Topicity classifies the relationship between apparently similar atoms, groups, or molecular faces by a replacement test. Replacing one member and then the other gives identical products for homotopic sites, enantiomers for enantiotopic sites, and diastereomers for diastereotopic sites.
For example, replacing either hydrogen on a suitable prochiral methylene carbon by deuterium can produce opposite enantiomers. Enantiotopic sites are equivalent in an achiral environment but can be distinguished by a chiral reagent or enzyme. Diastereotopic hydrogens can give different chemical shifts even in an achiral solvent. Topicity therefore connects molecular symmetry to spectroscopy and stereoselective reactions.
Two sites are enantiotopic if separately replacing one or the other by the same distinguishing group gives enantiomers. Achiral conditions do not distinguish them by chirality alone.
