Structural Isomerism (Cambridge (CIE) AS Chemistry): Revision Note

Structural Isomerism: Chain, Position & Functional Group

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

  • E.g. propene and cyclopropane

C₃H₆ structural isomers

• 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

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

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