Final answer:
Epimers are diastereomers that differ in the configuration of only one chiral carbon. Rotation around bonds cannot change this configuration; the interconversion of epimers requires a chemical reaction that breaks and reforms covalent bonds.
Step-by-step explanation:
Two sugars that are epimers of each other differ only in the configuration around one specific carbon atom. For example, D-glucose and D-mannose are epimers; they differ only at the second carbon atom. Similarly, D-glucose and D-galactose are epimers differing at the fourth carbon atom.
Epimers are a type of stereoisomer called diastereomers. In order to convert one epimer to another, a change in the configuration of a single chiral carbon is required. However, rotation around bonds, such as single-bond rotation, will not change the absolute configuration of the chiral carbon; hence, the configuration of a stereocenter cannot be altered by simple rotation. The only way to convert one epimer into the other is through a chemical reaction that breaks and reforms covalent bonds. Thus, simple physical rotation around single bonds cannot lead to the interconversion of epimers.