Question

Question:(a) Label compounds A, B, and C as D- or L-sugars.(b) How are compounds A and B related? A and C?B and C? Choose from enantiomers, diastereomers, or constitutional isomers.

Short answer

Answer

a.

• Compound A: L-sugar
• Compound B: L-sugar
• Compound C: D-sugar

b.

• A and B are diastereomers.
• A and C are enantiomers.
• B and C are diastereomers

Step-by-step solution

D and L configuration

D and L configuration is a configuration method used to describe the absolute configuration of a sugar.

The method predates the R and S configuration method. It is used in biological compounds.

The configuration of the molecule is assigned based on the configuration of the hydroxyl group on the chiral carbon farthest from the aldehyde group.

If the mentioned hydroxyl group is on the right, the sugar has a D configuration. If it is on the left side, the sugar has an L configuration.

Stereoisomers

Compounds that have atom connectivity with a difference in spatial arrangements are called stereoisomers.

If the stereoisomers are non-superimposable mirror images of each other, they are called enantiomers. If not, they are called diastereoisomers.

The configuration of enantiomers will be different in all chiral centers, and the configuration of some chiral centers will be different in diastereoisomers.

Isomerism in given compounds

a. The D and L configuration is determined by the configuration of the hydroxyl group in the lowest carbon atom.

Compound A:

The hydroxyl group is on the left. Therefore, it is an L-sugar.

Compound B:

The hydroxyl group is on the left. Therefore, it is an L-sugar.

Compound C:

The hydroxyl group is on the right. Therefore, it is a D-sugar.

b. Compounds A and B have different configurations only in the second carbon. They are not mirror images. Therefore, there are diastereoisomers.

Compounds A and C have different configurations in all the chiral centers. They are non-superimposable mirror images of each other. They are enantiomers.

Compounds B and C have different configurations only in third and fourth carbon. They are not mirror images. Therefore, there are diastereoisomers.