AQA A Level Chemistry

Revision Notes

7.1.1 Optical Isomerism

Optical Isomerism

Optical isomerism

  • Stereoisomers are molecules that have the same structural formula but have the atoms arranged differently in space
  • There are two types of stereoisomerism
    • Geometrical (E/Z)
    • Optical
  • A carbon atom that has four different atoms or groups of atoms attached to it is called a chiral carbon or chiral centre
    • Chira comes from a Greek word meaning hand, so we talk about these molecules having a handedness
  • Compounds with a chiral centre (chiral molecules) exist as two optical isomers which are also known as enantiomers


An Introduction to AS Level Organic Chemistry Enantiomers and Chiral Centre, downloadable AS & A Level Chemistry revision notes

A molecule has a chiral centre when the carbon atom is bonded to four different atoms or group of atoms; this gives rises to enantiomers

  • The enantiomers are non-superimposable mirror images of each other just like your left and right hand
  • Their physical and chemical properties are identical but they differ in their ability to rotate plane polarised light
    • Hence, these isomers are called ‘optical’ isomers

Organic Chemistry - Unpolarised Light, downloadable AS & A Level Chemistry revision notes

When unpolarised light is passed through a polariser, the light becomes polarised as the waves will vibrate in one plane only

  • 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
    • A common way to differentiate the isomers is to use (+) and (-), but there are other systems using d and l, D and L, or R and S
  • These enantiomers are therefore said to be optically active
  • 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 by a fixed number of degrees

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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