Question

Which of the following sets of compounds contain a pair of diastereomers?

(i) (1S,4R)-1,3-dichlorocyclohexane & (1R,4S)-1,3-dichlorocyclohexane

(ii) (R)-chloroiodomethanesulfonic acid & (S)-chloroiodomethanesulfonic acid

(iii) (1R,4S)-1,4-cyclohexanediol & (1S,4R)-1,4-cyclohexanediol

(iv) (1R,2R)-1,2-dimethylcyclopentane & (1R,2S)-1,2-dimethylcyclopentane

(v) (2R,3R)-2,3,4-trihydroxybutanal & (2S,3R)-2,3,4-trihydroxybutanal

Which of the following sets of compounds are enantiomers?

(i) (E)-1-chloro-1-butene & (Z)-1-chloro-1-butene

(ii) (1R,3S)-1,3-cyclopentanediol & (1S,3S)-1,3-cyclopentanediol

(iii) (S)-1-cyano-1-propanol & (R)-1-cyano-1-propanol

(iv) (1R,3S)-1,3-cyclopentanediol & (1R,3R)-1,3-cyclopentanediol

(v) (R)-2-tert-butyl-2-methylcyclobutanone & (S)-2-tert-butyl-2-methylcyclobutanone

Answer 1

Correct options for diastereomers:

(iv) (1R,2R)-1,2-dimethylcyclopentane & (1R,2S)-1,2-dimethylcyclopentane

(v) (2R,3R)-2,3,4-trihydroxybutanal & (2S,3R)-2,3,4-trihydroxybutanal

Explanation:

The absolute configuration of the stereoisomers are not mirror image of one another. Hence, they are not enantiomers but diastereomers.

For example in option (iv) the absolute configuration of one of the stereoisomer is (1R,2R) and the other is (1R,2S). The mirror image of 1R is 1S and the mirror image of 2R is 2S. But as you can see that the absolute configuration 1R in one is not changed to 1S in the other. Hence, they are not enantiomers. We know that, by definition, the stereoisomers that are not enantiomers, are diastereomers.

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Correct options for enantiomers:

(iii) (S)-1-cyano-1-propanol & (R)-1-cyano-1-propanol

(v) (R)-2-tert-butyl-2-methylcyclobutanone & (S)-2-tert-butyl-2-methylcyclobutanone

Explanation:

The absolute configuration of the stereoisomers are mirror images of one another. Hence, they are enantiomers.

Take option (iii) for example, the absolute configuration of one of the stereoisomer is (S) and the other is (R). The mirror image of R is S. Hence, the stereoisomers are enantiomers.