CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.1.7 Isomerism: Structural

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
  • 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
  • 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

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