Homologous Series
Chemistry: IB Diploma Higher Level, topic 3.6.8
Homologous series
A homologous series is a family of organic compounds in which each member differs from the next by CH2. They share similar chemical properties and show a trend in physical properties.
Homologous series
- A homologous series is a family of organic (carbon-based) compounds in which each member differs from the next by CH2.
- The members of a homologous series all have the same general formula (e.g alkanes: CnH2n+2).
- Members of a homologous series have the same functional group (e.g. alcohols: -OH).
Examples of homologous series
- Examples of homologous series are shown in the table above.
Chemical properties
- The chemical properties of organic molecules are determined by the functional group.
- Therefore, all members of a homologous series have similar chemical properties as they have the same functional group.
Physical properties
- Members of a homologous series show graduation in physical properties, such as boiling and melting point and solubility.
- Boiling point increases as the carbon-chain length is increased within a homologous series, due to the increasing strength of London dispersion forces.
- The different boiling points allow the compounds to be separated by distillation.
- For example, petroleum, a mixture of mainly hydrocarbons, is separated into useful fractions using fractional distillation.
Physical properties cont.
- The table above shows how the boiling point increases as the carbon-chain length is increased within a homologous series of alkanes.
Test yourself on Homologous Series
Which of the following is NOT an example of a homologous series?