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Updated Apr 7, 2026

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AQA A-Level Chemistry: Isomerism Explained

Hannah Egan

@hannahegan_xyym

Isomerism is a key concept in organic chemistry where molecules... Show more

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3.3.1.3 - Isomerism

Isomers = same molecular formula but different
displayed Jocmola (arrangement of atoms)

Structural Isomers
- Molecules

Structural Isomers - Same Formula, Different Connections

Structural isomers are molecules with identical molecular formulas but completely different structural arrangements. Think of them as having the same building blocks but assembled in different ways.

Chain isomers involve different carbon skeleton arrangements. Butane can exist as a straight chain or as methylpropane with a branched structure - same atoms, totally different shapes.

Positional isomers keep the same carbon chain but move functional groups to different positions. For example, 1-chlorobutane has chlorine on the first carbon, while 2-chlorobutane has it on the second carbon.

Functional group isomers rearrange the same atoms into completely different functional groups. Hex-1-ene (an alkene) and cyclohexane (a cycloalkane) contain identical atoms but form entirely different molecular structures.

Quick Tip: Remember that structural isomers have different displayed formulas - you can literally see the different connections when you draw them out!

Stereoisomers - Same Structure, Different Space Arrangement

Stereoisomers have identical structural formulas but arrange differently in three-dimensional space. This happens because double bonds can't rotate - the carbon atoms lock into the same plane.

E/Z isomerism occurs in alkenes when different groups attach to the carbon atoms of a double bond. Since the bond can't rotate, these groups get "stuck" in specific positions, creating distinct isomers.

Z-but-2-ene and E-but-2-ene demonstrate this perfectly - they're the same molecule but with methyl groups positioned differently around the double bond. The restricted rotation means they can't convert into each other.

To identify E and Z isomers, you'll need the Cahn-Ingold-Prelog priority rules. These help determine which groups take priority when naming these stereoisomers.

Remember: Single bonds CAN rotate freely, so they never form stereoisomers - don't let tricky exam questions fool you!

E/Z Priority Rules - Working Out Which Is Which

The Cahn-Ingold-Prelog priority rules help you assign E or Z labels to stereoisomers. Start by giving priority to the two atoms directly attached to each carbon of the double bond - higher atomic number equals higher priority.

For simple cases, fluorine beats chlorine, and chlorine beats hydrogen. When the highest priority groups sit across from each other, you've got an E isomer.

When the atoms directly bonded are identical, look at the next group along the chain. A methyl group attached to hydrogen has lower priority than an ethyl group attached to carbon - it's all about what comes next in the chain.

Master these priority rules and you'll confidently tackle any E/Z isomerism question. The key is working systematically through the atomic numbers and chain structures.

Exam Success: Always check for single vs double bonds first - this immediately tells you whether stereoisomerism is even possible!

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114

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Updated Apr 7, 2026

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3 pages

AQA A-Level Chemistry: Isomerism Explained

Hannah Egan

@hannahegan_xyym

Isomerism is a key concept in organic chemistry where molecules share the same molecular formula but have different arrangements of atoms. Understanding the different types of isomers will help you predict molecular behaviour and tackle exam questions with confidence.

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Structural Isomers - Same Formula, Different Connections

Chain isomers involve different carbon skeleton arrangements. Butane can exist as a straight chain or as methylpropane with a branched structure - same atoms, totally different shapes.

Quick Tip: Remember that structural isomers have different displayed formulas - you can literally see the different connections when you draw them out!

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Stereoisomers - Same Structure, Different Space Arrangement

To identify E and Z isomers, you'll need the Cahn-Ingold-Prelog priority rules. These help determine which groups take priority when naming these stereoisomers.

Remember: Single bonds CAN rotate freely, so they never form stereoisomers - don't let tricky exam questions fool you!

Sign up to see the contentIt's free!

Access to all documents

Improve your grades

Join milions of students

E/Z Priority Rules - Working Out Which Is Which

For simple cases, fluorine beats chlorine, and chlorine beats hydrogen. When the highest priority groups sit across from each other, you've got an E isomer.

Master these priority rules and you'll confidently tackle any E/Z isomerism question. The key is working systematically through the atomic numbers and chain structures.

Exam Success: Always check for single vs double bonds first - this immediately tells you whether stereoisomerism is even possible!