AQA A Level Chemistry

Revision Notes

7.1.3 Racemic Mixtures

Racemic Mixtures

  • Enantiomers are optical isomers that are mirror images of each other and are non-superimposable
  • They have similar chemical properties but differ from each other in their ability to rotate plane polarised light and in their biological activity

Optical activity

  • Let’s suppose that in a solution, there is 20% of the enantiomer which rotates the plane polarised light clockwise and 80% of the enantiomer which rotates the plane of polarised light anticlockwise
  • There is an uneven mixture of each enantiomer, so the reaction mixture is said to be optically active
    • The net effect is that the plane of polarised light will be rotated anticlockwise
  • Similarly, if the percentages of the enantiomers are reversed, the reaction mixture is still optically active but now the plane of polarised light will be rotated clockwise
    • In this case, there is 20% of the enantiomer, which rotates the plane anticlockwise
    • And 80% of the enantiomer, which rotates the plane clockwise
  • A racemic mixture is a mixture in which there are equal amounts of enantiomers present in the solution
  • A racemic mixture is optically inactive as the enantiomers will cancel out each others effect and the plane of polarised light will not change

Organic Chemistry - Optical Activity, downloadable AS & A Level Chemistry revision notes

When one of the enantiomers is in excess, the mixture is optically active; when there are equal amounts of each enantiomer the mixture is optically inactive

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