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The count of stereoisomers can be calculated by the chiral centers and has various classifications. The formula to calculate stereoisomers is: ${\text{2}}^n$,

where n is the count of chiral center.

• If the stereoisomers have opposite configurations (R isomer and S isomer), they are termed enantiomers.
• If the stereoisomers have cis-trans geometry, then they are termed diastereomers.

a. The pair has one chiral center due to the presence of four different groups. Therefore, the molecules are assigned S and S configurations due to the presence of groups.

The priority order of the groups is:

${\text{Br>CH}}_2{\text{OH>CH}}_3\text{>H}$

Therefore, the clockwise movement gives the S and S configurations for both of the molecules in the pair.

Identical

b. The molecule has one chiral center due to the presence of four different groups. Therefore, the given pair are the enantiomers due to the opposite configurations.

Enantiomers

c. The molecules in the pair have the same molecular formula, but the arrangement of the substituents is different. Therefore, the molecules in the pair are constitutional isomers.

Constitutional isomers

d. The molecule has two chiral centers due to the presence of four different groups at each chiral center.

One cis molecule and one trans molecule can be drawn.

Therefore, the number of stereoisomers is two. Therefore, the given pair is the diastereomers.

Diastereomers