Illustrated Glossary of Organic Chemistry

Epimer: One of a pair of stereoisomers that differ in the absolute configuration of a single stereocenter. When the molecule has only one stereocenter then the epimers are enantiomers. When the molecule has two or more stereocenters then the epimers are diastereomers.

(S)-2-Chlorobutane		(R)-2-Chlorobutane

(S)-2-Chlorobutane and (R)-2-chlorobutane are epimers because they differ in absolute configuration at the C2 stereocenter. Because 2-chlorobutane has just one stereocenter, these epimers are enantiomers.

(2R,3R)-Tartaric acid		(2R,3S)-Tartaric acid

(2R,3R)-Tartaric acid and (2R,3S)-tartaric acid are epimers because they differ in absolute configuration at the C3 stereocenter. Because tartaric acid has two stereocenters, these epimers are diastereomers.