We know that the alkyl group directly attached to benzene is known as an alkyl side-chain. Now, regardless of the length of that side-chain, it can be oxidized to benzoic acid using hot KMnO4. What's the kicker? There must be at least one benzylic hydrogen.
Concept #1: Benzylic Oxidation
Example #1: Which alkylbenzene would NOT yield benzoic acid, treated with KMnO4