Functional groups and isomerism (SACE Stage 2 Chemistry)
“Identify functional groups in organic molecules and describe structural isomerism, including chain, positional and functional-group isomers.”
A functional group is the reactive part of a molecule that defines its chemical family and behaviour (e.g. alcohol, carboxylic acid, aldehyde). Structural isomers share a molecular formula but differ in how the atoms are connected. The three types are chain isomerism (different carbon-skeleton branching), positional isomerism (same group in a different position), and functional-group isomerism (the atoms form a different functional group entirely).
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What this dot point is asking
SACE expects you to identify functional groups and the families they define, and to draw, name and classify structural isomers.
Lead worked calculation
Draw and name all the structural (chain) isomers of pentane, .
Start with the unbranched chain
The straight chain of five carbons is pentane: .
Shorten the main chain by one and add a methyl branch
A four-carbon chain with a methyl on gives 2-methylbutane: .
Shorten again and add two methyls
A three-carbon chain with two methyls on the central carbon gives 2,2-dimethylpropane: .
Conclude
There are exactly three chain isomers of . The method, systematically shortening the main chain and redistributing carbons as branches, ensures none are missed or double-counted (drawing the same structure two ways).
Functional groups define families
A functional group is a specific atom or group of atoms within a molecule that is responsible for its characteristic chemical reactions. Molecules with the same functional group behave similarly, so functional groups let us organise the millions of organic compounds into a small number of reactive families.
| Group | Family | Formula | Example |
|---|---|---|---|
| alkene | ethene | ||
| alcohol | ethanol | ||
| aldehyde | ethanal | ||
| (mid-chain) | ketone | propanone | |
| carboxylic acid | ethanoic acid | ||
| ester | ethyl ethanoate | ||
| (halogen) | haloalkane | chloroethane | |
| amine | ethylamine |
The three types of structural isomerism
Chain isomers differ in the branching of the carbon skeleton (e.g. butane versus 2-methylpropane). Positional isomers have the same functional group on a different carbon (e.g. propan-1-ol versus propan-2-ol). Functional-group isomers have the same molecular formula but a different functional group entirely (e.g. propanal, an aldehyde, versus propanone, a ketone; or an alcohol versus an ether). All three share one molecular formula but differ in connectivity, so they are different compounds with different properties.
Why isomers matter
Because the functional group governs reactivity, functional-group isomers can behave completely differently despite identical molecular formulae: an aldehyde is readily oxidised while its ketone isomer is not. Positional isomers can differ too, for instance a secondary alcohol oxidises to a ketone while a primary alcohol oxidises to an aldehyde then acid. Recognising the isomer type predicts the chemistry.
A useful structural check
To confirm two structures are genuinely different isomers rather than the same molecule drawn twice, name both by IUPAC rules: identical names mean identical molecules. Counting degrees of unsaturation (rings plus multiple bonds) from the formula also helps: is fully saturated, and each missing pair of hydrogens indicates one double bond or ring.
The most common SACE error is listing the same structure twice in a different orientation and counting it as two isomers, for example writing a branch on "the left" and "the right" end of a symmetric chain. Always name each structure: if two drawings give the same IUPAC name, they are the same compound. The opposite error is missing functional-group isomers entirely (e.g. forgetting that can be an aldehyde or a ketone).
Why it matters
Functional groups and isomerism underpin the rest of organic chemistry: they explain why molecules with the same formula can have different uses, and they are the basis for predicting reactions, interpreting spectra, and designing syntheses.
Exam-style questions
Questions in the style of SACE Board 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.
SACE 2021-style5 marksDraw and name all the structural isomers of that are alcohols, then state one isomer of that is **not** an alcohol and identify its functional group.Show worked answer →
Alcohols of (four exist): butan-1-ol (), butan-2-ol (), 2-methylpropan-1-ol () and 2-methylpropan-2-ol (). (4 marks, 1 per correct named structure)
A non-alcohol isomer: methoxypropane (an ether, ); its functional group is the ether linkage (). This is a functional-group isomer of the alcohols. (1 mark)
SACE 2019-style4 marksFor the molecular formula : (a) draw an aldehyde isomer and name it; (b) draw a ketone isomer and name it; (c) state what type of isomerism relates these two structures and explain why their properties differ.Show worked answer →
(a) Aldehyde: propanal, , with the group at the chain end. (1 mark)
(b) Ketone: propanone, , with the on the middle carbon. (1 mark)
(c) They are functional-group isomers: same molecular formula but different functional groups (aldehyde versus ketone). (1 mark) Because the functional group determines reactivity and physical properties, the two have different chemistry, for example aldehydes are readily oxidised whereas ketones are not. (1 mark)
Practise this
Sources & how we know this
- Stage 2 Chemistry subject outline (2025 onwards) — SACE Board
- Stage 2 Chemistry: external assessment — SACE Board