Q.18 Correctly match the following Monosaccharides with their respective Epimers.
Monosaccharide Epimer
P. D-mannose 1. C-3 epimer of D-glucose
Q. D-allose 2. C-4 epimer of D-glucose
R. D-galactose 3. C-4 epimer of D-mannose
S. D-talose 4. C-2 epimer of D-glucose
5. C-5 epimer of D-glucose
(A) P-4; Q-1; R-2; S-3
(B) P-5; Q-1; R-2; S-3
(C) P-4; Q-3; R-5; S-1
(D) P-1; Q-5; R-3; S-2
Unlocking Monosaccharide Epimers: D-Mannose, D-Allose, D-Galactose, D-Talose Matching Guide
D-mannose is the C-2 epimer of D-glucose, while D-allose, D-galactose, and D-talose each differ at specific chiral carbons, making the correct matching P-4, Q-1, R-2, S-3 for option (A).
Correct Answer
Option (A) P-4; Q-1; R-2; S-3 stands as the precise match. D-mannose (P) pairs with 4 (C-2 epimer of D-glucose), confirmed by its inverted configuration at C-2 in Fischer projection versus D-glucose. D-allose (Q) aligns with 1 (C-3 epimer), D-galactose (R) with 2 (C-4 epimer), and D-talose (S) with 3 (C-4 epimer of D-mannose).
Epimer Definitions
Epimers represent diastereomers differing solely at one chiral carbon in aldoses like D-glucose (standard aldohexose: OH right at C2, left at C3, right at C4/C5). Standard D-glucose Fischer projection sets the baseline: CHO-(H-C-OH)-(HO-C-H)-(H-C-OH)-(H-C-OH)-CH2OH. Inverting OH at C-2, C-3, or C-4 yields mannose, allose, or galactose, respectively.
Matching Breakdown
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P. D-mannose → 4. C-2 epimer of D-glucose: OH left at C-2 (vs. right in glucose); rest identical.
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Q. D-allose → 1. C-3 epimer of D-glucose: OH right at C-3 (vs. left); rare sugar in nature.
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R. D-galactose → 2. C-4 epimer of D-glucose: OH left at C-4 (vs. right); key in lactose.
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S. D-talose → 3. C-4 epimer of D-mannose: Starts from mannose (C-2 inverted), then inverts C-4; also C-2 epimer of galactose.
Option Analysis
| Option | P (D-mannose) | Q (D-allose) | R (D-galactose) | S (D-talose) | Status |
|---|---|---|---|---|---|
| (A) | 4 (Correct) | 1 (Correct) | 2 (Correct) | 3 (Correct) | ✅ Right |
| (B) | 5 (Wrong) | 1 (Correct) | 2 (Correct) | 3 (Correct) | ❌ P fails (C-5 is D-gulose) |
| (C) | 4 (Correct) | 3 (Wrong) | 5 (Wrong) | 1 (Wrong) | ❌ Q/R/S mismatch |
| (D) | 1 (Wrong) | 5 (Wrong) | 3 (Wrong) | 2 (Wrong) | ❌ All incorrect |
Exam Tips
Recall D-series (C-5 OH right). Practice Fischer flips: C-2 (mannose/talose family), C-3 (allose/altrose), C-4 (galactose/gulose). Visual mnemonics aid: “Man No Gal Today” for C2/C4 shifts.
