Homologous Series and Functional Groups (Edexcel AS Chemistry): Revision Note

Homologous Series & Functional Groups

Homologous series

• A homologous series is a group of organic compounds that have the same functional group, but each successive member differs by CH₂

Ethanol and propanol belong to the same homologous series

Homologous Series of Alkanes

• Things we can say about a homologous series:

  • Each member has the same functional group

  • Each member has the same general formula

  • Each member has similar chemical properties

  • Each successive member differs by  -CH₂ –

  • Members have gradually changing physical properties, for example, boiling point, melting point and density

• As a homologous series is ascended, the size of the molecule increases

• This has an effect on the physical properties, such as boiling point and density

Functional Groups

• Functional groups determine the physical and chemical properties of molecules

• The table below shows a summary of common functional groups found in compounds

• R is usually an alkyl group (in some families, such as aldehydes and carboxylic acids, R can be a hydrogen atom, e.g. methanal, HCHO)

Functional Groups found in Compounds Table