Question

When are epimers called diastereomers?

Answer 1

Epimers are always diastereomers.

Explanation:

Diastereomers are compounds that contain two or more chiral centres and are not mirror images of each other.

For example, the aldopentoses each contain three chiral centres.

Thus, D-ribose is a diastereomer of D-arabinose, D-xylose, and D-lyxose.

Epimers are diastereomers that contain more than one chiral center but differ from each other in the absolute configuration at only one chiral center.

Thus, D-ribose and D-arabinose are epimers (and diastereomers), because they differ in configuration only at `"C-2"`.

D-ribose and D-xylose are epimers (and diastereomers), because they differ in configuration only at `"C-3"`.

D-ribose and D-lyxose diastereomers, but they are not epimers, because they differ in configuration at both `"C-2"` and `"C-3"`.