Toluene

Ball-and-stick Stick Space-filling

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15 atoms, 15 bonds.

IUPAC name: toluene
Molar mass: 92.141(16) g/mol
Atoms: 15
Polarity: nonpolar
SMILES: CC1=CC=CC=C1
InChIKey: YXFVVABEGXRONW-UHFFFAOYSA-N
PubChem CID: 1140

Made of

Structural formula of toluene. 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.

One methyl group, CH₃, standing where a hydrogen atom would sit on a benzene ring, is the whole difference between toluene and its parent hydrocarbon. ChEBI, the Chemical Entities of Biological Interest database, lists it as the simplest member of the toluenes. Built from seven carbon and eight hydrogen atoms, it has a molar mass of 92.14 g/mol.

At room temperature toluene is a colourless liquid with a sweet, pungent smell rather like benzene’s. It boils at 110.6 °C and freezes at −94.9 °C. Because it is lighter than water and does not dissolve in it, a spill floats. On the Hazardous Substances Data Bank (HSDB) figure, only 526 mg of it dissolves in a litre of water at 25 °C. Yet it mixes freely with alcohol, ether and acetone. ChEBI gives it the role of a non-polar solvent.

Structure and bonding

The viewer shows 15 atoms joined by 15 bonds: a hexagon of six carbons, five holding one hydrogen each and the sixth carrying the methyl carbon. Turned edge-on, the ring, its five hydrogens and the methyl carbon lie in one plane. Only the three methyl hydrogens leave it, two on one side and one on the other.

Ball-and-stick draws the ring as one Kekulé form, three double bonds alternating with three single ones. The real ring is a resonance hybrid, a blend of the two equivalent ways of placing the three double bonds, as Chem1 describes for benzene’s ring. Each ring carbon keeps a p orbital pointing above and below the ring, and these merge into rings of electron density on both faces.

The computed geometry agrees. Every ring bond measures 1.395 Å however it is drawn, while the bond to the methyl carbon is an ordinary single bond of 1.492 Å. Stick thins the framework to rods, Space-filling shows a flat disc with a knobbly bulge at the methyl group, and Labels marks each atom C or H.

Where it turns up

Crude oil and the tolú tree both hold toluene naturally, according to ATSDR (Agency for Toxic Substances and Disease Registry). It also forms when crude oil is refined into petrol and other fuels, and when coal is turned into coke. Car exhaust contains it, so time spent in or near traffic can mean some exposure.

In the home it thins nail polish and nail glue, where California’s Safe Cosmetics Program says it keeps the colour even and the finish smooth. HSDB lists it as an adhesive solvent in plastic toys and model aircraft. It also turns up in car-care, hobby and home-maintenance products, according to the Consumer Product Information Database.

Released toluene does not usually linger. It evaporates from soil and surface water or is broken down by bacteria, and in air it reacts with oxygen and other chemicals.

Why it matters

Toluene is extracted from refinery and coke-oven streams rather than synthesised, for economic reasons according to HSDB. Catalytic reforming, a refinery step, yields 87 per cent of it, and another 9 per cent is separated from a liquid left over when steam crackers make ethylene and propylene. Coke ovens supply about 1 per cent, and up to 2 per cent arrives as a by-product of styrene manufacture. The US Environmental Protection Agency (EPA) puts American output in 2023 at between 10 and 15 billion pounds.

The EPA names its major use as blending into petrol to raise octane ratings, and ATSDR says it goes in alongside benzene and xylene. In HSDB’s 1985 estimate, benzene manufacture took 46 per cent, petrol blending 37 per cent, solvents 8 per cent and toluene diisocyanate, the route into polyurethane resins, 7 per cent.

HSDB also lists benzoic acid, saccharin, dyes, perfumes and medicines made from toluene, and ATSDR names nylon and plastics as well. As a solvent it dissolves paints, gums, resins, most oils and rubber, and it thins nitrocellulose lacquers.

Properties

Properties of toluene Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 1140.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₇H₈
Molar mass	92.141(16) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	7
Hydrogen atoms	8
Bonds	15
Single bonds	12
Double bonds	3
PubChem molecular weight	92.14 g/mol	g/mol	PubChem

Safety

Toluene is a highly flammable liquid whose vapour acts on the nervous system, and under the Globally Harmonised System (GHS) of hazard labels it carries the signal word Danger. Swallowed, it can be fatal if it enters the airways, and it is suspected of damaging the unborn child. Its vapour is heavier than air and can travel to a distant flame and flash back. The PubChem Laboratory Chemical Safety Summary and the Pocket Guide card from NIOSH, the US federal institute for occupational safety and health, are the references for anyone handling it.

Questions

Is toluene the same as benzene?

No, though they are close relatives. Toluene is a benzene ring with a methyl group, CH₃, in place of one hydrogen atom, and NIOSH, the US federal institute for occupational safety and health, lists methyl benzene as a synonym. The formula grows from C₆H₆ to C₇H₈. The link runs through industry too: in the Hazardous Substances Data Bank's 1985 estimate, making benzene took 46 per cent of toluene, and commercial toluene usually carries a little benzene as an impurity.

Why does the toluene model show double bonds in the ring?

The structure file behind the viewer stores fixed bond orders, so the ring appears as one Kekulé form: three double bonds alternating with three single ones. Chem1 describes the benzene ring, the core of toluene, as a resonance hybrid whose π electrons are spread around the whole ring. The computed geometry agrees, giving all six ring bonds one length, 1.395 Å, whichever way each is drawn.

Is toluene dangerous to breathe?

It can be. Its classification under the Globally Harmonised System (GHS) of hazard labels warns that the vapour may cause drowsiness or dizziness, and ATSDR (Agency for Toxic Substances and Disease Registry) says it may affect the nervous system, with symptoms that usually fade once exposure stops. Repeatedly inhaling it from glue or paint thinner may damage the brain permanently. ATSDR advises using paints, glues and nail polish that contain it in well-ventilated rooms.

What happens to toluene in the body?

Much of it is broken down in the liver, where a family of enzymes called P450s turns it mainly into benzyl alcohol, with small amounts of other products. Two further enzymes take benzyl alcohol on to benzoic acid, which is joined to the amino acid glycine and leaves in urine as hippuric acid. Part of the toluene is simply breathed out unchanged. ATSDR notes that blood and urine tests only help within several days of exposure.

Made of the same elements

C₈H₈ Styrene 16 atoms, 16 bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 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.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂ Acetylene 4 atoms, 3 bonds.
C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.