See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Concept: Osazone formation from a monosaccharide involves reaction of phenylhydrazine (PhNHNH2) with the sugar at carbons C1 and C2. Step 1 - Mechanism overview: The conversion of D-glucose to its osazone (glucosazone) proceeds in three distinct steps: (i) First equivalent of PhNHNH2 reacts with the aldehyde at C1 to form a phenylhydrazone (PhNH-N=CH-...), releasing water. (ii) A second equivalent of PhNHNH2 acts as an oxidant: it oxidizes the C2 hydroxyl to a ketone (C2=O), and in doing so, one molecule of PhNHNH2 is reduced to aniline (PhNH2) and ammonia (NH3). This second equivalent is consumed/destroyed in the oxidation step and does NOT appear in the final product. (iii) A third equivalent of PhNHNH2 reacts with the newly formed C2 ketone to give the second phenylhydrazone, completing the osazone (bis-phenylhydrazone at C1 and C2). Step 2 - Counting equivalents: - 1st equivalent: forms hydrazone at C1. - 2nd equivalent: oxidizes C2-OH to C2=O (this equivalent is consumed as a reducing agent/oxidant and is broken down to aniline + NH3). - 3rd equivalent: forms hydrazone at C2. Total = 3 equivalents of phenylhydrazine required. Step 3 - Why other options fail: - (a) two: Insufficient; only accounts for the two hydrazone-forming steps but ignores the oxidation step. - (c) four and (d) five: More than necessary; the reaction only requires exactly 3 equivalents. Therefore, the correct answer is B.