Physical Properties of Alkanes

A series of unbranched alkanes in which successive members differ by a −CH₂− group is called a homologous series, and each alkane in this series is a homologue.

Alkanes are nonpolar due to the small electronegativity difference between carbon and hydrogen, implying that only weak dispersion forces exist between their molecules.

The strength of these forces increases proportionally with carbon chain length. The first four alkanes are gases at room temperature and atmospheric pressure; moderate carbon chains, from C₅ to C₁₇, are liquids. Homologues beyond C₁₇ are solids.

Dispersion forces also influence the different physical properties of alkanes.

Within a homologous series, as surface area increases, the dispersion forces between molecules also increase. Because more energy is required to separate the molecules, the boiling points of unbranched alkanes rise with each additional carbon.

Unlike straight-chain alkanes, branched alkanes are compact and more spherical, which reduces the area of interaction and the strength of the intermolecular forces. Therefore, compared to the unbranched form, branched isomers boil at lower temperatures.

While the boiling points of straight-chain alka