Geometry-based classifications of isomeric molecules

Isomers can be defined as molecules which closely resemble each other, but fail to be identical due to one difference in their chemical structure. Thus, structural isomers are chemical entities which share the same molecular formula (i.e., the same atomic composition), but which differ in one aspect. When they differ in their constitution (i.e., in the connectivity of their atoms), they are called constitutional isomers, for example 1-propanol and 2-propanol. When structural isomers have identical constitution but differ in the spatial arrangement of their atoms, they are designated as stereoisomers. 

To the concepts of constitutional isomerism and stereoisomerism correspond those of regiochemistry and stereochemistry, respectively. Epiotis [17] has put forward the proposal to collectively describe regiochemistry and stereochemistry by the term chorochemistry (Greek choros = space). 

A fundamental subclassification is that of stereoisomers, which can be divided into enantiomers and diastereoisomers. Either two stereoisomers are related to each other as object and nonsuperimposable mirror image, or they are not. In the former case, they share an enantiomeric relationship. This implies that the molecules are dissymmetric (chiral), and chirality is the necessary and sufficient condition for the existence of enantiomers. An example of an enantiomeric relationship is illustrated in diagram III which shows the (R)- and (S)-enantiomers (see Section 4.b) of 

1-phenylethanol. Enantiomers are also referred to as optical isomers since they show optical rotations of opposite signs and ideally of identical amplitude. Note however that this optical activity may be too small to be detected, as is known in a few cases. 

Stereoisomers which are not enantiomers are diastereoisomers. While a given molecule may have one and only one enantiomer, it can have several diastereoisomers. However, two stereoisomers cannot at the same time be enantiomers and diastereoisomers of each other. Enantiomeric and diastereoisomeric relationships are thus mutually exclusive. Diagram IV shows the ( )- and (Z)-diastereoisomers (also 

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Fig. 2. Geometry-based classification of isomeric molecules. Upper half the conventional classification. Lower half the isometry-based classification. SP, superimposable SC, same constitution NSP, nonsuper-imposable mirror images I, isometric. Adapted from [18] and [19].

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