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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.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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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.

General formulae and the CH2 increment

A homologous series can be summarised by a single general formula, which is a powerful shortcut. Alkanes are $\text{C}_n\text{H}_{2n+2}$ , alkenes (one double bond) are $\text{C}_n\text{H}_{2n}$ , and alcohols are $\text{C}_n\text{H}_{2n+1}\text{OH}$ . Stepping from one member to the next always adds $\text{CH}_2$ , a mass increase of $14\ \text{g mol}^{-1}$ , which is why neighbouring members differ by $14$ in their molar masses and in their mass-spectrum molecular ions. This regular increment is also why members can be separated cleanly by fractional distillation: each extra $\text{CH}_2$ raises the boiling point by a predictable amount, so the series fractionates into a smooth sequence.

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.

Exam-style practice questions

Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WACE 20216 marksThe alkanes form a homologous series with general formula

\text{C}_n\text{H}_{2n+2}

. (a) State the general formula of the alkenes and of the alcohols. (b) The boiling points of the first four straight-chain alkanes are

-162, -89, -42

and

-1\ ^\circ\text{C}

. Explain the trend. (c) Predict whether pentane (the next member) boils above or below

-1\ ^\circ\text{C}

and justify.

Show worked answer →

A 6 mark question rewards the general formulae, the dispersion-force trend, and a justified prediction.

(a) Alkenes: $\text{C}_n\text{H}_{2n}$ . Alcohols: $\text{C}_n\text{H}_{2n+1}\text{OH}$ (equivalently $\text{C}_n\text{H}_{2n+2}\text{O}$ ).

(b) Each successive alkane has one more $\text{CH}_2$ unit, increasing molar mass and surface area. This strengthens the dispersion (London) forces between molecules, so more energy is needed to separate them and the boiling point rises steadily from methane to butane.

(c) Pentane has a longer chain than butane, so its dispersion forces are stronger still; it will boil above $-1\ ^\circ\text{C}$ (its actual boiling point is about $36\ ^\circ\text{C}$ ). The smooth trend within the homologous series lets us extrapolate confidently.

Markers reward both general formulae, the increasing-dispersion-force explanation, and the above- $-1\ ^\circ\text{C}$ prediction with reasoning.

WACE 20234 marksDefine the term homologous series and explain why all members of the alcohol series undergo similar chemical reactions but show a gradual change in physical properties.

Show worked answer →

A 4 mark answer needs the definition plus the chemical-versus-physical distinction.

A homologous series is a family of organic compounds that share the same functional group and general formula, where each member differs from the next by a $\text{CH}_2$ unit.

All alcohols undergo similar chemical reactions because chemistry is determined by the functional group, here the $\text{-OH}$ group, which is present in every member; so they all oxidise, dehydrate and burn in the same characteristic ways. Their physical properties change gradually because as the carbon chain lengthens the molar mass and surface area increase, strengthening dispersion forces and so raising boiling point and viscosity while lowering volatility and water solubility.

Markers reward the definition (same group, $\text{CH}_2$ increment), the functional-group reason for shared chemistry, and the dispersion-force reason for graded physical trends.