Optical Isomers

• Optical isomerism is a form is a form of stereoisomerism and occurs as a result of chirality in molecules, limited to molecules with a single chiral centre.
• An asymmetric carbon atom is chiral and gives rise to optical isomers (enantiomers), which exist as non super-imposable mirror images and differ in their effect on plane polarised light.
• A mixture of equal amounts of enantiomers is called a racemic mixture (racemate).
• Optical Isomers

  • A type of stereoisomerism
  • Stereoisomers have the same structure but the atoms are arranged differently in space
  • Optical isomers occur when there is a chiral carbon atom, which has 4 different groups attached.
• Optical isomers

  • This leads to 2 different orientations of the groups around the atom
  • These are known as enantiomers, or optical isomers
  • These enatiomers are non-superimposable mirror images of each other (like hands)
• Chiral centres

  • These are carbon atoms with 4 different groups attached
  • To find the chiral centre look for a carbon with 4 different groups, these could be functional groups, of different length carbon chains.
• Properties of Enantiomers

  Enantiomers have identical chemical properties and the same boiling temperatures and solubilities.

  They differ in 2 ways:

  • Optical isomers are optically active, meaning they rotate plane-polarised light (light that only vibrates in 1 direction). One enantiomer rotates the light clockwise and the other anticlockwise.
  • Optical isomers often have different biochemical reactions.
• Importance of enantiomers

  The action of many pharmaceutical drugs depend on the stereochemistry of the molecules.
• Racemates

  • A racemate (or racemic mixture) contains equal quantities of each enantiomer (optical isomer). These cancel the light-rotating effect of eachother.
  • Many reactions produce optical isomers but these appear as racemates as there is an equal chance of producing the isomers.
  • You can produce a single enantiomer but this is expensive and difficult.
  • Reactions involving planar bonds usually produce racemic mixtures e.g. alkenes, ketones and aldehyde reactiosn
• Racemates

  A racemic mixture is often obtained when an addition reaction takes place on a planar double bond (carbon-carbon or carbon-oxygen) as the reaction can occur with equal probability on either side of the plane.
• Where might optical isomers be important?

  Enzyme controlled reactions