33.1.1—Preparation of benzoic acid

An alkylbenzene with at least one benzylic hydrogen can be oxidised by hot alkaline KMnO₄. After acidification, the side chain is converted to a carboxylic acid attached directly to the ring.

The carbon skeleton beyond the benzylic carbon is removed during vigorous oxidation, so different alkylbenzenes can give the same benzoic-acid ring product.

Methylbenzene + hot alkaline KMnO₄, then dilute acid → benzoic acid. The methyl carbon becomes the carboxyl carbon.

Do not oxidise the aromatic ring itself or stop at an alcohol; this is a strong side-chain oxidation followed by acidification.