See image — Biomolecules Chemistry Question

💡 Solution & Explanation

Concept: Osazone formation requires phenylhydrazine reacting with carbonyl compounds that have an alpha-hydroxy carbonyl group (i.e., a hydroxyl group on the carbon adjacent to a carbonyl group, as found in alpha-hydroxy aldehydes or alpha-hydroxy ketones). The reaction proceeds by first forming a phenylhydrazone at the carbonyl carbon, then oxidizing the alpha-carbon and forming a second phenylhydrazone there. This is only possible when the alpha-carbon bears a hydroxyl group. Step 1: Identify the structural requirement. Osazone formation needs an alpha-hydroxy carbonyl group: C(=O)-C(OH)- or equivalently a 1,2-diol adjacent to a carbonyl, i.e., the compound must have a -CO-CHOH- or -CO-CH2OH motif. Step 2: Analyze option (a) CH3CH2COCH2OH. This is 1-hydroxy-2-butanone (or methyl ethyl ketone with a hydroxymethyl group at C1). The structure is: CH3CH2-C(=O)-CH2OH. Here the carbonyl is at C3 (counting from OH end) and the alpha carbon to the carbonyl on one side is -CH2OH, which bears an -OH group. This is an alpha-hydroxy ketone motif (-CO-CH2OH), satisfying the structural requirement for osazone formation. Step 3: Analyze option (b) CH3COCH2CH2OH. This is 4-hydroxy-2-butanone: CH3-C(=O)-CH2-CH2OH. The alpha carbon to the carbonyl is -CH2- (not bearing OH); the OH is on the beta carbon. This does NOT have an alpha-hydroxy carbonyl, so osazone will not form. Step 4: Analyze option (c) CH3CH2CHOHCH2OH. This is a 1,2-diol (butane-1,2-diol): CH3CH2-CHOH-CH2OH. There is no carbonyl group present, so osazone cannot form in the classical sense. Step 5: Analyze option (d) CH3CH2COCH2OCH3. This is a beta-methoxy ketone: CH3CH2-C(=O)-CH2-OCH3. The alpha carbon has an -OCH3 group, not a free -OH group. Osazone formation requires a free hydroxyl, so this compound does not form an osazone. Conclusion: Only option (a) CH3CH2COCH2OH contains the alpha-hydroxy ketone (-CO-CH2OH) motif necessary for osazone formation. Therefore, the correct answer is A.