A homologous series is one of the simplest—but most foundational—ideas in IB Chemistry. It explains why organic compounds behave in predictable ways, why properties change gradually, and how functional groups determine reactions. Once you understand what a homologous series is, the entire structure of organic chemistry becomes easier to navigate.
What Is a Homologous Series?
A homologous series is a family of organic compounds that share the same functional group and general formula, with each successive member differing by a CH₂ unit.
This means every compound in the series:
- Has the same functional group (e.g., –OH, –COOH)
- Has the same chemical properties
- Differs by one carbon and two hydrogens from the next member
- Shows gradual, predictable trends in physical properties
Examples of homologous series:
- Alkanes: CₙH₂ₙ₊₂
- Alkenes: CₙH₂ₙ
- Alcohols: CₙH₂ₙ₊₁OH
- Carboxylic acids: CₙH₂ₙ₊₁COOH
These repeating patterns make organic chemistry systematic rather than random.
Why Homologous Series Differ by CH₂
The –CH₂– unit is the structural building block of carbon chains.
Adding a CH₂:
- Increases the carbon chain by one
- Increases molar mass
- Slightly changes physical properties
- Leaves functional groups and reactivity unchanged
This repetition creates predictable patterns across the series.
Characteristics of a Homologous Series
A homologous series has several defining features.
