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Structural isomerism and E-Z isomerismAQA A-Level Chemistry: Revision notes

Section 1

Isomers and structural isomerism

Isomers are compounds with the same molecular formula but a different arrangement of atoms. Structural isomers have the same molecular formula but different structural formulae, meaning the atoms are joined together in a different order.

There are three types you must be able to draw and name:

  • Chain isomers differ in the carbon skeleton (straight or branched). C₅H₁₂ has three: pentane, 2-methylbutane and 2,2-dimethylpropane.
  • Position isomers have the same carbon skeleton and the same functional group, but the group is attached at a different position. Butan-1-ol and butan-2-ol are position isomers.
  • Functional group isomers have the same molecular formula but different functional groups. Butan-1-ol (an alcohol, –OH) and ethoxyethane (an ether, C–O–C) are both C₄H₁₀O.
Key termsisomerstructural isomerchain isomerposition isomerfunctional group isomer
Common mistake

Rotating a drawing is not a new isomer. If you can turn one structure into the other by rotating bonds, they are the same molecule drawn differently. Always name the longest chain to check.

Section 2

Drawing and spotting structural isomers

To find all the isomers of a formula, work systematically:

  1. Start with the longest straight chain.
  2. Shorten the chain by one carbon and place that carbon as a branch on each possible position, checking that you do not repeat a molecule.
  3. Repeat with a shorter chain.

Example, C₄H₈ (alkenes): but-1-ene, but-2-ene and 2-methylpropene. But-1-ene and but-2-ene are position isomers; 2-methylpropene is a chain isomer of both. Cyclobutane has the same formula, but with no C=C, so it is a functional group isomer.

For a functional group isomer, the molecule is in a different homologous series, so its chemical properties differ. For example, an alcohol reacts with sodium to release hydrogen, but an ether does not.

Key termssystematic approach
Exam tip

Check that each structure has the right number of bonds: four for carbon, one for hydrogen. A quick count of H atoms confirms you still have the original molecular formula.

Section 3

Stereoisomerism and E-Z isomerism

Stereoisomers have the same structural formula, but a different arrangement of atoms in space. E-Z isomerism is one type of stereoisomerism.

It occurs because of restricted rotation about the planar C=C double bond. The π bond is formed by sideways overlap of p orbitals, so to rotate the bond the π bond would have to break. The two forms cannot interconvert at room temperature.

For E-Z isomerism you need two different groups attached to each carbon atom of the C=C bond. But-2-ene (each carbon has H and CH₃) shows it. But-1-ene and 2-methylpropene do not, because one carbon has two identical groups.

Key termsstereoisomerE-Z isomerismrestricted rotation
Common mistake

E-Z isomerism is not just 'two groups the same'. Each carbon of the double bond must have two different groups. A carbon with two H atoms rules it out.

Section 4

The CIP priority rules

To label an isomer E or Z, use the Cahn-Ingold-Prelog (CIP) priority rules.

  1. On each carbon of the C=C, look at the two atoms directly attached to it. The atom with the higher atomic number has the higher priority. (Br > Cl > O > N > C > H.)
  2. If the two atoms are the same, move outwards to the next atoms and compare them. For example, CH₂CH₃, which is C(C,H,H), has a higher priority than CH₃, which is C(H,H,H).
  3. If the two higher-priority groups are on the same side of the double bond, the isomer is Z (from German zusammen, together).
  4. If they are on opposite sides, the isomer is E (entgegen, opposite).

Worked example: in CH₃(Br)C=C(Cl)CH₂CH₃, Br beats CH₃ on one carbon and Cl beats CH₂CH₃ on the other. If Br and Cl are on the same side, the isomer is Z.

Key termsCIP priority rulesZ isomerE isomer
Exam tip

Cis-trans is a special case of E-Z. It applies when the two carbons share a pair of identical groups (such as H on both). When the groups are all different, only E-Z works.

Must know

  • Structural isomers: same molecular formula, different structural formulae (chain, position, functional group).
  • Stereoisomers: same structural formula, different arrangement in space.
  • E-Z isomerism needs restricted rotation about C=C and two different groups on each carbon.
  • CIP rules: higher atomic number = higher priority; same side = Z, opposite sides = E.
  • Functional group isomers have different chemical properties.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Structural isomerism and E-Z isomerism

  1. A refinery chemist is analysing the C₅ fraction of a petroleum sample. The fraction is a mixture of alkanes with the molecular formula C₅H₁₂, which are blended into petrol.
    Explain why pentane and 2-methylbutane are structural isomers of each other.2 marks
  2. A student is comparing four alkenes: but-1-ene, CH₂=CHCH₂CH₃; but-2-ene, CH₃CH=CHCH₃; 2-methylpropene, (CH₃)₂C=CH₂; and propene, CH₃CH=CH₂.
    In one isomer of but-2-ene, both methyl groups are on the same side of the double bond. Use the CIP priority rules to explain why this isomer is the Z isomer.2 marks
  3. But-1-ene, CH₂=CHCH₂CH₃, is an alkene used to make polymers. Its molecular formula, C₄H₈, is shared by several other compounds.
    Explain why but-1-ene does not show E-Z isomerism but but-2-ene does.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).