Toluene is heated with alkaline $\mathrm{KMnO}{}_4$ and the mixture is then acidified with $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The product is

Hot alkaline permanganate oxidises any alkyl side chain that has a benzylic hydrogen completely to $-\mathrm{COO}{}^{-}$. Toluene gives potassium benzoate, and acidification liberates benzoic acid.