IUPAC Nomenclature and Homologous Series Revision Notes | A Level AQA Chemistry

1. Why Naming Matters in Organic Chemistry

Imagine ordering a chemical for a laboratory and receiving the wrong compound because the name was ambiguous. That is the exact problem IUPAC nomenclature was designed to solve. Every organic compound gets one unique, internationally recognised name — and reading that name tells you precisely what the molecule looks like.

At A Level, you need to move fluently in both directions: given a structure, produce the correct IUPAC name; given a name, draw the correct structure. These two skills are tested repeatedly across AQA papers, so building a reliable system now pays dividends throughout the course.

2. Homologous Series — The Families of Organic Chemistry

Before naming individual compounds, it helps to understand how organic molecules are organised into families. A homologous series is a series of compounds that all contain the same functional group and share the same general formula, with each successive member differing by exactly one CH2 unit.

Because the functional group stays the same across the series, all members share similar chemical reactions. Physical properties — such as boiling point and viscosity — change gradually and predictably as the carbon chain lengthens, because the strength of London dispersion forces increases with molecular size.

Notice that the general formula changes between series — this is because the functional group itself contributes different atoms. Within a single series, however, the formula increments by CH2 each time you add one more carbon to the chain.

3. The Building Blocks of IUPAC Names

Every IUPAC name is built from three components: a prefix (substituents and their positions), a stem (number of carbons in the longest chain), and a suffix (the functional group). Master these three layers and you can decode — or construct — any name the exam throws at you.

The suffix tells you the functional group. For alkanes it is -ane; for alkenes it is -ene; for alcohols it is -ol; for carboxylic acids it is -oic acid; for aldehydes it is -al; for ketones it is -one. When you see a name, the suffix immediately tells you which homologous series the compound belongs to.

4. IUPAC Rules for Naming Chains — Step by Step

With the stems and suffixes in place, you need a reliable procedure for applying IUPAC rules to any structure. The rules below apply to chains with up to six carbons, which is the AQA specification limit at this stage.

• Rule 1 — Longest chain: Identify the longest continuous carbon chain. This gives the stem.
• Rule 2 — Functional group priority: The suffix is determined by the highest-priority functional group present.
• Rule 3 — Lowest locants: Number the chain from the end that gives the functional group (or substituent) the lowest possible position number.
• Rule 4 — Substituent prefixes: Name any branches (substituents) as prefixes. Methyl (–CH3) becomes 'methyl-'; ethyl (–C2H5) becomes 'ethyl-'.
• Rule 5 — Alphabetical order: When two or more different substituents are present, list them alphabetically in the name.
• Rule 6 — Multiplying prefixes: Use di-, tri-, tetra- when the same substituent appears more than once. These prefixes are ignored when alphabetising.
• Rule 7 — Cyclic compounds: Add the prefix 'cyclo-' before the stem for ring structures (e.g. cyclohexane for a six-carbon ring).

Worked example: Naming a Branched Alkane from its Structure

Name the following compound: a five-carbon chain with a methyl branch on carbon 2.

1. Count the longest continuous carbon chain: C1–C2–C3–C4–C5 = 5 carbons — The stem is 'pent-'. The branch (–CH3) is not part of this longest chain.
2. No double bonds, no heteroatoms — this is an alkane. Suffix = '-ane' — The compound belongs to the alkane homologous series.
3. Number from the end closest to the branch: branch is on C2 (not C4 from the other end) — Numbering from the left gives the branch position 2; from the right it would be 4. Choose 2 — the lower number.
4. –CH3 branch = methyl group, at position 2 → '2-methyl' — The prefix '2-methyl' is placed before the stem.
5. 2-methyl + pent + ane = 2-methylpentane — No space between prefix and stem; hyphen separates the locant from the substituent name.

Answer: 2-methylpentane

Worked example: Drawing a Structure from an IUPAC Name

Draw the full structural formula of 3-methylhexan-1-ol.

1. 'hex-' = 6 carbons in the longest chain. Draw: C1–C2–C3–C4–C5–C6 — Always start by drawing the main chain before adding anything else.
2. '-ol' = hydroxyl group (–OH). 'hexan-1-ol' means the –OH is on C1. — The locant '1' in 'hexan-1-ol' tells you the –OH is attached to carbon 1.
3. Attach –OH to C1: HO–C1–C2–C3–C4–C5–C6 — Carbon 1 now carries the hydroxyl group.
4. '3-methyl' = a –CH3 branch on C3 — Attach a methyl group to C3 of the main chain.
5. Each carbon must have four bonds total. Fill remaining bonds with H atoms. — C1: –OH, 2H, bond to C2. C3: bond to C2, C4, CH3 branch, and 1H. Check every carbon satisfies tetravalency.

Answer: HO–CH2–CH2–CH(CH3)–CH2–CH2–CH3 (3-methylhexan-1-ol, a six-carbon alcohol with a methyl branch at C3)

5. Naming Alkenes, Alcohols, and Other Key Series

The same stem-and-suffix logic applies across all homologous series, but you need to include a locant (position number) whenever the functional group could be on more than one carbon. For alkenes, the locant indicates which carbon the double bond starts on; for alcohols, it indicates which carbon carries the –OH group.

For example, a three-carbon chain with a double bond starting at C1 is prop-1-ene, while the double bond starting at C2 would give but-2-ene (in a four-carbon chain). The locant is placed immediately before the suffix it modifies: 'but-2-ene', not '2-butene' — AQA mark schemes use the former format.

Carboxylic acids are a special case: the –COOH group is always on carbon 1 by definition, so no locant is needed for the acid group itself. The carbon of the –COOH counts as part of the main chain, so ethanoic acid has two carbons in total (the –CH3 and the –COOH carbon).

6. Cyclic Compounds and the 'Cyclo-' Prefix

When the carbon chain forms a closed ring, the prefix cyclo- is added before the stem. A six-carbon ring of alkane carbons is cyclohexane; a six-carbon ring with one double bond is cyclohexene. The ring carbons are counted in the same way as a chain — the stem reflects how many carbons are in the ring.

For substituted cyclic compounds, the ring carbon bearing the substituent (or functional group) is numbered C1, and subsequent carbons are numbered to give the lowest possible locants to all substituents. A methyl group on a cyclohexane ring gives methylcyclohexane — no locant is needed when there is only one substituent, because all ring positions are equivalent until a second substituent is added.

7. Exam Technique and Common Mistakes

AQA mark schemes for nomenclature questions are unforgiving about specific details. The most common mark-losing errors are predictable — which means they are entirely avoidable once you know what to watch for.

The ability to move between name and structure — and to justify your choices using IUPAC rules — is the core skill this topic tests. Every rule exists to eliminate ambiguity, so when you apply them correctly, the name you produce is the only possible correct answer.

Homologous series, IUPAC stems, suffixes, locants, and the cyclo- prefix together form a complete system for naming and drawing any organic compound within the AQA specification range. Practise applying all seven rules in sequence on unfamiliar structures, and the process becomes automatic under exam conditions.