Diastereoisomers

A stereoisomer that contains more than one chiral center is called a diastereoisomer. Even when the diastereoisomer contains only two chiral centers, the identification of the different structures can be quite complex. Consider the different structures of 2,3 dichloropentane shown in figure 18.

Figure 18. Different Isomers of the Diastereoisomer 2,3 Dichloropentane

There are 4 apparent structures; structure 1 is the mirror image of structure 2 but the two structures are not superimposable and, thus, structures 1 and 2 are enantiomers. Furthermore, structure 2 cannot be converted into structure 1 by any form of rotation and, thus, both structure 1 and structure 2 are optically active. Similarly, structures 3 and 4 are mirror images, not superimposable and not interconvertable by rotation and, thus, they are enantiomers and are optically active. However, structures 3 and 4 are not mirror images of structure 1 and, thus, they are not enantiomers but only diastereoisomers of structure 1. In a similar way structures 3 and 4 are not mirror images of structure 2 and, thus, these are also not enantiomers but diastereoisomers of structure 2. Diastereoisomers have different chemical properties and, thus, can often be separated by conventional techniques.

Anomers

There are two further terms that require defining regarding diastereoisomers and they are anomers and epimers. Consider the pair of aldoses, a D (+) glucose and b D (+) glucose depicted in figure 19.

Figure 19 The Aldoses a D (+) Glucose and b D (+) Glucose

It is seen in figure 19 that the two diastereoisomers only differ in the configuration of the (H) atom and the (OH) group about the first carbon atom and have the same configuration about he rest of the carbon atoms. A pair of diastereoisomers that differ only in the configuration of the atoms or groups about the first carbon atom are called anomers.