Homologous series

A homologous series is a family of carbon compounds that share the same general formula and the same functional group, whose successive members differ by a –CH₂– unit, and which show similar chemical properties with a gradual change in physical properties. Malay: siri homolog · Chinese: 同系列.

Because every member is built around the same functional group, the whole series reacts in the same characteristic way, so you can predict the reactions of a compound you have never met simply from the series it belongs to. The physical properties, in contrast, drift steadily down the series: as each extra –CH₂– is added, the relative molecular mass rises, the intermolecular forces grow stronger, and so the melting point and boiling point increase while volatility falls. The step from one member to the next is always the same single –CH₂–, which is why these physical changes are smooth and predictable rather than jumping about, and why a graph of boiling point against the number of carbon atoms gives a steadily rising curve.

Example. The alkanes form a homologous series: methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀) each differ from the next by CH₂, all fit the general formula CₙH₂ₙ₊₂, and all burn in oxygen the same way. Their boiling points climb steadily from methane to butane, which is exactly the gradual physical trend the definition promises.

Confusion to avoid. Do not treat a homologous series as any group of carbon compounds. Two compounds belong to the same series only if they share the general formula and functional group; ethanol and ethanoic acid are not in one series because their functional groups (–OH and –COOH) differ.

This idea is the backbone of the whole Carbon Compounds chapter: once you can name a compound’s homologous series, its general formula, functional group and typical reactions all follow, which is the joined-up thinking that SPM Chemistry rewards.