AQA A Level Chemistry

Revision Notes

7.1.2 Identifying Chiral Centres

Identifying Chiral Centres

  • Molecules with a chiral centre exist as optical isomers
  • These isomers are also called enantiomers and are non-superimposable mirror images of each other
  • The major difference between the two enantiomers is that one of the enantiomers rotates plane polarised light in a clockwise manner and the other in an anticlockwise fashion
    • The enantiomer that rotates the plane clockwise is called the R enantiomer
    • The enantiomer that rotates the plane anticlockwise is called the S enantiomer
  • These enantiomers are therefore said to be optically active
  • Therefore, the rotation of plane polarised light can be used to determine the identity of an optical isomer of a single substance
    • For example, pass plane polarised light through a sample containing one of the two optical isomers of a single substance
    • Depending on which isomer the sample contains, the plane of polarised light will be rotated either clockwise or anti-clockwise
    • No effect will be observed when the sample is a racemic mixture

Organic Chemistry - Effect of Optical Isomers on Plane of Polarised LightEffect of Optical Isomers on Plane of Polarised Light, downloadable AS & A Level Chemistry revision notes

Each enantiomer rotates the plane of polarised light in a different direction

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