← Unit 4: Structure, synthesis and design
Topic 1: Properties and structure of organic materials
Apply IUPAC nomenclature to name and write structural formulas for organic compounds including alkanes, alkenes, haloalkanes, alcohols, aldehydes, ketones, carboxylic acids, esters, amines and amides, and classify organic compounds by their functional groups
A focused answer to the QCE Chemistry Unit 4 dot point on IUPAC nomenclature and functional groups. Covers the ten core homologous series, the suffix/prefix priority order, locant numbering rules, and worked names for substituted alkenes, alcohols and esters. Includes the structural-formula skeletal/condensed conventions QCAA accepts.
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What this dot point is asking
QCAA wants you to apply IUPAC nomenclature consistently across the ten homologous series in the Unit 4 syllabus, write structural formulas (skeletal, condensed, or full) for any compound you name, and recognise functional groups from structures. This dot point underpins almost every Unit 4 question; if you cannot name the species, you cannot describe its reactions, its spectrum, or its role in a synthesis pathway.
The answer
IUPAC nomenclature builds a systematic name from three pieces: the parent chain (longest carbon chain that contains the principal functional group), the principal-group suffix, and locants and prefixes for substituents.
The ten homologous series
| Series | Functional group | Suffix or class name | Example |
|---|---|---|---|
| Alkane | C-C single bonds only | -ane | propane (CH3-CH2-CH3) |
| Alkene | C=C double bond | -ene | propene (CH3-CH=CH2) |
| Haloalkane | C-X (X = F, Cl, Br, I) | halo- prefix | 2-chloropropane |
| Alcohol | C-OH | -ol | propan-1-ol |
| Aldehyde | terminal -CHO | -al | propanal |
| Ketone | internal C=O | -one | propan-2-one |
| Carboxylic acid | -COOH (terminal) | -oic acid | propanoic acid |
| Ester | -COO-R | -yl -oate | methyl propanoate |
| Amine | C-NH2 (or C-NHR, C-NR2) | -amine | propan-1-amine |
| Amide | -CO-NH2 (or -CO-NHR) | -amide | propanamide |
QCAA past papers expect you to identify any of these from a structural formula and to write the matching name and formula on demand.
Naming rules in priority order
- Identify the principal functional group (highest in the suffix priority list). Order from highest to lowest: carboxylic acid, ester, amide, aldehyde, ketone, alcohol, amine, alkene, alkane. Haloalkanes are always prefixes, never suffixes.
- Find the longest carbon chain containing the principal group. That is the parent.
- Number the chain so the principal group gets the lowest possible locant. For aldehydes and carboxylic acids the suffix carbon is C1 by definition.
- Add prefixes for substituents in alphabetical order, each with a locant. Ignore di-, tri-, tetra- for alphabetising but keep them in the name.
- Use hyphens between numbers and letters, commas between numbers. No spaces except between the two words of an ester (methyl propanoate).
Worked names
CH3-CH(OH)-CH2-CH3. Principal group: -OH (alcohol). Longest chain containing it: 4 carbons. Number so OH gets the lowest locant: from the left, OH is on C2; from the right, OH is on C3. Use C2. Name: butan-2-ol.
(CH3)2CH-CH2-CH2-COOH. Principal group: -COOH (highest priority). Longest chain containing COOH: 5 carbons (pentanoic acid). COOH carbon is C1. Methyl branch on C4. Name: 4-methylpentanoic acid.
CH3-CH2-O-CO-CH3. Two oxygens between the ester carbon and the next chain identify this as an ester. The acid side (with C=O) is ethanoic; the alcohol side (CH3CH2-) is ethyl. Esters are named alcohol-side first as the alkyl, acid-side second as the -oate. Name: ethyl ethanoate.
CH3-CO-NH-CH3. Amide. Parent is the acid chain bearing C=O (ethan-: 2 carbons). The nitrogen has one methyl substituent, prefixed with "N-" in italics. Name: N-methylethanamide.
Structural-formula conventions
QCAA accepts three structural-formula styles:
- Full structural formula. Every bond shown explicitly. Slow to draw but unambiguous.
- Condensed structural formula. Bonds inferred from sequence, e.g. CH3-CH2-CH2-OH or CH3CH2CH2OH. Use brackets for branches: (CH3)2CHCH2OH for 2-methylpropan-1-ol.
- Skeletal formula. Carbons implied at vertices and chain ends; hydrogens on carbon implied. Heteroatoms and their hydrogens drawn explicitly. Fast and clean for larger molecules.
A question asking for "the structural formula" accepts any of the three unless it specifies "full" or "skeletal". The QCAA EA preference for short responses is condensed; skeletal is preferred for organic synthesis diagrams.
Common nomenclature traps
Numbering from the wrong end. Always number so the principal suffix gets the lowest locant. Substituents come second in the tie-breaker.
Forgetting locants on the suffix. "But-2-ene" and "but-1-ene" are different compounds; "butan-2-ol" and "butan-1-ol" are different compounds. The locant goes immediately before the suffix.
Naming esters backwards. The alkyl group is from the alcohol; the -oate name is from the acid. Methyl propanoate is the ester of propanoic acid with methanol (CH3CH2COOCH3), not vice versa.
Treating amines like amides. An amine has C-NH2 with no carbonyl. An amide has C(=O)-NH2. Different functional groups, different reactivity, different IUPAC class.
Confusing aldehyde and ketone numbering. Aldehyde CHO is always on C1 (and the "1" is omitted). Ketone C=O is always internal and needs an explicit locant (butan-2-one, pentan-3-one).
Why this matters in IA3 and the EA
IA3 research investigations frequently include comparisons of compounds named by functional group (esters in flavour chemistry, polymers built from specific monomers). The EA Paper 1 multiple choice typically includes one or two nomenclature items, and Paper 2 short response routinely asks students to draw structures from names or vice versa. Errors here propagate into every downstream answer about reactivity or spectroscopy.
In one sentence
IUPAC nomenclature follows a fixed priority for identifying the principal functional group, choosing the parent chain, numbering for lowest locants, and adding substituent prefixes alphabetically; mastering the ten Unit 4 homologous series and the structural-formula conventions is the precondition for every subsequent organic dot point.
Past exam questions, worked
Real questions from past QCAA papers on this dot point, with our answer explainer.
2023 QCAA-style4 marksDraw the structural formula of (a) 3-methylbut-1-ene, (b) 2-chloropropan-1-ol, (c) methyl propanoate, (d) N-methylethanamide. For each, state the homologous series.Show worked answer →
A 4-mark answer needs each structural formula correctly drawn and each series identified. One mark per part.
(a) 3-methylbut-1-ene. Four-carbon chain with a double bond between C1 and C2 and a methyl branch on C3. CH2=CH-CH(CH3)-CH3. Series: alkene.
(b) 2-chloropropan-1-ol. Three-carbon chain; OH on C1, Cl on C2. HOCH2-CHCl-CH3. Series: haloalkanol (both alcohol and haloalkane functional groups; QCAA accepts either, prefer alcohol because OH has higher suffix priority).
(c) Methyl propanoate. Ester of propanoic acid and methanol. CH3-CH2-COO-CH3. Series: ester.
(d) N-methylethanamide. Amide derived from ethanoic acid; the nitrogen carries a methyl group. CH3-CO-NH-CH3. Series: amide (N-substituted).
Markers reward correct connectivity in each formula and the right series name. Common slips: writing methyl propanoate backwards (so it becomes propyl methanoate), or labelling propan-1-ol when the chlorine is on C2.
2022 QCAA-style3 marksAn organic compound has the structural formula CH3-CH2-CH(CH3)-CHO. (a) Identify the homologous series. (b) Give the IUPAC name. (c) Explain why the locant for the carbonyl group is not stated in the name.Show worked answer →
A 3-mark answer needs the series, the IUPAC name, and the locant convention reasoning.
(a) Series. Aldehyde (terminal -CHO group).
(b) IUPAC name. Number from the carbonyl carbon (C1). Longest chain containing the CHO is four carbons (butanal). A methyl branch sits on C2. Name: 2-methylbutanal.
(c) Locant convention. Aldehydes always have the CHO on C1 because the carbonyl carbon is at the chain end (it carries an H). With only one possible position, the "1" is omitted in the parent name (you write butanal, not butan-1-al). Compare ketones (propanone vs butan-2-one), where internal carbonyl positions require an explicit locant.
Markers reward the IUPAC name with the methyl on C2 (not C3, which is the wrong direction of numbering), and the aldehyde-position reasoning. A common error is naming this 3-methylbutanal, which numbers from the wrong end.
Related dot points
- Describe and explain structural isomerism (chain, position and functional group isomers) and stereoisomerism (cis-trans / geometric isomerism in alkenes) in organic compounds
A focused answer to the QCE Chemistry Unit 4 dot point on isomerism. Distinguishes chain, position and functional-group isomers, sets out the conditions for cis-trans isomerism in alkenes, and works through C4H8O3 and 1,2-dichloroethene examples. Highlights the property differences QCAA tests in IA3 secondary data.
- Describe and explain trends in the physical properties of organic compounds (melting point, boiling point and solubility in water) in terms of molecular structure, functional groups and intermolecular forces
A focused answer to the QCE Chemistry Unit 4 dot point on physical properties of organic compounds. Connects boiling point and solubility trends to dispersion forces, dipole-dipole, and hydrogen bonding. Compares alcohols, aldehydes, ketones, carboxylic acids and amides at matched Mr and explains chain length and branching effects for IA3 and EA.
- Predict and explain the products of substitution reactions of alkanes with halogens and addition reactions of alkenes with halogens, hydrogen halides, hydrogen and water
A focused answer to the QCE Chemistry Unit 4 dot point on alkane and alkene reactivity. Sets out free-radical substitution of alkanes by halogens (UV initiation) and electrophilic addition of alkenes (halogens, hydrogen halides, hydrogen, water) with Markovnikov's rule for unsymmetrical alkenes. Includes the bromine-water test and the IA3 / EA expected products for each.