From alkyl benzene

Aromatic carboxylic acids can be prepared by oxidising an alkylbenzene with dilute HNO3, or with alkaline or acidic KMnO4, or with chromic acid. A key feature of this oxidation: regardless of how LONG the alkyl side chain actually is, the ENTIRE chain gets oxidised down to a single -COOH group directly on the ring -- so a long alkyl chain gives exactly the same acid product as a short one, provided both are attached at the same ring position. The one deliberate exception is a TERTIARY alkyl substituent on the ring, which resists this oxidation entirely and is NOT converted to -COOH. Two worked examples given: toluene (methylbenzene), with alkaline KMnO4 and heat, gives potassium benzoate first, which on acidification with H3O+ gives benzoic acid; and cumene (isopropylbenzene), treated exactly the same way, has its whole three-carbon isopropyl side chain oxidised down to a single -COOH, again g …