Question

Draw the stereoisomers of the following amino acids. Indicate pairs of enantiomers and pairs of diastereomers.

Short answer

a. 2 stereoisomers- S and R

b. 4 stereoisomers- RR, SS, RS and SR.

Step-by-step solution

Definition of stereoisomers-

Stereoisomers are isomers that differ in atom spatial arrangement rather than atomic connection order.

Definition ofenantiomers-

If the configuration of the asymmetric centres in one stereoisomer is the opposite of the configuration of the asymmetric centres in the other stereoisomer, the stereoisomers are called enantiomers.

Definition of diastereomers-

If one of the asymmetric centres has the same configuration in both stereoisomers and the other asymmetric centre has the opposite configuration in both stereoisomers, stereoisomers with two asymmetric centres are called diastereomers.

a) Definition of Perspective formulae-

Perspective formulae are chemical compounds that have their components orientated in three dimensions. The Fischer projection is another name for it.

a)Determination of stereoisomers of leucine.

Leucine has one asymmetric center, so it will have two stereoisomers.

b)Determination of stereoisomers of isoleucine.

• Isoleucine has two asymmetric centers, so it has 4 stereoisomers.
• We need to draw the perspective formula of isoleucine.
• Determine the configuration of each of the asymmetric centres.
• We get RR, SS, RS and SR.
• Note that the asymmetric centers in the mirror image have the opposite configuration.

And