chain instead: a carbon with one hydrogen, joined by a double bond to a last carbon with two. In Ball-and-stick every angle inside the ring measures 120.0°. The ring is drawn with alternating single and double bonds, though its electrons are really spread evenly around it, just as in benzene.

The side-chain double bond is a real one, like the double bond in ethylene, and at 1.349 Å it is the shortest carbon–carbon bond in the model. The single bond tying it to the ring is the model’s longest carbon–carbon bond, 1.455 Å. Seen edge-on, the side chain does not lie in the ring’s plane: PubChem’s computed shape twists it about 31° out of line. Stick trims the atoms back to their bonds and leaves that tilt plain; Space-filling swells every atom to full size and shows a flat slab with a short, bent tail.

Where it turns up

Styrene is primarily synthetic, according to ATSDR, the US Agency for Toxic Substances and Disease Registry, but it occurs in nature as well. ChEBI records its isolation from benzoin, the resin of Styrax species, and the LOTUS natural-products database reports it in Punica granatum and Oecophylla smaragdina. Fruits, vegetables, nuts, drinks and meats contain it naturally too.

Indoors, its vapour comes off building materials, photocopiers and cigarette smoke, and car exhaust carries it outside. A little can pass into food from packaging made with it. Insulation, fibreglass, plastic pipes, car parts, shoes, drinking cups and carpet backing are made with it, and most of these hold it both chained into polystyrene and loose. Paints, floor waxes, adhesives, putty and car-body fillers contain it too, according to HSDB. Outdoors, air usually breaks it down within one or two days.

Why it matters

About 85 per cent of commercial styrene comes from dehydrogenation, stripping hydrogen from ethylbenzene, according to HSDB. Ethylbenzene in turn is made from ethylene and benzene. A smaller share leaves a process for propylene oxide as a co-product.

Most styrene goes into polystyrene plastics and resins, the US Environmental Protection Agency (EPA) reports. Packaging is the largest single use of styrene-containing resins, on HSDB’s account, above all foams that fill spaces and cushion goods. Polystyrene also makes take-out trays, foam packing peanuts, CD cases and foam-walled drink cups. The plastic is transparent but rather brittle, and it yellows under ultraviolet light. It stays a good electrical insulator even at ultrahigh radio frequencies.

Styrene also joins other monomers, the small molecules that link into polymers, in mixed chains called copolymers. ABS adds acrylonitrile and butadiene; SAN, styrene with acrylonitrile alone, is suited to transparent drink containers. In 1985, on HSDB’s figures, polystyrene took 55 per cent of styrene and ABS 9 per cent.

Properties

Properties of styrene Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 7501.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₈H₈
Molar mass	104.152(18) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	8
Hydrogen atoms	8
Bonds	16
Single bonds	12
Double bonds	4
PubChem molecular weight	104.15 g/mol	g/mol	PubChem

Safety

Styrene is a flammable liquid, harmful if inhaled, and its classification under the Globally Harmonized System carries the signal word Danger. It irritates skin and eyes, is suspected of harming an unborn child, and can damage organs through prolonged or repeated exposure. Breathing it mainly affects the nervous system, ATSDR says, and the International Labour Organization warns that it may worsen noise-induced hearing loss. The PubChem Laboratory Chemical Safety Summary gathers the classifications, and the NIOSH card covers workplaces, where ATSDR says styrene workers are likely to breathe it.

Questions

Is styrene the same as polystyrene?

No. Styrene is the small molecule, an oily liquid, and polystyrene is the plastic formed when very many styrene units link into long chains. The two often travel together: most styrene-based products hold some unlinked styrene alongside the polymer. Building the chains is a partly random process that never matches its reactants in exact proportion, so some unreacted monomer, the single-unit starting molecule, can stay in the finished plastic.

Why is styrene also called vinylbenzene?

The name lists the molecule's two parts. Six of its eight carbons form a benzene ring that has given up one hydrogen, and the other two form a vinyl group, CH=CH₂, joined by a double bond. Named the other way round, the same molecule is phenylethylene: ethylene with a benzene ring in place of one hydrogen. Its formula, C₆H₅CH=CH₂, shows both halves.

How does the body break down styrene?

It is absorbed by every route, but mainly through the lungs. In people, CYP2E1 is the main cytochrome P450 enzyme that breaks it down, and its known metabolites, the compounds the body turns it into, include styrene 7,8-oxide. Mandelic acid and phenylglyoxylic acid in urine serve as markers of exposure. Styrene leaves the body quickly, yet it crosses the placenta and can reach nursing infants in breast milk.

Does styrene cause cancer?

The US National Toxicology Program classes styrene as reasonably anticipated to be a carcinogen in humans. On the California Safe Cosmetics Program's list, its rating from IARC, the International Agency for Research on Cancer, is Group 2A, probably carcinogenic to humans. The US Environmental Protection Agency has not formally classified it. In mice it causes lung tumours.

Made of the same elements

C₇H₈ Toluene 15 atoms, 15 bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₄H₁₀ Isobutane 14 atoms, 13 single bonds.

All 18 compounds made of carbon and hydrogen

C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂ Acetylene 4 atoms, 3 bonds.