AQA A Level Chemistry

Revision Notes

7.1.1 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 (cis/tras)
    • Optical

Optical isomerism

  • A carbon atom that has four different atoms or groups of atoms attached to it is called a chiral carbon or chiral centre
  • 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
  • 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
    • One of the optical isomers will rotate the plane of polarised in the clockwise direction
    • Whereas the other isomer will rotate it in the anti-clockwise direction

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

Biological activity of enantiomers

  • Enantiomers also differ from each other in terms of their biological activity
  • Enzymes are chiral proteins that speed up chemical reactions by binding substrates
  • They are very target-specific as they have a specific binding site (also called active site) and will only bind molecules that have the exact same shape
  • Therefore, if one enantiomer binds to a chiral enzyme, the mirror image of this enantiomer will not bind nearly as well if at all
  • It’s like putting a right-hand glove on the left hand!

Organic Chemistry - Biological Activity of Enantiomers, downloadable AS & A Level Chemistry revision notes

Enantiomers differ from each other in their biological activity


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