11.3 - Isomerism

• Isomers are molecules that have the same molecular formula but whose atoms are arranged differently. There are two basic types of isomerism in organic chemistry - structural isomerism and stereoisomerism.
• Structural isomers are defined as having the same molecular formula but different structural formulae. There are 3 sub-divisions. Structural isomers can have:

  1. the same functional group attached to the main chain at different points - this is called positional isomerism
  2. functional groups that are different - this is called functional group isomerism
  3. a different arrangement of the hydrocarbon chain (such as different branching) - this is called chain isomerism.
• Positional isomerism


  The functional group is attached to the main chain at different points. For example, C3H7Cl could represent 1-chloropropane or 2-chloropropane.
• Functional group isomerism


  There are different functional groups. For example, the molecular formula C2H6O could represent ethanol (an alcohol) or methoxymethane (an ether).
• Chain isomerism


  The hydrocarbon chain is arranged differently. For example, the molecular formula C4H9OH could represent butan-1-ol or 2-methylpropan-1-ol. These isomers are called chain-branching isomers.
• Stereoisomerism is where two (or more) compounds have the same structural formula. They differ in the arrangement of bonds in space. There are two types:

  • E-Z isomerism
  • optical isomerism
• E-Z isomerism tells us about the positions of substituents at either side of a carbon-carbon double bond. Two substituents may either be on the same side of the bond z or on opposite sides E.