Definition and shared features

A homologous series is a family of organic compounds with the same functional group and general formula. Members have similar chemical properties because reactions occur at the same functional group. Successive members differ by one CH2 unit.

Physical-property trends

As chain length increases, intermolecular attractions generally become stronger. Boiling points rise, volatility falls and viscosity tends to increase. The change is gradual, so neighbouring members have similar but not identical physical properties.

Chemical similarity

Methane and ethane both combust and undergo chlorination; ethene and propene both decolourise aqueous bromine; ethanol and propan-1-ol both combust. Equations differ in coefficients and products, but the reaction type is shared.

Using the series idea

General formulae help test whether a molecular formula belongs to a series. A formula that fits CnH2n may be an alkene at this level, but structural information is still needed to locate the double bond and distinguish isomers.

High-value recall and connections

Essential recall: By what unit do successive members differ? CH2. Why do members have similar chemical properties? They contain the same functional group. Give one physical trend as chain length increases. Boiling point generally increases, volatility decreases or viscosity increases. Practical connection: Plot boiling point against carbon number for a supplied series and describe the trend. Identify any anomaly without forcing it onto a straight line. Examination connection: Do not state that members have identical physical properties; they show a trend.

Review questions and suggested answers