Benzene diazonium chloride on reaction with KCN in the presence of CuCN yields X. X on hydrolysis yields Y. Y can also be obatained from

When benzene diazonium chloride is treated with CuCN, the diazonium group is replaced with CN group to form benzonitrile (X) & on  hydrolysis forms benzoic acid (compound Y)                 $\mathrm{Ph}-\mathrm{N}=\mathrm{N}-\mathrm{Cl}\stackrel{\mathrm{CuCN}}{\to }\mathrm{Ph}-\mathrm{CN}\stackrel{{\mathrm{H}}_2\mathrm{O}}{\to }\mathrm{Ph}-\mathrm{COOH}$

Methyl benzene on by oxidation by KMnO₄ to form Benzoic acid