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Octane

Hydrocarbon compound with the formula C8H18

Wikipedia contributorsAdapted for Fathomly · View authors
1 min read
Source updated September 15, 2026
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Octane is a hydrocarbon and also an alkane with the chemical formula C8H18, and the condensed structural formula CH3(CH2)6CH3. Octane has many structural isomers that differ by the location of branching in the carbon chain. One of these isomers, 2,2,4-trimethylpentane (commonly called iso-octane), is used as one of the standard values in the octane rating scale.

Octane is a component of gasoline and petroleum. Under standard temperature and pressure, octane is a colorless liquid. Like other short-chained alkanes with a low molecular weight, it is volatile, flammable, and toxic. Octane is 1.2 to 2 times more toxic than heptane.

01Isomers

N-octane has 23 constitutional isomers. 8 of these isomers have one stereocenter; 3 of them have two stereocenters.

Achiral isomers:

Chiral isomers:

(3S,4S)-3,4-Dimethylhexane (top left) and (3R,4R)-3,4-Dimethylhexane (top right) are non-superimposable mirror images, so they are chiral enantiomers. (meso)-3,4-Dimethylhexane (bottom) has a superimposable mirror image, so it is an achiral meso compound.
(3S,4S)-3,4-Dimethylhexane (top left) and (3R,4R)-3,4-Dimethylhexane (top right) are non-superimposable mirror images, so they are chiral enantiomers. (meso)-3,4-Dimethylhexane (bottom) has a superimposable mirror image, so it is an achiral meso compound.
N-octane is the octane isomer that has the longest carbon skeleton. Unlike its constitutional isomers, it has a very low knock resistance.
N-octane is the octane isomer that has the longest carbon skeleton. Unlike its constitutional isomers, it has a very low knock resistance.

02Production and use

In petrochemistry, octanes are not typically differentiated or purified as specific compounds. Octanes are components of particular boiling fractions.

A common route to such fractions is the alkylation reaction between iso-butane and 1-butene, which forms iso-octane.

Octane is commonly used as a solvent in paints and adhesives.

The octane isomer, iso-octane, is used as one of the standards for octane ratings. It has a rating of 100 by definition.
The octane isomer, iso-octane, is used as one of the standards for octane ratings. It has a rating of 100 by definition.
The octane isomer 2,3,3-Trimethylpentane has an octane rating exceeding 100.
The octane isomer 2,3,3-Trimethylpentane has an octane rating exceeding 100.
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Sources and credits

This article is adapted from the Wikipedia article “Octane”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.

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Octane rating

An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without causing engine knocking. The higher the octane number, the more compression the fuel can withstand before auto-igniting.