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Functional groups and homologous series: WACE Year 12 Chemistry

Syllabus dot point

“Identify the functional groups that define the main organic families and explain the trends within a homologous series”

8 min read

Quick answer

A functional group is the reactive part of a molecule that defines its chemical family and its characteristic reactions. A homologous series is a family of compounds with the same functional group and general formula, where each member differs from the next by a CH2\text{CH}_2 unit. Members react alike chemically but show smooth trends in physical properties as the chain lengthens.

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.

Functional group

A specific atom or group of atoms within a molecule that determines its chemical properties and characteristic reactions. The rest of the molecule (the carbon skeleton) is comparatively unreactive.

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

Homologous series

A family of organic compounds that share the same functional group and the same general formula, in which each successive member differs from the previous by a CH2\text{CH}_2 (methylene) unit.

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).

Same chemistry, graded physical trends

Members of a homologous series react in the same way chemically because they share the functional group. Their physical properties (boiling point, melting point, viscosity) change gradually and predictably as the carbon chain lengthens, because the strength of dispersion forces increases with molecular size.

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.

Predicting a trend

Question

Explain why the boiling point of alcohols increases from methanol to butan-1-ol, and why ethanol has a much higher boiling point than ethane.

Solution

Within the alcohol homologous series, each step adds a CH2\text{CH}_2 unit, increasing molar mass and surface area. This strengthens the dispersion forces between molecules, so more energy is needed to separate them and the boiling point rises from methanol to butan-1-ol.

Ethanol boils far higher than ethane (a similar-sized molecule) because the -OH functional group allows hydrogen bonding between ethanol molecules, a much stronger intermolecular force than the dispersion forces in ethane. This shows the functional group controls properties, not just chain length.

General formulae and the CH2 increment

A homologous series can be summarised by a single general formula, which is a powerful shortcut. Alkanes are CnH2n+2\text{C}_n\text{H}_{2n+2}, alkenes (one double bond) are CnH2n\text{C}_n\text{H}_{2n}, and alcohols are CnH2n+1OH\text{C}_n\text{H}_{2n+1}\text{OH}. Stepping from one member to the next always adds CH2\text{CH}_2, a mass increase of 14 g mol−114\ \text{g mol}^{-1}, which is why neighbouring members differ by 1414 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 CH2\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.

Confusing the functional group with the whole molecule

A common error is treating the alkyl chain as the reactive part; it is the functional group that drives the chemistry. Another is assuming members of a homologous series have identical physical properties; they share chemical reactions but differ in boiling point and solubility. Always identify the functional group first when classifying or predicting reactions.

Exam-style questions

Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

WACE 2021-style6 marks
The alkanes form a homologous series with general formula CnH2n+2\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,−42-162, -89, -42 and −1 ∘C-1\ ^\circ\text{C}. Explain the trend. (c) Predict whether pentane (the next member) boils above or below −1 ∘C-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: CnH2n\text{C}_n\text{H}_{2n}. Alcohols: CnH2n+1OH\text{C}_n\text{H}_{2n+1}\text{OH} (equivalently CnH2n+2O\text{C}_n\text{H}_{2n+2}\text{O}).

(b) Each successive alkane has one more CH2\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 ∘C-1\ ^\circ\text{C} (its actual boiling point is about 36 ∘C36\ ^\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 ∘C-1\ ^\circ\text{C} prediction with reasoning.

WACE 2023-style4 marks
Define 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 CH2\text{CH}_2 unit.

All alcohols undergo similar chemical reactions because chemistry is determined by the functional group, here the -OH\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, CH2\text{CH}_2 increment), the functional-group reason for shared chemistry, and the dispersion-force reason for graded physical trends.

Practise this

Sources & how we know this

Based on the official SCSA syllabus. Not yet individually reviewed by a person; always check SCSA and your school's materials for current rules.

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