Q.No.21. Which one of the following hexoses will give an osazone that has a different
melting point from that of the osazone obtained from D (+) glucose?
D-galactose (option C) is the hexose that gives an osazone with a different melting point from D-glucose. Osazone formation involves C1 and C2, making configurations at C3-C6 determine differences, and D-galactose differs at C4.
Option Analysis
D-glucose reference: Standard aldohexose with OH groups right at C2, left at C3, right at C4/C5 (Fischer projection). Forms glucosazone with characteristic melting point.
Option A: Matches D-mannose (flipped OH at C2). C2 epimers like glucose/mannose yield identical osazones since reaction obliterates C1/C2 differences. Same melting point as D-glucose.
Option B: Matches D-allose (flipped OH at C3 vs. glucose). Same configuration from C4-C6 as glucose, so identical osazone and melting point.
Option C: Matches D-galactose (flipped OH at C4 vs. glucose). Differs at C4; osazone crystals differ in structure/time, yielding distinct melting point.
Option D: Complex structure, likely D-gulose or D-talose (differs at C3/C4). But C3-C6 match glucose’s osazone group? Wait, gulose differs at C3; talose at C2/C4—both same osazone as glucose.
Osazone Mechanism
Phenylhydrazine reacts with carbonyl (C1), oxidizes C2 to ketone, forms second hydrazone—identical for sugars matching C3-C6. Melting points distinguish: glucosazone ~200°C, galactosazone lower/distinct.
D-glucose, a key aldohexose in CSIR NET Life Sciences, forms characteristic osazones with phenylhydrazine, but which hexose gives different osazone from D-glucose based on melting point? This question tests understanding of stereochemistry and reaction specificity at C1-C2.
Understanding Osazone Formation
The osazone test identifies reducing sugars by forming phenylhydrazones at C1 and C2 via oxidation, erasing stereodifferences there. Aldohexoses with identical C3-C6 configurations yield same crystalline osazones (shape, melting point). D-glucose, D-mannose (C2 epimer), D-fructose share one; D-galactose differs.
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Mechanism steps: Aldehyde + PhNHNH2 → hydrazone (C1); C2 H oxidized; second hydrazone.
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Key: C3-C6 unchanged, dictate identity.
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Melting points: Glucosazone/mannosazone ~188-200°C; galactosazone distinct (~170°C).
Detailed Option Breakdown
Fischer projections reveal configurations:
| Option | Likely Identity | Differs from D-Glucose at | Osazone Same as Glucose? |
|---|---|---|---|
| A | D-Mannose | C2 | Yes |
| B | D-Allose | C3 | Yes |
| C | D-Galactose | C4 | No |
| D | D-Gulose/Talose | C3 or C2/C4 | Yes |
Answer: C (D-galactose). Its C4 flip alters osazone structure/melting point.
CSIR NET Exam Tips
Practice drawing Fischer projections and epimers (glucose-galactose C4; glucose-mannose C2). Osazone distinguishes galactose from glucose group. Relate to biotech: enzyme specificity mirrors stereo-sensitivity.
