Chain isomers (Figure 6.2(i)) occur because functional groups can branch off from the main carbon backbone. The number of carbon and hydrogen atoms remains the same but the structures are very different.

Figure 6.2 The different forms of structural isomerism. (i) Chain (ii) positional (iii) mixture of chain and positional.

In positional isomers (Figure 6.2(ii)) the structure of the carbon backbone remains the same but the functional groups or side chains are moved around the backbone.

It is possible to have a mixture of chain and positional isomerism (Figure 6.2(iii)). Positional isomerism can occur around an aromatic ring (see Figure 5.3, p. 31), hence the need for precise nomenclature.

Enantiomers are stereoisomers that are mirror images of each other and all thecomponents of the compound radiating from one point are different. The term forthis is chiral. The position of the compound around which this occurs is a chiral centre (see Figures 6.3, and Figures 6.7). Take, for example, natural alanine, which is foundin only one isomeric form: L -alanine ( Figures 6.3). If alanine is produced synthetically, then both forms L and D alanine, are found. This is signifi cant when it comes togiving a patient nutritional supplementation, as the L isomer is more likely to fit intothe receptor site of an enzyme, which explains why natural products are favoured innutritional supplementation.

Figure 6.3 The two isomers of alanine.