If the mixture has a 40% enantiomeric excess, 40% of the mixture is excess S enantiomer and 60% is a racemic mixture. Half of the racemic mixture plus the amount of excess S enantiomer equals the amount of the S enantiomer present in the mixture. Thus, 70% of the mixture is the S enantiomer and 30% is the R enantiomer.

Another class of stereoisomers is the so called diastereomers which have different chemical and physical properties. Such compounds may include geometrical isomers- the cis and trans isomers (Figutre 4).

Figure 4

A different type of isomerism may exist in disubstituted cyclic compounds. Thus, in 4-tert-butylcyclohexanol, two isomers-cis and trans exist (Fig 5).

Figure 5

Diastereomeric compounds may or may not be chiral. The above two examples are both achiral, each having a plane running through them. However, when cis and trans epoxides are compared, it can be easily seen that they may be chiral compounds. As an example, the comparison of cis and trans isomers of 2,3-dimethyloxirane, the cis-isomer is achiral having a plane of symmetry in the molecule. However, the trans isomer does not have any plane of symmetry through the molecule and as such it is a chiral molecule (Figure 6).

Figure 6