Step 1: 2,4-Dinitrophenylhydrazine reacts with carbonyl compounds to form hydrazones. This reagent can distinguish acetophenone from benzophenone.
Step 2: Acetophenone reacts to form a hydrazone, whereas benzophenone does not.
Final Answer:
\[
\boxed{2,4\text{-dinitrophenylhydrazine}}
\]