If an atom has two identical ligands on it, such that changing one of them to something else creates a chiral center, it is a prochiral atom or center
If replacing the two ligands one at a time creates a pair of enantiomers, the ligands are said to be enantiotopic
The two hydrogens of either CH2 group in butane are enantiotopic. Here we replace them separately with an OH:
Butane
(R)-2-Butanol
S-2-Butanol
Ligands that are enantiotopic will behave identically under all circumstances, except in a chiral environment
In either enantiomer of 2-butanol, the hydrogens of the remaining CH2 are diastereotopic Here we replace them in turn with iodine:
(R)-2-Butanol
(R,R)-3-Iodo-2-butanol
((S,R)-3Iodo-2-butanol
Ligands that are diastereotopic will behave differently under all circumstances; for example, they will have different absorptions in the 1H nmr spectrum.
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