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.
How functional groups define organic families and determine reactivity, and how structural isomers (chain, positional and functional-group) share a molecular formula but differ in structure.
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What this dot point is asking
You must recognise the common functional groups and classify pairs of structural isomers.
Functional groups
The functional groups you should recognise:
| Family | Functional group | Example |
|---|---|---|
| Alkene | C=C | propene |
| Haloalkane | C-X (F, Cl, Br, I) | chloroethane |
| Alcohol | -OH | ethanol |
| Aldehyde | -CHO (terminal) | ethanal |
| Ketone | C=O (internal) | propanone |
| Carboxylic acid | -COOH | ethanoic acid |
| Ester | -COO- | ethyl ethanoate |
| Amine | -NH | ethanamine |
Structural isomerism
There are three types to distinguish at SACE level.
Chain isomers
Same formula, different carbon skeleton (straight versus branched). Example: exists as butane (straight chain) and 2-methylpropane (branched).
Positional isomers
Same carbon chain and same functional group, but the group is in a different position. Example: exists as propan-1-ol and propan-2-ol.
Functional-group isomers
Same molecular formula but a different functional group entirely. Example: is either ethanol (an alcohol) or methoxymethane (an ether).
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.
2023 SACE Stage 21 marksSalicylic acid is added to face creams. The structural formula has a -COOH group indicated by a dashed oval. Name the functional group indicated by the dashed oval in the diagram.Show worked answer →
The dashed oval surrounds a -COOH group, which is the carboxyl group (carboxylic acid functional group).
One mark for "carboxyl" or "carboxylic acid". (The phenol -OH on the ring is a separate group; the COOH highlighted is the carboxyl.)
2022 SACE Stage 21 marksThe structural formula of a thermoplastic polymer being investigated for composite materials is shown, with a -CO-O- group inside a dashed box. Name the functional group indicated by the dashed box on the diagram.Show worked answer →
The dashed box highlights a -C(=O)-O- linkage between two carbon-containing groups, which is the ester functional group.
One mark for "ester". (Distinguish from a carboxylic acid, which would have an -OH; here the oxygen is bonded to another carbon chain.)
2024 SACE Stage 21 marksThe structural formula of sorbitol (a chain with an aldehyde-derived carbon reduced to an alcohol and many -OH groups) is shown. With reference to its structural formula, state why sorbitol should not be classified as a carbohydrate.Show worked answer →
A carbohydrate must contain a carbonyl group (an aldehyde or ketone) in addition to its hydroxyl groups.
Sorbitol has many hydroxyl (-OH) groups but no carbonyl group (no aldehyde or ketone). Because it lacks the carbonyl functional group, it is a polyalcohol (sugar alcohol) rather than a carbohydrate. One mark for identifying the absence of the carbonyl/aldehyde/ketone group.