CID 8078

C₆H₁₂

Cyclohexane

C 6
H 12

Ball-and-stick Stick Space-filling

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18 atoms, 18 single bonds.

IUPAC name: cyclohexane
Molar mass: 84.162(14) g/mol
Atoms: 18
Polarity: nonpolar
SMILES: C1CCCCC1
InChIKey: XDTMQSROBMDMFD-UHFFFAOYSA-N
PubChem CID: 8078

Made of

C H

Structural formula of cyclohexane. Each unlabelled corner is a carbon atom, and the 12 hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

A loop of six carbon atoms, each holding two hydrogen atoms, is the whole of cyclohexane. ChEBI, the Chemical Entities of Biological Interest database, calls it a hydrocarbon ring, the cyclic form of hexane. Its molar mass is 84.16 g/mol. Older names on the NIOSH card, hexahydrobenzene and benzene hexahydride among them, record its tie to benzene, which carries six fewer hydrogen atoms.

At room temperature cyclohexane is a clear liquid. Its odour is petroleum-like to CAMEO Chemicals, a hazard database, and sweet, like chloroform, to NIOSH, the US National Institute for Occupational Safety and Health. It melts at 6.47 °C, so on a cold day it can freeze solid, and it boils at 80.7 °C, according to the Hazardous Substances Data Bank (HSDB). ChEBI files it as a volatile organic compound and a cycloalkane.

Structure and bonding

Cyclohexane has 18 atoms and 18 single bonds: six C–C bonds close the ring, and twelve C–H bonds give every carbon two hydrogens. Turned edge-on in the Ball-and-stick view, the ring is no flat hexagon. Its carbons step alternately up and down, 0.23 Å either side of the ring’s mean plane, in the shape called the chair. In the model each C–C–C angle is 111.6° and each C–C bond 1.526 Å long.

In the chair, the twelve C–H bonds fall into two sets, as HSDB describes them. Six are axial, parallel to the ring’s axis of symmetry, pointing straight up or down; the six equatorial ones fan out around the rim. Every drawn bond is single, as it is in the real molecule: CAMEO files cyclohexane among the saturated hydrocarbons, those whose carbons hold only single bonds. In Chem1’s account, a carbon forming four bonds uses four sp³ hybrid orbitals, mixed orbitals aimed at the corners of a tetrahedron.

The model holds the chair only; the real molecule, HSDB notes, can also take a boat form, and the two interconvert. The Stick view shows the ring’s zig-zag, and Space-filling closes its centre.

Where it turns up

Crude oil holds it naturally: petroleum contains 0.5 to 1.0 per cent cyclohexane, on HSDB’s figure, and distillation recovers it. LOTUS, a natural-products database, also reports it in the plants Manilkara zapota and Helichrysum odoratissimum.

In manufactured goods it mostly works as a solvent. JECFA, a joint United Nations expert committee on food additives, classes it as an extraction solvent, and cosmetics use it as a solvent too. The US Consumer Product Information Database lists 61 consumer products that contain it, from car-care and hobby goods to home-maintenance supplies. Workplace contact comes with painting, glues and adhesives, and leather tanning, according to Haz-Map, an occupational health database.

It reaches people outside work as well. In one study summarised by HSDB, the breast milk of five of eight mothers contained cyclohexane, thought to come from pollution, since the mothers lived near chemical plants.

Why it matters

Nylon takes nearly all of it. Over 98 per cent of the cyclohexane produced goes into nylon intermediates, according to HSDB: adipic acid, caprolactam and hexamethylenediamine. Adipic acid leads on to nylon 66 and caprolactam to nylon 6. Those two outlets took 57 and 29 per cent respectively, with 13 per cent exported, in an HSDB chemical profile.

That demand runs at a large scale. The US Environmental Protection Agency’s Chemical Data Reporting places the reported national product volume for each year from 2020 to 2023 between 1 billion and under 2.5 billion pounds. From benzene’s side, 15.1 per cent of its 1981 uses outside petrol went into making cyclohexane, on HSDB’s figure in benzene’s PubChem record.

A few uses are narrower. Cyclohexane serves in the industrial recrystallisation of steroids, in solid fuel for camp stoves, and in analytical chemistry for measuring molecular weights (HSDB). It was once given as an anaesthetic, a use dropped because of the risk of explosion.

Properties

Properties of cyclohexane Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 8078.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₆H₁₂
Molar mass	84.162(14) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	6
Hydrogen atoms	12
Bonds	18
PubChem molecular weight	84.16 g/mol	g/mol	PubChem

Safety

Cyclohexane is a highly flammable liquid, and its vapour is heavier than air, spreading along the ground into low or enclosed spaces, where distant ignition is possible. Under EU rules its label bears the signal word Danger, and it may be fatal if swallowed and enters the airways. It causes skin irritation, may cause drowsiness or dizziness, and is very toxic to aquatic life, with effects that last. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card set out the full hazards and the workplace exposure limits.

Questions

Is cyclohexane dangerous to breathe or touch?

It can be. NIOSH lists irritation of the eyes, skin and respiratory system, drowsiness, dermatitis, narcosis (a stupor) and coma among its symptoms. The International Chemical Safety Card says repeated skin contact may cause dryness, cracking and dermatitis. Swallowed liquid can be breathed into the lungs and cause chemical pneumonitis, an inflammation of the lungs. The NIOSH Pocket Guide card for cyclohexane gives the full picture.

What is cyclohexane used for besides nylon?

It is a working solvent. Haz-Map, an occupational health database, lists it for lacquers, resins and glues, including glues in leather manufacturing, and as an intermediate for organic synthesis. It also strips paint and varnish and extracts essential oils, according to HSDB. Chemical plants also turn it into cyclohexanol, cyclohexanone, cyclohexyl chloride and nitrocyclohexane, and HSDB also lists it for glass substitutes.

How is cyclohexane made?

Cyclohexane is made by two routes. In one, benzene is hydrogenated with the help of a nickel, platinum or palladium catalyst, as a liquid or as a vapour. In the other, a petroleum naphtha holding 10 to 14 per cent cyclohexane is superfractionated, a very fine distillation, to an 85 per cent concentrate, both as HSDB describes them. Commercial grades can contain unconverted benzene and methylcyclopentane as impurities.

Does cyclohexane mix with water?

Barely. Only 55 mg of it dissolves in a litre of water at 25 °C, on HSDB's measurement, yet it mixes completely with ethanol, ether, acetone, benzene and olive oil. It is also less dense than water, about 0.78 times as dense at 20 °C, so a spill floats on the surface, as CAMEO Chemicals, a hazard database, notes. ChEBI gives it the role of a non-polar solvent.

Made of the same elements

C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.

All 18 compounds made of carbon and hydrogen

C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
C₂H₂ Acetylene 4 atoms, 3 bonds.