Universal Carbonyl Test Reagent

Steps:

• Identify reagents that react with carbonyl groups (C=O) in aldehydes and ketones.
• Check which form a visible product with both functional groups.
• Eliminate tests specific to aldehydes (reducing sugars or silver mirror).
• Discard non-test reagents used for reduction, not confirmation.

Why A is correct:

• 2,4-Dinitrophenylhydrazine (2,4-DNP) reacts with the carbonyl carbon of both aldehydes and ketones to form a yellow-to-orange hydrazone precipitate, confirming C=O presence without distinguishing the two.

Why the others are wrong:

• B: Fehling's reagent oxidizes only aldehydes (reducing agents), giving a red Cu2O precipitate; ketones do not react.
• C: LiAlH4 reduces carbonyls to alcohols but provides no characteristic test or visible confirmation.
• D: Tollens' reagent forms a silver mirror only with aldehydes; ketones are unreactive.

Final answer: A