CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.1.10 Structural Isomerism

Structural Isomerism: Chain, Position & Functional Group

  • Structural isomers are compounds that have the same molecular formula but different structural formulae
    • Eg. propene and cyclopropane

 

An Introduction to AS Level Organic Chemistry Propene and Cyclopropane, downloadable AS & A Level Chemistry revision notes

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
    • Eg. pentane and 2,2-dimethylpropane

 

An Introduction to AS Level Organic Chemistry Chain Isomerism, downloadable AS & A Level Chemistry revision notes

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
    • For example, butanol and 2-butanol

 

An Introduction to AS Level Organic Chemistry Positional Isomerism, downloadable AS & A Level Chemistry revision notes

Both compounds have an alcohol group and are made up of 4 carbon, 10 hydrogen and one oxygen atom however in butanol the functional group is located on the first carbon and in 2-butanol 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
    • For example, butanol and ethoxyethane

 

An Introduction to AS Level Organic Chemistry Functional Group Isomerism, downloadable AS & A Level Chemistry revision notes

Both compounds have the same molecular formula however butanol contains an alcohol functional group and ethoxyethane an ether functional group

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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