13.2 Homologous Series

This topic explains the homologous series — a family of related organic compounds that differ by a –CH₂– unit and share the same functional group and general formula.

Definition

A homologous series is a series of related compounds in which any two adjacent members differ by a –CH₂– unit.

Simple explanation: A homologous series is a family of organic compounds that differ from each other by one CH₂ group.

Main Idea

There exists a close relationship between different organic compounds. This similarity in behavior has made the study of millions of organic compounds easier. They can be classified into a few families.

A series of related compounds in which adjacent members differ by –CH₂– is called a homologous series.

Examples: Alkanes, alcohols, and many other classes of organic compounds form homologous series.

Simple explanation: Organic compounds can be grouped into families — each family is a homologous series.

Core Concept

How Adjacent Members Differ

Consider the alkanes:

Note the difference between adjacent alkanes — they differ by the same unit –CH₂–.

Simple explanation: Each member of the series is one CH₂ group bigger than the previous one.

Alcohols Also Form Homologous Series

These compounds also differ by the same unit –CH₂–.

Simple explanation: Alcohols also form a homologous series — each member differs by CH₂.

All Classes of Organic Compounds Form Homologous Series

• Alkanes
• Alkenes
• Alkynes
• Alcohols
• Aldehydes
• Ketones
• Carboxylic acids

form homologous series.

Simple explanation: Every family of organic compounds is a homologous series.