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Isomerism

A concise revision guide to structural isomerism in organic chemistry, including chain isomerism, position isomerism and functional group isomerism.

Paper 2
4.1.1: Basic Concepts of Organic Chemistry
H432/02
Dr. Mohammed Al-Fatah

Written by: Dr. Mohammed Al-Fatah

Chemistry specialist revision notes for A Level Chemistry.

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Before you start

GCSE Recap: Molecular and Structural Formulae

Check that you can count atoms in a structural formula and know what a molecular formula leaves out.

1

What Are Structural Isomers?

Structural isomers are compounds that have the same molecular formula but different structural formulae.

This means the compounds contain the same number of each type of atom, but the atoms are connected in a different order. In organic chemistry, this usually changes the carbon skeleton, the position of a functional group or the functional group itself.

Isomers

Definition: Structural isomers have the same molecular formula but different structural formulae. The difference is in how the atoms are arranged and bonded together.

Exam focus: To decide whether two compounds are structural isomers, compare the molecular formula first, then compare how the atoms are connected.

Check your understanding

Quick Check: Same Molecular Formula?

Work out each molecular formula to find the one true isomer of hexane.

2

Chain Isomerism

Chain isomerism occurs when compounds have the same molecular formula but different structures of the carbon skeleton.

The carbon chain may be straight or branched. For example, pentane, 2-methylbutane and 2,2-dimethylpropane all have the molecular formula C5H12, but their carbon skeletons are different.

Structural formula of pentane.

Pentane

A straight-chain alkane with five carbon atoms.

Structural formula of 2-methylbutane.

2-methylbutane

A branched isomer with the same molecular formula as pentane.

Structural formula of 2,2-dimethylpropane.

2,2-dimethylpropane

A more highly branched isomer of C5H12.

Check your understanding

Quick Check: Isomers or the Same Compound?

Decide whether each pair shows chain isomers, one compound drawn twice, or two compounds that are not isomers.

3

Position Isomerism

Position isomerism occurs when compounds have the same molecular formula but different structures because the same functional group is in a different position on the same carbon skeleton.

For example, 1-bromopropane and 2-bromopropane both have the formula C3H7Br, but the bromine atom is attached to a different carbon atom.

Structural formula of 1-bromopropane.

1-bromopropane

The bromine atom is attached to carbon 1 of the propane chain.

Structural formula of 2-bromopropane.

2-bromopropane

The bromine atom is attached to carbon 2 of the propane chain.

Structural formula of 2-bromobutane.

Haloalkanes are useful examples for practising position isomerism because the halogen can occupy different positions on the carbon chain.

Check your understanding

Quick Check: Spot the Position Isomer

Only one of these four structures is a position isomer of pentan-1-ol.

Check your understanding

Quick Check: Counting Isomers

Draw the isomers on paper and count them before you flip each card.

4

Functional Group Isomerism

Functional group isomerism occurs when compounds have the same molecular formula but the atoms are arranged to give different functional groups.

This is more than a change in the position of the same group. The compounds belong to different homologous series, so they may have different chemical reactions and physical properties.

Structural formula of butan-1-ol.

Butan-1-ol

An alcohol, identified by the hydroxyl functional group, -OH.

Skeletal formula of cyclohexane.

Cyclohexane

A cyclic hydrocarbon structure, useful for recognising that connectivity and functional group type both matter.

Important: Two structures are only isomers if they have the same molecular formula. Do not classify compounds as isomers simply because their structures look similar or contain related atoms.

Check your understanding

Quick Check: Every Isomer of Pent-1-ene

Click all the compounds that share the molecular formula of pent-1-ene.

5

Comparing the Types of Structural Isomerism

The table below summarises the difference between the three common types of structural isomerism used in this topic.

Type of isomerism What changes? Typical example
Chain isomerism The arrangement of the carbon skeleton changes. Pentane, 2-methylbutane and 2,2-dimethylpropane.
Position isomerism The same functional group is attached at a different position on the same carbon skeleton. 1-bromopropane and 2-bromopropane.
Functional group isomerism The atoms are arranged to give different functional groups. Compounds with the same molecular formula but different homologous-series membership.

Exam focus: Always check the molecular formula before naming the type of isomerism. Same formula first, then decide what structural feature has changed.

Check your understanding

Quick Check: Which Type of Isomerism?

Pick the accurate statement about each new pair of compounds.

6

Common Exam Mistakes

  • Do not say structural isomers have the same structural formula. They have the same molecular formula but different structural formulae.
  • Do not confuse chain isomerism with position isomerism. Chain isomerism changes the carbon skeleton.
  • Do not describe position isomerism as a different functional group. It is the same functional group in a different position.
  • Do not ignore the molecular formula. If the molecular formula is different, the compounds are not isomers.

Isomerism Summary

Structural isomers

Same molecular formula, different structural formulae.

Pentane structure used as a structural isomerism example.

Chain isomerism

The carbon skeleton changes from straight-chain to branched.

2-methylbutane structure used as a chain isomerism example.

Position isomerism

The same functional group is in a different position on the carbon skeleton.

2-bromopropane structure used as a position isomerism example.

Functional group isomerism

The same atoms are arranged to give different functional groups.

Butan-1-ol structure used as an alcohol functional group example.

Related Topics

Continue revising OCR Module 4 Core Organic Chemistry with these linked pages.

Aliphatic, Alicyclic and Aromatic Compounds Introduction to Organic Chemistry Alkanes from Crude Oil Reactions of Alkanes
OCR A Level Chemistry A 4.1.3 Basic Concepts of Organic Chemistry and Alkanes interactive course banner
OCR H432 | 4.1.1 and 4.1.2 Course
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Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.