Structural Isomerism: Types and Drawing Revision Notes | A Level AQA Chemistry

1. The Same Formula, Different Molecule

Here is a puzzle: two compounds both have the molecular formula $C_4H_{10}$, yet one boils at $-1^{\circ}C$ and the other at $-12^{\circ}C$. They are not the same substance — they are structural isomers. Understanding why this happens is the foundation of organic chemistry at A Level.

Organic molecules are built from the same atoms, but the way those atoms are connected changes everything: the shape, the boiling point, the reactivity, and even the smell. Structural isomerism is the formal name for this phenomenon, and AQA expects you to define it precisely, recognise all three types, and draw them accurately.

A key point to anchor this: the molecular formula tells you what atoms are present, but the structural formula tells you how they are connected. Structural isomers share the first but differ in the second. With that definition locked in, you can now look at the three distinct types AQA requires you to know.

2. Chain Isomerism — Rearranging the Carbon Skeleton

The first type of structural isomerism is chain isomerism. Here, the carbon skeleton itself changes — you can have a straight, unbranched chain or a branched arrangement. The functional group (if any) stays the same; it is purely the backbone that differs.

Think of the carbon atoms as a set of building blocks. You can line them all up in a row, or you can attach some of them as side branches. Both arrangements use exactly the same number of carbon and hydrogen atoms, so the molecular formula is identical — but the structures are genuinely different molecules.

The clearest example uses $C_4H_{10}$. Butane has all four carbons in a straight chain: $CH_3CH_2CH_2CH_3$. Methylpropane has a central carbon bonded to three methyl groups: $(CH_3)_3CH$. Both are alkanes, both are saturated, but their different shapes mean different intermolecular forces and therefore different boiling points.

Worked example: Drawing All Chain Isomers of $C_5H_{12}$

Identify and draw all chain isomers of pentane ($C_5H_{12}$).

1. $CH_3 - CH_2 - CH_2 - CH_2 - CH_3$ — All five carbons in a single unbranched chain. This is pentane. Always start here — it is the parent chain and the most straightforward isomer.
2. $CH_3 - CH(CH_3) - CH_2 - CH_3$ — A four-carbon backbone with a methyl ($-CH_3$) branch on carbon 2. This is methylbutane. Check: 5 C and 12 H — correct.
3. $CH_3 - C(CH_3)_2 - CH_3$ — A three-carbon backbone with two methyl branches on the central carbon. This is 2,2-dimethylpropane. Check: 5 C and 12 H — correct.
4. A two-carbon backbone would require three branches, but carbon can only form four bonds — already satisfied by the backbone and H atoms. No further chain isomers are possible. — Systematically reducing the chain length confirms three chain isomers total for $C_5H_{12}$.

Answer: Three chain isomers: pentane, methylbutane, and 2,2-dimethylpropane.

Chain isomers are the most straightforward type to spot: ask yourself whether the carbon backbone has changed shape. If the answer is yes, and the functional group is the same, you have chain isomers. With that type secured, the next step is to keep the skeleton fixed and move the functional group instead.

3. Position Isomerism — Moving the Functional Group

Position isomers share the same carbon skeleton and the same functional group — the only difference is where on the chain that functional group is attached. The name itself is a clue: the position changes.

A helpful way to picture this: imagine a row of numbered seats on a bus. The passenger (functional group) is the same person, but they can sit in seat 1, seat 2, or seat 3. The bus (carbon chain) has not changed. The passenger has not changed. Only the seat number has.

• Propan-1-ol ($CH_3CH_2CH_2OH$) and propan-2-ol ($CH_3CH(OH)CH_3$) — both $C_3H_8O$, both have an $-OH$ group, but on C-1 versus C-2
• But-1-ene ($CH_2=CHCH_2CH_3$) and but-2-ene ($CH_3CH=CHCH_3$) — both $C_4H_8$, both have a $C=C$ double bond, but between C1–C2 versus C2–C3
• 1-bromopropane ($CH_3CH_2CH_2Br$) and 2-bromopropane ($CH_3CHBrCH_3$) — both $C_3H_7Br$, both have a $-Br$ substituent, but on C-1 versus C-2

Notice that in every example above, the molecular formula is identical and the functional group is the same type — only its location on the numbered carbon chain differs. This distinction matters enormously in exam questions, where you must justify why two structures are position isomers rather than chain or functional group isomers.

Position isomerism is particularly common with alcohols, halogenoalkanes, and alkenes — all functional groups that can sit at different numbered positions on a chain. When drawing position isomers in an exam, always number the carbon chain from the end that gives the functional group the lowest possible number, and make sure the chain length and branching are identical in both structures.

4. Functional Group Isomerism — A Different Chemical Identity

The third type is the most striking: functional group isomers share the same molecular formula but contain entirely different functional groups. This means they belong to different homologous series and have fundamentally different chemical properties — they react in completely different ways.

The classic AQA example is the molecular formula $C_2H_6O$. This formula can represent ethanol ($CH_3CH_2OH$), an alcohol with an $-OH$ group, or methoxymethane ($CH_3OCH_3$), an ether with a $-O-$ linkage. Both contain two carbons, six hydrogens, and one oxygen — but one reacts with sodium to produce hydrogen gas, and the other does not. Different functional group, different chemistry.

The propanal/propanone pair ($C_3H_6O$) is another high-frequency AQA example. Propanal is an aldehyde — it has the $-CHO$ group at the end of the chain and will give a positive result with Tollens' reagent. Propanone is a ketone — the carbonyl group sits between two carbon atoms and does not react with Tollens' reagent. Same formula, completely different reactivity.

5. Comparing All Three Types — A Systematic Approach

Now that all three types are in place, it helps to see them side by side. The key diagnostic question for any pair of structural isomers is: what has changed? Work through the carbon skeleton first, then the functional group type, then its position.

A systematic three-question check works reliably in exams: (1) Is the molecular formula the same? If yes, they are structural isomers of some kind. (2) Is the functional group the same type? If no, they are functional group isomers. (3) Is the carbon skeleton the same? If yes and the functional group is the same type, check the position — position isomers if the location differs, chain isomers if the skeleton itself has changed.

6. Drawing Structural Isomers — Exam Technique and Common Errors

Being able to draw isomers accurately is a separate skill from identifying them. AQA mark schemes award marks for correct displayed or structural formulae, and they penalise structures where the atom count does not match the molecular formula or where carbon has the wrong number of bonds.

Worked example: Drawing All Structural Isomers of $C_4H_8O$ Containing a Carbonyl Group

Draw the structural isomers of $C_4H_8O$ that contain a carbonyl group ($C=O$). Identify the type of isomerism between each pair.

1. Aldehydes ($-CHO$) and ketones ($-CO-$) both contain a carbonyl group. Both are possible with $C_4H_8O$. — This immediately tells you that functional group isomers are possible here, as well as position isomers within each functional group class.
2. $CH_3CH_2CH_2CHO$ — butanal. The $-CHO$ group must be at C-1 by definition for an aldehyde. — Aldehydes always have the carbonyl at the terminal carbon, so there is only one straight-chain aldehyde for a four-carbon chain.
3. $(CH_3)_2CHCHO$ — 2-methylpropanal. A three-carbon chain with a methyl branch at C-2 and the $-CHO$ at C-1. — This is a chain isomer of butanal — same functional group (aldehyde), different carbon skeleton.
4. $CH_3COCH_2CH_3$ — butan-2-one. The carbonyl sits between C-2 and C-3. Check: 4 C, 8 H, 1 O — correct. — Butan-2-one is a functional group isomer of butanal and 2-methylpropanal — same molecular formula, but a ketone rather than an aldehyde. Note: butan-3-one would be the same compound as butan-2-one when numbered correctly from the nearest end.
5. Butanal vs 2-methylpropanal: chain isomers (same functional group, different skeleton). Butanal vs butan-2-one: functional group isomers (aldehyde vs ketone). — Always state the type of isomerism and justify it — AQA mark schemes often award a mark specifically for the correct classification with reasoning.

Answer: Three structural isomers containing a carbonyl group: butanal ($CH_3CH_2CH_2CHO$), 2-methylpropanal ($(CH_3)_2CHCHO$), and butan-2-one ($CH_3COCH_2CH_3$). Butanal and 2-methylpropanal are chain isomers; both are functional group isomers of butan-2-one.

7. Exam Pivot — Securing Every Mark

Structural isomerism questions appear across AQA papers in several formats: 'draw two structural isomers of...', 'state and explain the type of isomerism shown', or 'suggest a structural isomer of compound X'. Each format has a slightly different mark-winning strategy.

For 'draw' questions, the mark scheme checks connectivity, not presentation. A skeletal formula, a structural formula, or a displayed formula are all acceptable — but every atom must be accounted for. For 'state and explain' questions, naming the type alone is rarely sufficient; you must link the name to the structural feature that defines it (skeleton, functional group type, or functional group position).

For 'suggest' questions, AQA is testing whether you can apply isomerism to an unfamiliar context. Work through the three-question diagnostic: same formula? same functional group type? same skeleton? Your answer will fall naturally into one of the three categories. Always write the molecular formula of your suggested isomer alongside the structure to demonstrate it matches the original — this is a mark-winning habit that costs no extra time.

• State the molecular formula of both compounds at the start of any isomerism explanation — this confirms the shared formula and earns the first mark.
• Use the exact AQA terminology: 'chain isomers', 'position isomers', 'functional group isomers' — not informal alternatives.
• When drawing isomers, annotate each structure with its name or a brief label to show the examiner you know what you have drawn.
• For functional group isomers, name both functional groups explicitly — 'alcohol and ether' or 'aldehyde and ketone' — rather than describing them vaguely.
• Check that cycloalkanes are considered when listing isomers of alkenes with the formula $C_nH_{2n}$ — they are functional group isomers and are frequently overlooked.

Structural isomerism underpins much of the organic chemistry you will encounter throughout the A Level course — from alcohols and halogenoalkanes to carbonyl compounds and esters. Every time you meet a new molecular formula, the habit of asking 'how many structural isomers exist, and what type are they?' will sharpen both your understanding and your exam performance.