Protons that are not interchangeable by rotational symmetry but are interchangeable by reflectional symmetry.
Constitutionally identical atoms or groups in molecules which are related by symmetry elements of the second kind only (mirror plane, inversion center or rotation-reflection axis).
For example the two groups c in a grouping Cabcc are enantiotopic.
Replacement of one of a pair of enantiotopic groups forms one of a pair of enantiomers.
Analogously, if complexation or addition to one of the two faces defined by a double bond or other molecular plane gives rise to a chiral species, the two faces are called enantiotopic.