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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.