Q.Is the following sugar, D–sugar or L–sugar? Fischer projection: CHO at C1; C2 – OH on the right (H on left); C3 – OH on the right (H on left); C4 – OH on the right (H on left); C5 – CH₂OH.

Epimers are diastereomers that differ in spatial configuration at just one of several chiral (stereogenic) carbon atoms; $D$-glucose/$D$-mannose (C2 epimers) and $D$-glucose/$D$-galactose (C4 epimers) are the standard sugar examples.

When a molecule contains more than one chiral (asymmetric) carbon atom, it can exist as several diastereomers - stereoisomers that are not mirror images of each other. Epimers are a special sub-class of diastereomers: two epimers have identical configuration at every chiral centre except one, at which their configurations are opposite (inverted).

Examples (from carbohydrate chemistry):

• $D$-Glucose and $D$-Mannose differ in configuration only at carbon $C2$ (the $-OH$ points in opposite directions at $C2$ in their Fischer projections, all other centres, $C3$, $C4$, $C5$, being identical) - so they are epimers, specifically 'C2-epimers'.
• $D$-Glucose and $D$-Galactose differ in configuration only at carbon $C4$ - so they too are epimers, 'C4-epimers'. …