9701 MCQ Solution 61

Question 61: [Organic > Introductory Organic]

The isomers, citric acid and isocitric acid, are intermediates in the Krebs cycle of the oxidation of glucose in living cells.

How many chiral centres does each acid possess?

	citric acid	isocitric acid
A	0	1
B	0	2
C	1	1
D	1	2

Reference: Past Exam Paper –June 2003 Paper 1 Q21

Solution 61:

Answer: B.

All carbon atoms in CO₂H group are not chiral. All carbon atoms in CH₂ groups are not chiral. In citric acid the only remaining left-middle carbon atom is also not chiral since the bottom group exactly matches the top, cancelling out options C and D. In isocitric acid the left-middle carbon atom is chiral since the groups attached to it are: H, CO₂H, CH₂CO₂H and C(OH)CO₂H, which are all different. The bottom-left carbon atom is also chiral since the groups attached to it are: H, OH, CO₂H and the remaining top, which are also all different. Thus the correct answer is option B.