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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