Structural Isomerism (CIE A Level Chemistry)

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

C₃H₆ 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

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

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

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