Homotopic Ligands and Faces

We have indicated in the previous section that some apparently alike ligands are, in fact, not equivalent towards enzymes or in their NMR signals. How does one 

Superposable molecules 25 (by rotation of CH2CI groups, see text) 

4 The term equivalent is overly general and therefore bland and of equivocal meaning. Thus the methylene hydrogen atoms in propionic acid (Fig. 1) are equivalent when detached (i.e. they are homomorphic), but, as already explained, they are not equivalent in the CH3CH2C02H molecules because of their placement — i.e. they are heterotopic. Ligands that are equivalent by the criteria to be described in the sequel are called homotopic from Greek homos = same and topos = place 6 , those that are not are called heterotopic . 

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The two criteria used to spot homotopic ligands and faces may also be used to detect those which are enantiotopic. 

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