Question 26.3: How many stereoisomers can exist for the following compounds...

(a) Carbons 2 and 3 are chiral, so there can be a maximum of four
stereoisomers (2^2=4). Write structures around the chiral carbons:

There are four stereoisomers: two pairs of enantiomers (I and II, and III and IV) and no meso compounds. Structures I and III, I and IV, II and III, and II and IV are diastereomers.

(b) Carbons 2 and 3 are chiral, so there can be a maximum of four stereoisomers. Write structures around the chiral carbons:

There are three stereoisomers: one pair of enantiomers (VII and VIII) and one meso compound (V). Structures V and VI represent the meso compound because there is a plane of symmetry between carbons 2 and 3, and turning VI 180° makes it superimposable on V. Structures V and VII and V and VIII are diastereomers.