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

The benzylic $\mathrm{CH}{}_2$ carries hydrogen, so the whole side chain is oxidised and cleaved at the benzylic carbon. The ring keeps only one carbon, as $-\mathrm{COOH}$, and the terminal $\mathrm{CH}{}_3$ is lost as $\mathrm{CO}{}_2$. The product is benzoic acid.