Stereoisomerism Isomers with the same structural formula but different arrangements of atoms
Optical Isomerism A form of stereoisomerism where compounds have non-superimposable, mirror
images. Enantiomers (optical isomers) arise when there are four different
groups around a tetrahedral carbon atom
Chiral Centre The central carbon atom of optically active compounds with four different
groups bonded to it
Racemate A mixture of equal amounts of enantiomers is called a racemic mixture
There are two ways of arranging the four different groups around a
tetrahedral carbon atom, and these give mirror images. These isomers are
optical isomers, or enantiomers: isomers which are non-superimposable,
mirror images of each other.
Optical isomerism occurs when molecules have an asymmetric carbon atom bonded to four different
atoms (or groups). This carbon atom is called a chiral centre – e.g. the central C on the butan-2-ol above.
Optical isomers have the same chemical and physical properties to each other, and differ only in their
effect on plane-polarised light, as the direction in which they rotate the plane varies:
• If the light appears to be rotated clockwise/to the right: dextrorotatory, or + isomer
• If the light appears to be rotated anticlockwise/to the left: laevororotatory, or – isomer
A 50:50 mixture of two enantiomers is called a racemic mixture or a racemate. Racemates are optically
inactive as the opposite effects of each isomer cancel out so there is no effect on plane-polarised light.
Racemic mixtures are produced during nucleophilic addition. This is because there is equal chance of
nucleophilic attack above or below the planar carbonyl group.
Almost all amino acids are chiral, with only – enantiomers found naturally. This means that active sites
in enzymes and protein receptor sites are all chiral. Drug action may be determined by the
stereochemistry of the molecule. Enantiomers will interact differently with other chiral compounds,
meaning only one optical isomer of certain drugs is effective. Where only one isomer can be used and
the other is toxic, catalysts must be used to synthesise an enantiomerically pure drug.