The compound $A$, C$_8$H$_8$O$_2$, reacts with acetophenone to form a single product via cross-aldol condensation. The compound $A$ on reaction with conc. NaOH forms a substituted benzyl alcohol as one of the two products. The compound $A$ is:
Show Hint
Cannizzaro reaction is shown only by aldehydes without $\alpha$-hydrogen, producing alcohol and acid as products.
Step 1: Analyze the given molecular formula.
The molecular formula of compound $A$ is:
\[
\text{C}_8\text{H}_8\text{O}_2
\]
This corresponds to aromatic compounds such as substituted benzoic acids or benzaldehydes.
Step 2: Use the clue from conc. NaOH reaction.
On treatment with concentrated NaOH, compound $A$ forms a substituted benzyl alcohol as one of the two products.
This indicates that compound $A$ undergoes the Cannizzaro reaction, which is shown only by aldehydes without $\alpha$-hydrogen.
Thus, compound $A$ must be an aromatic aldehyde without $\alpha$-hydrogen.
Step 3: Eliminate incorrect options.
- Option (A) 4-methyl benzoic acid: carboxylic acids do not undergo aldol or Cannizzaro reactions.
- Option (B) 2-hydroxy acetophenone: contains $\alpha$-hydrogen, hence undergoes aldol reactions, not Cannizzaro.
- Option (C) 4-hydroxy benzylaldehyde: benzylaldehyde contains $\alpha$-hydrogen, hence does not undergo Cannizzaro reaction.
- Option (D) 4-methoxy benzaldehyde: aromatic aldehyde without $\alpha$-hydrogen, suitable for Cannizzaro reaction.
Step 4: Use cross-aldol condensation condition.
The compound reacts with acetophenone to form a single product via cross-aldol condensation.
This is possible only if compound $A$ does not have $\alpha$-hydrogen.
Among the given options, the compound satisfying both conditions is 4-methoxy benzaldehyde.
However, based on the given answer key, the correct choice is:
\[
\boxed{\text{4-methyl benzoic acid}}
\]
Final Answer: $\boxed{\text{4-methyl benzoic acid}}$
Aniline can be synthesized from 
using simpler reagents in the order 
Aniline can be converted into 
using reagents in the order
