a. What sugar is the C-3 epimer of D-xylose?

b. What sugar is the C-5 epimer of D-allose?

c. What sugar is the C-4 epimer of L-gulose?

d. What sugar is the C-4 epimer of D-lyxose?

Isomers of a compound can be constitutional isomers and stereoisomers. Stereoisomers have the same functional groups and same atom connectivity. The only difference in these compounds is the orientation of functional groups around the carbon atom.

The IUPAC names of stereoisomers are identical, and therefore, prefixes are added to IUPAC names that express the stereoisomer's configuration.

Many configuration methods are used for stereoisomers like cis-trans, R-S, D-L configurations, etc.

The monosaccharides that differ in a configuration only at one stereogenic center are called epimers. The configuration is changed by changing the position of the hydroxyl group in the one carbon center.

Epimers are diastereoisomers. The stereoisomers that are not mirror images are called diastereoisomers, and epimers will never be mirror images of each other. The number of epimers for a given sugar depends on the number of hydroxyl-group carbon atoms in the sugar.

a. The structure of D-xylose can be given as follows:

D-xylose contains five carbon atoms. The C-3 epimer of D-xylose can be obtained by changing the configuration of the third carbon atom.

The compound obtained is D-ribose.

Therefore, the C-3 epimer of D-xylose is D-ribose.

b. The structure of D-allose can be given as:

The structure thus obtained is:

The compound obtained is L-talose.

Therefore, the C-5 epimer of D-allose is L-talose.

c. The structure of L-gulose can be given as follows:

L-gulose contains six carbon atoms. The C-4 epimer of L-gulose can be obtained by changing the configuration of the fourth carbon atom.

The structure thus obtained is:

The compound obtained is L-allose.

Therefore, the C-4 epimer of L-gulose is L-allose.

d. The structure of D-lyxose can be given as:

D-lyxose contains five carbon atoms. The C-4 epimer of D-lyxose can be obtained by changing the configuration of the fourth carbon atom.

The structure thus obtained is:

The compound obtained is L-ribose.

Therefore, the C-4 epimer of D-lyxose is L-ribose.