The compound formed as a result of oxidation of ethyl benzene by ${\text{KMnO}}_4$ is:

Powerful oxidizing agents like acidified or alkaline ${\text{KMnO}}_4$ oxidize the side chains of aromatic hydrocarbons to a carboxylic acid group ($\text{-COOH}$), provided the benzylic carbon (the carbon directly attached to the ring) has at least one hydrogen atom.Ethyl benzene (${\text{C}}_6{\text{H}}_5{\text{-CH}}_2{\text{CH}}_3$) contains benzylic hydrogens. Regardless of the length of the side chain, it is oxidized completely to benzoic acid (${\text{C}}_6{\text{H}}_5\text{COOH}$).