octanes

Octanes are eight-carbon alkanes with the chemical formula C8H18, belonging to the paraffin (alkane) family. They exist as a family of constitutional isomers with varying degrees of branching. There are 18 structural isomers of octane, ranging from the straight-chain n-octane to more branched forms such as iso-octane (2,2,4-trimethylpentane).

The octane isomers differ in physical properties, but all are saturated, nonpolar hydrocarbons. Common examples include n-octane, which has a linear carbon backbone, and iso-octane, a highly branched isomer widely used as a reference compound. Physical properties vary by structure; for instance, n-octane has a boiling point of about 125.6 degrees Celsius and a melting point around −56 degrees Celsius, with a density near 0.70 g/mL at room temperature. Branched isomers generally have different boiling points and melting points but share typical hydrocarbon characteristics.

In fuels, octanes are important representatives of gasoline components. The octane rating of a fuel measures its resistance to knocking in internal combustion engines, expressed on a scale using reference fuels: iso-octane (2,2,4-trimethylpentane) assigned a rating of 100 and n-heptane assigned 0. Real-world gasoline blends typically exhibit octane ratings in the 90s, reflecting the combined contributions of multiple octane isomers and other hydrocarbon species. The term “octane” in everyday context often relates to this rating rather than a single molecular property.

Safety and handling follow standard hydrocarbon guidelines: octanes are flammable liquids with low water solubility and should be stored and used with appropriate precautions.