Which reagent is used to detect the presence of a carbonyl group?

Solution

1. Carbonyl compounds react with 2,4-DNP to form a hydrazone derivative.
2. The reaction is ${\text{R}}_2\text{C=O}+{\text{NH}}_2{\text{NHC}}_6{\text{H}}_3({\text{NO}}_2{)}_2\to {\text{R}}_2{\text{C=NNHC}}_6{\text{H}}_3({\text{NO}}_2{)}_2+{\text{H}}_2\text{O}$.
3. The formation of a precipitate confirms the presence of the carbonyl group.
4. Hence the answer is (A).