heptane

C₇H₁₆. An alkane hydrocarbon with seven carbon atoms. There are nine structural isomers, most of which occur in the gasoline fraction of petroleum. The straight-chain compound, boiling point 98°C, n-heptane, CH₃CH₂CH₂CH₂CH₂CH₂CH₃, is particularly bad for causing ‘knocking’ in petrol engines.

Heptane has nine isomers:

Heptane (n-heptane) 2-Methylhexane 3-Methylhexane 2,2-Dimethylpentane 2,3-Dimethylpentane 2,4-Dimethylpentane 3,3-Dimethylpentane 2,2,3-Trimethylbutane 3-Ethylpentane

The straight-chain isomer n-heptane is the zero point of the octane rating scale. Its choice for the zero point of the scale was due to the availability of very high purity n-heptane, unmixed with other isomers of heptane or other alkanes, distilled from the resin of Jeffrey pine. Other sources of heptane and octane, produced from crude oil, contain a mixture of different isomers with greatly differing ratings, so do not give a precise zero point.