How Many Stereoisomers Would Be Expected from a Ketohexose

Question

How many stereoisomers would be expected from a ketohexose?
A)4
B)8
C)16
D)32
E)64

Accepted Answer

A ketohexose has six carbon atoms, one of which is a carbonyl group at position two. Thus, there are five possible chiral centers. Each chiral center can have two possible configurations (R or S), so the total number of stereoisomers is 2^5 = 32. However, if we consider the fact that ketoses exist in equilibrium with their corresponding aldoses through a process called tautomerization, we can eliminate half of the configurations. Therefore, the number of stereoisomers would be 16.