Question

Consider Newman projections (A–D) for four-carbon carbohydrates. How is each pairof compounds related: (a) A and B; (b) A and C; (c) A and D; (d) C and D? Choose fromidentical molecules, enantiomers, or diastereomers.

Short answer

a. Compounds A and B are identical molecules.

b. Compounds A and C are enantiomers.

c. Compounds A and D are diastereomers.

d. Compounds C and D are diastereomers.

Step-by-step solution

Newman projections

The conformation of a particular molecule can be recognized with the help of Newman projection.The front carbons are indicated with a dot, and the back carbons are indicated with a circle in a Newman projection.

Enantiomers, diastereomers, and identical molecules

Molecules possessing opposite configurations about the stereogenic center are enantiomers. Diastereomers possess a different configuration at a particular stereogenic center.

The configuration is similar while considering identical molecules.

Relation between compounds A and B

a. The compounds A and B are redrawn to evaluate the R and S configuration.

Relation between compounds A and B

Here you can observe the configuration is similar at the stereogenic centers. Hence, these molecules are identical.

Relation between compounds A and C

b. The Newman projection can be redrawn to obtain the R and S configuration as follows:

Relation between compounds A and C

In the compounds drawn above, it is observed that the compounds have an opposite configuration around the stereogenic center. Hence the compounds A and C are enantiomers.

Relation between compounds A and D

c. The R and S configuration can be evaluated by redrawing the compounds A and D.

Relation between compounds A and D

One stereogenic center in the compounds A and D have the same configuration. Another stereogenic center has an opposite configuration in the compounds A and D. Hence, compounds A and D are diastereomers.

Relation between compounds C and D

d. The compounds C and D can be redrawn to obtain the R and S configuration as follows:

Relation between compounds C and D

The configuration of one particular stereogenic center is similar for compounds C and D. An opposite configuration is observed for another stereogenic center in these compounds. Hence, compounds C and D are diastereomers.