4-Methylbenzamide is treated successively with (i) Br2/KOH, (ii) NaNO2/HCl at 273-278 K, (iii) CuCN, and (iv) H3O^+, heat.

(i) Hofmann bromamide degradation removes the carbonyl carbon: $\mathrm{ArCONH_2\to ArNH_2}$ (4-methylaniline, one carbon fewer). (ii) Diazotisation gives the diazonium chloride. (iii) Sandmeyer reaction with CuCN gives 4-methylbenzonitrile, restoring a carbon on the ring. (iv) Acid hydrolysis converts the nitrile to the carboxylic acid: 4-methylbenzoic acid. Note that the product has the same carbon count as the starting amide but is the acid, not the amide.