Optical Isomerism (AQA A Level Chemistry): Revision Note

Optical Isomerism

• Stereoisomers are molecules that have the same structural formula but differ in the arrangement of atoms in space

• There are two main types of stereoisomerism:

  • Geometrical isomerism (E/Z isomerism)

  • Optical isomerism

• A carbon atom bonded to four different atoms or groups is called a chiral carbon or chiral centre

  • The term “chiral” comes from the Greek word for hand, reflecting the idea that these molecules have “handedness”

• Compounds containing a chiral centre can exist as two optical isomers, known as enantiomers

  • These are non-superimposable mirror images of each other

    

• Enantiomers are non-superimposable mirror images of each other, in the same way that your left and right hands are mirror images but cannot be perfectly aligned

• They have identical physical properties (such as melting point and boiling point) and identical chemical properties when reacting with achiral substances

  • However, they differ in their effect on plane-polarised light

  • One enantiomer rotates plane-polarised light in one direction, while the other rotates it by the same amount in the opposite direction

  • For this reason, these stereoisomers are known as optical isomers

• The key difference between the two enantiomers is that one rotates plane-polarised light in a clockwise direction, and the other rotates it in an anticlockwise direction

• Because they rotate plane-polarised light, enantiomers are described as optically active

• The direction and magnitude of rotation can be used to help identify an optical isomer

  • When plane-polarised light is passed through a pure sample of one enantiomer, the plane of polarisation is rotated either clockwise or anticlockwise by a specific number of degrees characteristic of that compound