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

a.

b.

The stereoisomers are classified into optical and geometrical isomers. The optical isomers consist of enantiomers and diastereomers. The geometrical isomers consist of different geometry.

The enantiomers are the non-superimposable mirror images. The R has S enantiomer, and S has R enantiomer.

The diastereomers are not mirror images of each other. The R, R has R, S diastereomers.

a. The given compound has two stereogenic centers (as there are two centers that have four different groups). Therefore, it has four stereoisomers.

The stereochemistry is inverted in the A and B. Therefore, A and B are enantiomers.

• The C and D are identical as they have the same stereochemistry of the substituents.
• The A and C are diastereomers as they differ in stereochemistry at only one carbon (left carbon containing four groups).
• The B and C are diastereomers as they differ in stereochemistry at only one carbon (right carbon containing four groups).

Stereoisomers

b. The given compound also has 2 stereogenic centres (as two centers have four different groups). Therefore, it has four stereoisomers.

The stereochemistry is inverted in the A and B. Therefore, A and B are enantiomers.

• The C and D also have inverted stereochemistry. Therefore, C and D are enantiomers.
• The A and C are diastereomers as they differ in stereochemistry at only one carbon (left carbon containing four groups).
• The A and D are diastereomers as they differ in stereochemistry at only one carbon (right carbon containing four groups).
• The B and C are diastereomers as they differ in stereochemistry at only one carbon (left carbon containing four groups in C).
• The B and D are diastereomers as they differ in stereochemistry at only one carbon (right carbon containing four groups).

Stereoisomers