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How do functional groups define organic families, and how can one formula give different compounds?

Identify functional groups in organic molecules and describe structural isomerism, including chain, positional and functional-group isomers.

Recognising the main functional groups, naming the families they define, and the three types of structural isomerism, with worked SACE-style isomer-drawing and degree-of-unsaturation examples.

Generated by Claude Opus 4.89 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
Lead worked calculation
Functional groups define families
The three types of structural isomerism
Why isomers matter
A useful structural check
Why it matters

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

Counting and drawing isomers of C5H12

Draw and name all the structural (chain) isomers of pentane, $\text{C}_5\text{H}_{12}$ .

Start with the unbranched chain

The straight chain of five carbons is pentane: $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3$ .

Shorten the main chain by one and add a methyl branch

A four-carbon chain with a methyl on $\text{C-2}$ gives 2-methylbutane: $(\text{CH}_3)_2\text{CHCH}_2\text{CH}_3$ .

Shorten again and add two methyls

A three-carbon chain with two methyls on the central carbon gives 2,2-dimethylpropane: $(\text{CH}_3)_4\text{C}$ .

Conclude

There are exactly three chain isomers of $\text{C}_5\text{H}_{12}$ . 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

Group	Family	Formula	Example
$\text{C}=\text{C}$	alkene	$\text{R-CH}=\text{CH-R}$	ethene
$\text{-OH}$	alcohol	$\text{R-OH}$	ethanol
$\text{-CHO}$	aldehyde	$\text{R-CHO}$	ethanal
$\text{C}=\text{O}$ (mid-chain)	ketone	$\text{R-CO-R}$	propanone
$\text{-COOH}$	carboxylic acid	$\text{R-COOH}$	ethanoic acid
$\text{-COO-}$	ester	$\text{R-COO-R}$	ethyl ethanoate
$\text{-X}$ (halogen)	haloalkane	$\text{R-X}$	chloroethane
$\text{-NH}_2$	amine	$\text{R-NH}_2$	ethylamine

The three types of structural isomerism

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: $\text{C}_n\text{H}_{2n+2}$ is fully saturated, and each missing pair of hydrogens indicates one double bond or ring.

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 practice questions

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

SACE 20215 marksDraw and name all the structural isomers of

\text{C}_4\text{H}_{10}\text{O}

that are alcohols, then state one isomer of

\text{C}_4\text{H}_{10}\text{O}

that is **not** an alcohol and identify its functional group.

Show worked answer →

Alcohols of $\text{C}_4\text{H}_{10}\text{O}$ (four exist): butan-1-ol ( $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$ ), butan-2-ol ( $\text{CH}_3\text{CH}_2\text{CH(OH)CH}_3$ ), 2-methylpropan-1-ol ( $(\text{CH}_3)_2\text{CHCH}_2\text{OH}$ ) and 2-methylpropan-2-ol ( $(\text{CH}_3)_3\text{COH}$ ). (4 marks, 1 per correct named structure)

A non-alcohol isomer: methoxypropane (an ether, $\text{CH}_3\text{OCH}_2\text{CH}_2\text{CH}_3$ ); its functional group is the ether linkage ( $\text{C-O-C}$ ). This is a functional-group isomer of the alcohols. (1 mark)

SACE 20194 marksFor the molecular formula

\text{C}_3\text{H}_6\text{O}

: (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, $\text{CH}_3\text{CH}_2\text{CHO}$ , with the $\text{-CHO}$ group at the chain end. (1 mark)

(b) Ketone: propanone, $\text{CH}_3\text{COCH}_3$ , with the $\text{C}=\text{O}$ 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)