JEE Main 2025 · 22 January · evening · Single correct · +4 / −1
Identify the number of structure/s from the following which can be corelated to D-glyceraldehyde.
- (a) three
- (b) two
- (c) one
- (d) four
Answer and solution
Answer (a) three
Solution
Chemistry ma’am · 3 steps
Look only at the chiral carbon farthest from CHO, and put CHO on top before you judge it.
- D or L rule. D-glyceraldehyde has its OH on the right in the Fischer projection. A sugar belongs to the D series when the chiral carbon farthest from the CHO group, the one next to CH₂OH, also has its OH on the right. The other chiral carbons do not decide D or L.
- Fix structure D. In (D) the CHO is at the bottom. Rotating a Fischer projection by 180° in the plane of the paper keeps the configuration. After the rotation, CHO is on top and every OH swaps sides. The farthest chiral carbon had HO on the left, so it now has OH on the right.
Structure CHO on top? OH on the chiral carbon farthest from CHO Series (A) yes right D (B) yes right D (C) yes left L (D) no, rotate 180° left before rotating, right after D - So. (A), (B) and (D) can be correlated to D-glyceraldehyde, and (C) cannot. That makes three structures, option (a).
The idea to keep. D or L is set by one carbon only: the chiral carbon farthest from CHO. Always bring CHO to the top first. A 180° turn in the plane is allowed and moves every OH to the opposite side.
Answer from the NTA final answer key. Solution by PrepWiser.