enantiotopic

Enantiotopic describes a situation in stereochemistry where two or more chemically identical sites in an achiral molecule are related by the molecule’s symmetry such that replacing one of the sites with a different substituent yields enantiomeric products. The concept relies on the parent molecule being achiral and on a symmetry operation that maps one site onto the other without altering the rest of the structure. Enantiotopic sites are distinct from homotopic sites, which produce identical products on substitution, and from diastereotopic sites, which give diastereomers.

A common example involves a methylene group adjacent to a halogen. In ethyl chloride (CH3-CH2-Cl), the two hydrogens on the CH2 group are enantiotopic. Replacing one hydrogen with deuterium yields CH3-CHD-Cl, a chiral molecule. The other hydrogen, if replaced, produces the enantiomer of that same pair of products. Thus, the two potential substitutions lead to enantiomeric products.

In broader practice, enantiotopicity is a key concept in identifying prochiral centers. Replacing an enantiotopic site with a given group can generate a pair of non-superimposable mirror images, whereas replacing homotopic sites would not alter the overall stereochemical outcome. In achiral environments, the enantiotopic sites are often chemically equivalent in NMR and other measurements; in chiral environments (solvents or catalysts), they can become diastereotopic and thus distinguishable.