Why do members of a homologous series share chemical properties, and how does a functional group define a family?
Identify the functional groups that define the main organic families and explain the trends within a homologous series
A focused answer to the WACE Year 12 Chemistry dot point on functional groups and homologous series, identifying the group that defines each organic family and explaining why members share chemical properties but show graded physical trends, with a worked example and common exam mistakes.
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What this dot point is asking
Organic chemistry is organised into families. Each family is defined by a functional group, the cluster of atoms responsible for the molecule's characteristic chemistry.
The main functional groups
The families you must recognise and the group that defines each:
- Alkene: C=C double bond.
- Haloalkane: a halogen atom (such as -Cl, -Br).
- Alcohol: hydroxyl group, -OH.
- Aldehyde: -CHO (a carbonyl at the end of the chain).
- Ketone: C=O carbonyl within the chain.
- Carboxylic acid: -COOH.
- Ester: -COO- linkage.
- Amine: -NH2 (or substituted nitrogen).
- Amide: -CONH2 linkage.
What a homologous series is
For example, the alcohols methanol, ethanol and propan-1-ol all contain -OH and form the homologous series of alcohols. Because they share the functional group, they undergo the same chemical reactions (the chemistry is set by the group, not the chain).
Physical trends along a series
As the chain gets longer, the molar mass and surface area increase, so the dispersion forces between molecules strengthen. This raises boiling and melting points and viscosity, and lowers volatility and water solubility (the non-polar chain becomes a larger fraction of the molecule). These smooth trends let you predict an unknown member's properties from its neighbours.
Why this matters
Recognising functional groups is the key skill that unlocks the whole of Unit 4: naming, predicting reactions, designing synthesis routes, and interpreting spectra all depend on identifying which family a molecule belongs to. The homologous series idea lets you generalise from one member to a whole family.