Q.15 Which of the following is the CORRECT statement about hexoses?
(A) D-mannose is C- 4 epimer of D-glucose
(B) D-galactose is C- 2 epimer of D-glucose
(C) D-glucose and L-glucose are diastereomers
(D) D-glucose and D-galactose are diastereomers
D-glucose and D-galactose are diastereomers, making option (D) correct. Epimers are diastereomers differing at only one chiral carbon, while D-glucose and L-glucose are enantiomers as non-superimposable mirror images. D-mannose is actually the C-2 epimer of D-glucose, and D-galactose is its C-4 epimer.
Option Analysis
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(A) Incorrect: D-mannose differs from D-glucose at C-2 (OH left in Fischer projection for mannose, right for glucose), so it is the C-2 epimer, not C-4.
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(B) Incorrect: D-galactose differs from D-glucose at C-4, making it a C-4 epimer, not C-2.
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(C) Incorrect: D-glucose and L-glucose are mirror images differing at all chiral centers (C-2, C-3, C-4, C-5); they are enantiomers, not diastereomers.
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(D) Correct: D-glucose and D-galactose differ only at C-4 but are diastereomers (stereoisomers that are not mirror images).
Hexoses epimers such as D-mannose and D-glucose form key topics in CSIR NET Life Sciences, testing stereochemistry in carbohydrates. Understanding relationships like D-galactose as a diastereomer clarifies molecular biology concepts.
Core Concepts
Hexoses are six-carbon monosaccharides like D-glucose (aldose with aldehyde at C-1). Epimers differ at one chiral carbon; diastereomers differ at two or more but are not mirror images; enantiomers are non-superimposable mirrors.
Detailed Comparisons
| Sugar Pair | Chiral Difference | Relationship | Example |
|---|---|---|---|
| D-mannose & D-glucose | C-2 only | Epimers (C-2) & diastereomers | OH axial in mannose pyranose |
| D-galactose & D-glucose | C-4 only | Epimers (C-4) & diastereomers | Galactose in lactose |
| D-glucose & L-glucose | All (C2-C5) | Enantiomers | L-glucose non-digestible |
D-glucose Fischer projection: CHO-(H-C-OH)-(HO-C-H)-(H-C-OH)-(H-C-OH)-CH2OH. Invert C-2 for mannose, C-4 for galactose.
Exam Relevance
For CSIR NET, recall: epimers specify position (e.g., D-mannose C-2 epimer D-glucose). Diastereomers include epimers but broader. Practice drawing projections.
