Selective carbonyl test distinguishes ketone from alcohol

Steps:

• Identify functional groups: propanone contains a carbonyl (ketone), ethanol contains an alcohol group.
• Recall 2,4-DNPH reagent forms orange hydrazone precipitates with carbonyl compounds.
• Note ethanol lacks a carbonyl, so it shows no reaction with 2,4-DNPH.
• Verify other reagents fail to differentiate based on their specificity.

Why A is correct:

• 2,4-Dinitrophenylhydrazine reacts with ketones like propanone to form a yellow-orange precipitate (hydrazone), but alcohols like ethanol do not react.

Why the others are wrong:

• B. Bromine water detects alkenes; neither propanone nor ethanol decolorizes it.
• C. Fehling's reagent tests for aldehydes; neither propanone (ketone) nor ethanol reacts.
• D. Tollen's reagent tests for aldehydes; neither propanone nor ethanol gives a silver mirror.

Final answer: A