Question

The reagent which does NOT react with both acetone and benzaldehyde is

• A. Sodium hydrogen sulphite
• B. Benedict's reagent
• C. Grignard reagent
• D. Phenyl hydrazine

Hint:

Benedict's and Fehling's reagents oxidise only aliphatic aldehydes, not ketones or aromatic aldehydes.

Answer 1

The Correct Option is B

Step 1: Split the four reagents into two groups: general carbonyl reagents versus a selective oxidising reagent. Any reagent that attacks the C=O group directly will react with both a ketone and an aldehyde.
Step 2: NaHSO₃, the Grignard reagent and phenyl hydrazine all act on the carbonyl carbon (addition or condensation). Acetone and benzaldehyde both contain a C=O group, so all three of these give products with both compounds.
Step 3: Benedict's reagent is not a general carbonyl reagent; it needs an easily oxidisable aliphatic -CHO group. Acetone has no -CHO, and benzaldehyde's -CHO is aromatic and resists this mild oxidation, so Benedict's reagent stays unreacted with both.
Result: The reagent that reacts with neither is Benedict's reagent, alternative (ii).