Q45. Considering the open chain forms, which of the following pair(s) represent/s an
epimer?
(A) D–mannose and D–fructose
(B) D–glucose and D–mannose
(C) D–glucose and D–fructose
(D) D–galactose and D–glucose
D-glucose and D-mannose are epimers in their open-chain forms, making option (B) correct. D-galactose and D-glucose are also epimers, so option (D) is correct.
Epimer Definition
Epimers are stereoisomers differing in configuration at only one chiral carbon in their open-chain structures. All listed sugars are aldohexoses except D-fructose, a ketohexose, but epimers must share identical functional groups and differ at exactly one chiral center.
Option Analysis
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(A) D-mannose and D-fructose: Incorrect. D-mannose (aldose, aldehyde at C1) and D-fructose (ketose, ketone at C2) have different functional groups in open chains, so they are not stereoisomers or epimers.
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(B) D-glucose and D-mannose: Correct. Both are aldohexoses; they differ only at C2 (D-glucose: OH right; D-mannose: OH left in Fischer projection).
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(C) D-glucose and D-fructose: Incorrect. Different functional groups (aldose vs. ketose) prevent epimer status.
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(D) D-galactose and D-glucose: Correct. Both aldohexoses; differ only at C4 (D-glucose: OH right; D-galactose: OH left).
Answer: (B) and (D)
D-glucose and D-mannose serve as classic epimer open chain forms examples in carbohydrate stereochemistry, vital for CSIR NET Life Sciences preparation. This detailed breakdown of Q45 analyzes each pair using Fischer projections.
Understanding Epimers
Epimers represent diastereomers differing at one chiral carbon, typically compared in open-chain Fischer projections for aldoses. CSIR NET emphasizes this for hexose identification.
Detailed Pair Comparison
| Pair | Open-Chain Difference | Epimer? | Reason |
|---|---|---|---|
| D-mannose & D-fructose | Functional group (C1 aldehyde vs. C2 ketone) | No | Not stereoisomers |
| D-glucose & D-mannose | C2 configuration only | Yes (C2-epimer) | Identical elsewhere |
| D-glucose & D-fructose | Functional group mismatch | No | Ketose vs. aldose |
| D-galactose & D-glucose | C4 configuration only | Yes (C4-epimer) | Single chiral difference |
CSIR NET Relevance
Such questions test Fischer projection analysis and stereoisomer classification. Practice drawing open chains: D-glucose (all OH right except C3/C5), mannose (C2 flipped), galactose (C4 flipped).
