homologous series
/ huh-MOL-uh-gus /
Line up the alkanes by size — methane, ethane, propane, butane — and you notice each one is just the previous one with one more CH2 unit stitched in. That orderly staircase is a homologous series: a family of compounds sharing the same functional group, each member differing from the next by a constant CH2. It is nature being tidy, and chemists love it.
Formally, a homologous series is a set of compounds with the same general formula and the same functional group, in which consecutive members differ by a CH2 (a methylene) unit. The alkanes follow CnH2n+2, the primary alcohols follow CnH2n+1OH, and so on. Because every member carries the identical functional group, they share the same characteristic chemical reactions — all alcohols behave like alcohols. And because they differ only in the length of an inert hydrocarbon tail, their physical properties change smoothly and predictably as you go up the series: boiling point, melting point and viscosity generally rise with chain length (more surface area means stronger dispersion forces), while water solubility usually falls as the greasy tail grows.
The homologous series is a powerful organizing and predictive tool. Measure a property for a few members and you can interpolate or extrapolate for the rest, because the trend is so regular. It also captures a deep simplifying truth of organic chemistry: chemistry is governed mainly by the functional group, while the carbon chain mostly just tunes the physical behaviour. One honest caveat — the very first member of a series can be a little anomalous, and trends are smooth but not perfectly linear.
Among straight-chain alkanes, methane (CH4), ethane and propane are gases at room temperature, pentane (C5) is a volatile liquid, and around C18 they become waxy solids — boiling point climbing steadily by roughly 20-30 °C per added CH2.
Add CH2 units one at a time and physical properties trend smoothly up the series.
Members share chemistry because they share a functional group; their physical properties trend with chain length. But the trend is smooth, not perfectly linear, and the first member of a series can be slightly anomalous.