Epimers

Consider the structure of the aldoses (+) glucose and (+) mannose shown in figure 20 .

Figure 20 The Aldoses (+) Glucose and (+) Mannose

The two diastereoisomers only differ in the configuration of the (H) atom and the (OH) group about the second carbon atom and have the same configuration around the rest of the carbon atoms. Thus, a pair of diastereoisomers that differ only in the configuration about the second carbon atom are called epimers.

The more important nomenclature used generally in chiral chemistry are summarized in figure 21.

It should be emphasized that whereas all enantiomers are stereoisomers, not all stereoisomers are enantiomers.

It should be noted that in this book, the main subject of interest will be concerned with the separation of enantiomers. However, the techniques described will be appropriate for the separation of all types of isomers and, it is worth noting, that the separation of m and p xylene was first achieved in 1958 using gas chromatography employing a 50 ft long packed column.

Figure 21. Nomenclature for Different Types of Isomers.