Question

What is the relationship of the two structures in each set? That is, are they identical molecules, enantiomers, or diastereomers?

Answer 1

Concepts and reason

This problem is based on the concept of absolute configuration to determine the relationship of stereoisomers.

Enantiomers are the two stereoisomers which are non-superimposable images of each other. These compounds are optically active. Absolute configuration (R, S configuration) of asymmetric carbons in these two compounds are exactly opposite whereas, diastereomers are the two stereoisomers which are not the mirror images of each other but have different absolute configuration at asymmetric centers and same absolute configuration at one carbon.

Fundamentals

If a compound has two asymmetric centers then changing the absolute configuration at one asymmetric carbon will result in the generation of diastereomers whereas changing the absolute configuration at both asymmetric carbons will result in the generation of enantiomers.

Part a

The absolute configuration of the following compounds is as follows:

In the first pair, absolute configuration at one chiral centers in both the compounds is same but different at other chiral centers. Therefore, these two compounds are diastereomers.

Part b

The absolute configuration of the following compounds is as follows:

In the second pair, absolute configuration of asymmetric carbons of one compound is entirely different from the asymmetric carbons present in another compound. Thus, they are enantiomers.

Part c

The absolute configuration of the following compounds is as follows:

In the third pair, absolute configuration of asymmetric carbons of one compound are exactly the same as the absolute configuration of asymmetric carbons. Thus, they are identical.

Ans: Part a

The relationship of this pair is as follows:

Part b

The relationship of this pair is as follows:

Part c

The relationship of this pair is as follows: