Organic molecules can display many different types of isomerism. In the last tutorial, we saw examples of both structural and geometrical isomerism. In this tutorial we will consider a third type of isomerism known as optical isomerism. The different types of isomers are summarized below.

• Structural isomers - two molecules with the same molecular formula, but different structural formulas (different structures)
• Stereoisomers - two molecules with the same structural formula (same connectivity) but different three-dimensional arrangements of atoms
  • geometrical isomers - two different arrangements (cis and trans) around a bond with restricted rotation
  • optical isomers - different three-dimensional arrangements of atoms around one or more chiral carbon atoms
    • enantiomers - mirror images that are not superimposable upon one another
    • diastereomers - optical isomers that are not enantiomers (requires more than one chiral carbon)

As can be seen from the above summary, this new type of isomerism is similar to geometrical isomerism in that two optical isomers differ only in the three-dimensional arrangement of their atoms.

Geometrical isomers required the presence of restricted rotation. Optical isomers require the presence of a chiral carbon atom.