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Structural Isomerism (Oxford AQA International A Level Chemistry): Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Stewart Hird

Updated on

Structural Isomerism

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

    • E.g. propene and cyclopropane, C3H6

C3H6 structural isomers

Diagram showing that propene and cyclopropane are structural isomers
Both propene and cyclopropane are made up of 3 carbon and 6 hydrogen atoms but the structure of the two molecules differs

  • There are three different types of structural isomerism:

    • Chain isomerism

    • Positional isomerism

    • Functional group isomerism

Chain isomerism

  • Chain isomerism is when compounds have the same molecular formula, but their longest hydrocarbon chain is not the same

  • This is caused by branching

    • E.g. pentane and 2,2-dimethylpropane

C5H12 branch / chain isomers

Diagram showing that pentane and 2,2-dimethylpropane are branched chain isomers
Both compounds are made up of the same atoms however the longest carbon chain in pentane is 5 and in 2,2-dimethylpropane 3 (with two methyl branches)

Positional isomerism

  • Positional isomers arise from differences in the position of a functional group in each isomer

    • The functional group can be located on different carbons

    • E.g. butan-1-ol and butan-2-ol / 2-butanol

C4H10O positional isomers

Diagram showing that butanol and 2-butanol are positional isomers
Both compounds have an alcohol group and are made up of 4 carbon, 10 hydrogen and one oxygen atom however in butan-1-ol the functional group is located on the first carbon and in butan-2-ol on the second carbon

Functional group isomerism

  • When different functional groups result in the same molecular formula, functional group isomers arise

  • The isomers have very different chemical properties as they have different functional groups

    • E.g. butan-1-ol and ethoxyethane

C4H10O functional group isomers

Diagram showing that butanol and ethoxyethane are functional group isomers
Both compounds have the same molecular formula however butanol contains an alcohol functional group and ethoxyethane an ether functional group

  • You should be able to deduce all possible isomers for organic compounds knowing their molecular formula

Worked Example

How many isomers are there of C3H6Br2 ?

Answer:

Step 1: Draw the displayed formula of the compound:

An Introduction to AS Level Organic Chemistry Step 1 Isomers of dibromopropane, downloadable AS & A Level Chemistry revision notes

 Step 2: Determine whether there is functional group, branch-chain or positional isomerism

  • Functional group? No, as Br is the only functional group possible

  • Branch-chain? No, as the longest chain can only be 3

  • Positional? Yes, as the two bromine atoms can be bonded to different carbon atoms

An Introduction to AS Level Organic Chemistry Step 3 Isomers of dibromopropane, downloadable AS & A Level Chemistry revision notes

Worked Example

How many isomers are there of the compound with molecular formula C4H10?

Answer:

Step 1: Draw the displayed formula of the compound

An Introduction to AS Level Organic Chemistry Step 1 Deducing isomers of C4H10, downloadable AS & A Level Chemistry revision notes

Step 2: Determine whether it is a functional group, chain or positional isomerism

  • Functional group? No, as there are no functional groups

  • Positional? No, as there are no functional groups which can be positioned on different carbon atoms

  • Chain? yes!

An Introduction to AS Level Organic Chemistry Answer Deducing isomers of C4H10, downloadable AS & A Level Chemistry revision notes

Examiner Tips and Tricks

Don't be fooled by molecules bending and turning through 90 degrees - that does not make them isomers. The best test is to try and name them - isomers will have a different name.

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    Author
    Reviewer
    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.