A homologous series is a family of organic compounds with the same functional group and general formula, in which successive members differ by a unit. Members have similar chemical properties but show gradual changes in physical properties.
This definition is required at both SL and HL in Structure 3.2: Functional groups: classification of organic compounds.
Each homologous series is defined by several connected features:
| Feature | Meaning | Example for alkanes |
|---|---|---|
| Same functional group | Members contain the same reactive structural feature | Alkanes contain only carbon to carbon and carbon to hydrogen single bonds |
| Same general formula | One formula represents every member of the series | |
| Successive members differ by | Each next member has one additional carbon and two additional hydrogens | , , |
| Similar chemical properties | The shared functional group determines characteristic reactions | Alkanes undergo combustion and radical substitution |
| Gradual physical-property trend | Increasing molecular size strengthens London dispersion forces | Boiling point generally increases with carbon-chain length |
For example, methane, ethane and propane are consecutive members of the alkane homologous series. Moving from one member to the next adds , increasing the molar mass by approximately .
A common misconception is that compounds in a homologous series have identical physical properties. They do not: their physical properties change gradually because molecular size and the strength of intermolecular forces change. Their chemical properties are similar because they share the same functional group, although reaction rates may differ.
Exam technique: If asked to “state” what a homologous series is, include the same functional group, same general formula, and the difference. If asked to “explain” a boiling-point trend, link increasing molecular size to stronger London dispersion forces and therefore more energy required to separate the molecules.