Question

Draw all possible stereoisomers for each compound. Label pairs of enantiomers and diastereomers.

a.

b.

c.

Short answer

a. The A and B pair are diastereomers. The B and C pair are enantiomers.

b. The A and B, and C and D pairs are enantiomers. The A and C, A and D, B and C, B and D are diastereomers.

c. The A and B pair are diastereomers.

Step-by-step solution

Stereoisomers  

The count of stereoisomers can be calculated by the chiral centers. The more the number of chiral centers, the more will be the stereoisomers.

The formula to calculate stereoisomers is: , where n is the count of chiral center.

Enantiomers and diastereomers

If the stereoisomers have the mirror images non-superimposable and, therefore, they have opposite configurations (R isomer and S isomer), then the isomers are termed as enantiomers.

If the stereoisomers are not associated with any kind of mirror image, and they have cis-trans geometry then, the isomers are termed as diastereomers.

Explanation

a. 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. The cis molecule has a plane of symmetry due to the presence of two methyl groups above the plane. The trans molecule has its enantiomer because there is no plane of symmetry.

Therefore, the number of stereoisomers is three.

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

Stereoisomers

b. The molecule has two chiral centers due to the presence of four different groups at each chiral center. Therefore, it should have four stereoisomers maximum.

One cis molecule and one trans molecule can be drawn. The cis molecule has its enantiomers due to the absence of any plane of symmetry (methyl and OH groups are there).

The trans molecule has its enantiomer because there is no plane of symmetry.

Therefore, the number of stereoisomers is four.

• The A and B, C, and D pairs are enantiomers due to opposite configurations or non-superimposable mirror images.
• The A and C, A and D, B and C, B and D are diastereomers due to cis and trans geometry.

Stereoisomers

Stereoisomers

c. 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. The cis molecule has a plane of symmetry due to the presence of two chloride groups above the plane.

The trans molecule also has a plane of symmetry due to the presence of two chloride groups.

Therefore, the number of stereoisomers is two.

• The A and B pair are diastereomers.

Stereoisomers