3.7 Optical isomerism

Nomenclature organic chemistry
To apply IUPAC rules for nomenclature not only to the simple organic compounds, limited to chains with up to 6 carbon atoms, met at AS, but also to benzene and the functional groups listed in this unit.


Defined previously      Structural isomerism       E-Z stereoisomerism

Optical isomerism is another form of stereoisomerism 

Compounds that contain an asymmetric carbon atom form stereoisomers that differ in their effect on plane polarised light. This type of isomerism is called optical isomerism.

Optical isomerism 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 superimposable mirror images and differ in their effect on plane polarised light.

A mixture of equal amounts of enantiomers is called a racemic mixture (racemate).

Drug action may be determined by the stereochemistry of the molecule and that different optical isomers may have very different effects.

Students should be able to:

• draw the structural formulas and displayed formulas of enantiomers

• understand how racemic mixtures (racemates) are formed and why they are optically inactive.

Assessment and practical opportunities

  • Students could be asked to recognise the presence of a chiral centre in a given structure in 2D or 3D forms.
  • They could also be asked to draw the 3D representation of chiral centres in various species.
  • Students understand the origin of optical isomerism.
  • Passing polarised light through a solution of sucrose.