first ethyl is electron donating group, so push electrons out to protonate Electrophiles, which is H+ on this reaction on ortho or para position. Now electron deficiency occur on the benzene ring, but resonance stablized (remember ! benzene is stable compound which always wants to go back to benzene). You probably remember cation ion will be most stable form on tertiary, so positive charge will be tertiary carbon directly attached with ethyl group. Through the complex mechanism (you don't need to know this mechanism for dat), (K agents is strong oxidizing agents) so oxidation occur which leaves the ethyl benzene into benzoic acid. No matter what carbon length, for example, ethyl, methy, propyl group, it gives the same benzoic acid.
guys correct me if i am wrong!
