Octane

C 8
H 18

Ball-and-stick Stick Space-filling

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26 atoms, 25 single bonds.

IUPAC name: octane
Molar mass: 114.232(20) g/mol
Atoms: 26
Polarity: nonpolar
SMILES: CCCCCCCC
InChIKey: TVMXDCGIABBOFY-UHFFFAOYSA-N
PubChem CID: 356

Made of

C H

Structural formula of octane. 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.

ChEBI (Chemical Entities of Biological Interest) describes octane as a straight chain alkane of eight carbon atoms. Alkanes are hydrocarbons with the general formula C_nH_2n+2, so eight carbons carry eighteen hydrogen atoms: C₈H₁₈, 114.23 g/mol. ChEBI also gives it the role of xenobiotic, a substance foreign to the body.

Octane has a gasoline-like odour, as NIOSH, the US institute for workplace safety and health, describes it. HSDB, the Hazardous Substances Data Bank, lists it as miscible with gasoline, benzene and petroleum ether, able to mix with each in any proportion. It melts at −56.73 °C and boils at 125.62 °C, so at room temperature it is a colourless liquid.

Acids and alkalis dissolved in water leave octane unchanged, and so do most oxidising and reducing agents, according to CAMEO Chemicals, a hazard database. Strong oxidants are the exception: the International Chemical Safety Cards list a reaction with them.

Structure and bonding

Eight carbon atoms zigzag down the length of the model, and 18 hydrogens surround them, 26 atoms in all. Of the 25 single bonds, seven join carbon to carbon and 18 hold a hydrogen each. In PubChem’s computed model the chain is stretched flat: all five C–C–C–C torsion angles, the twist about each inner C–C bond, lie within 3° of 180°. Each C–C–C angle measures 111.2° to 113.4°.

With Labels on, the chain reads CH₃(CH₂)₆CH₃. Seen edge-on in Ball-and-stick, all eight carbons lie almost in one plane, and Space-filling fuses the 26 atoms into a slender rod, grey carbon showing between white hydrogens.

Every bond is single, as CAMEO Chemicals says of the saturated hydrocarbons, and HSDB gives octane a dipole moment of 0, which makes it nonpolar. Chem1 calls dispersion forces additive and ties their strength largely to the number of electrons. By that account a longer straight chain should attract its neighbours more strongly and boil higher. HSDB’s boiling points climb with the chain: methane −161.50, ethane −88.6, propane −42.1, butane −0.50, pentane 36.06, hexane 68.73 and octane 125.62 °C.

Where it turns up

Octane is found in petroleum, and HSDB states that it is obtained by fractional distillation and refining. Above six carbons, straight-chain alkanes come off not one by one but as mixtures of homologues, chains of neighbouring lengths. Octane’s neighbours are heptane and nonane, one carbon shorter and one longer. HSDB puts the three straight chains together at 18.35 per cent by weight of a naphtha, a petroleum liquid, used to thin bitumen from oil sands. Selective separations, crystallisation with urea among them, can draw off straight chains at 95 per cent linearity or better.

Haz-Map, an occupational health database, ties octane exposure to painting solvents. Five products in the Consumer Product Information Database contain it, sold for cars, home maintenance and indoor use. LOTUS, a natural-products database, records octane in Humulus lupulus and Aster scaber, among other organisms.

Why it matters

HSDB lists octane as an industrial solvent, a diluent for lacquers and a carrier solvent in polymer manufacture, as well as a raw material for organic synthesis. It also serves in calibrations and as a working agent in the petroleum industry. Under the US Environmental Protection Agency’s Chemical Data Reporting, its recorded functions include solvent, fuel, fuel agent and intermediate. US production reported for 2023 lay between 100 million and 500 million pounds, and HSDB lists octane as a High Production Volume chemical.

The octane rating on fuel pumps is not a measure of octane itself: EIA explains that fuels are rated against reference blends of heptane, toluene and a different compound, isooctane. Gasoline itself is a blend. ATSDR, the US federal agency on toxic substances, says a typical gasoline holds more than 150 chemicals. The US Energy Information Administration describes it as a product of crude oil and other petroleum liquids.

Properties

Properties of octane Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 356.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₈H₁₈
Molar mass	114.232(20) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	8
Hydrogen atoms	18
Bonds	25
PubChem molecular weight	114.23 g/mol	g/mol	PubChem

Safety

Octane is highly flammable as liquid and vapour, and its vapour, 3.86 times as dense as air on HSDB’s figure, can spread along the ground to a distant flame. The EU classifies it with the signal word Danger. Swallowed liquid that enters the airways may be fatal, and octane irritates the skin, may cause drowsiness or dizziness and is very toxic to aquatic life. The PubChem Laboratory Chemical Safety Summary covers these hazards, and the NIOSH Pocket Guide card lists workplace exposure limits.

Questions

How much heat does octane release when it burns?

HSDB, the Hazardous Substances Data Bank, gives octane a heat of combustion of 1,302.7 kilocalories per mole, measured at 20 °C and 760 mm Hg. Heat of combustion is the heat given out when a compound burns completely in oxygen. A mole of octane weighs 114.23 g, so each gram burned releases about 11.4 kilocalories.

What happens to octane in the body?

Breathing it in is the main route of absorption, according to HSDB, and like most alkanes it collects most readily in fatty tissue. A cytochrome P450 system, a set of oxidising enzymes, attacks an end carbon of the chain; in rat liver preparations the main product is 1-octanol. That alcohol is then joined to glucuronic acid or oxidised further to octanoic acid, also called caprylic acid. Rats given octane also excrete 2-octanol and 3-octanol.

Why is octane called octane?

The name follows the rule for alkanes, hydrocarbons with the general formula C_nH_2n+2: a numerical prefix for the count of carbon atoms, joined to the ending -ane. Oct- stands for eight, so the formula is C₈H₁₈. The IUPAC (International Union of Pure and Applied Chemistry) name is the same word. It is also written n-octane, for normal octane, a form that marks the chain as unbranched.

Made of the same elements

C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.

All 18 compounds made of carbon and hydrogen

C₄H₁₀ Isobutane 14 atoms, 13 single 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.
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.