In the above reaction, the reagent A is:

An aromatic ring imparts extra stability to the nearest carbon atom of its side chains. The aromatic ring and one carbon atom of a side chain can survive a vigorous permanganate oxidation. The product is a carboxylate salt of benzoic acid. (Hot chromic acid can also be used for this oxidation.) This oxidation is occasionally useful for making benzoic acid derivatives, as long as any other functional groups are resistant to oxidation.

Functional groups such as - NO2, halogens, - COOH, and -SO3H usually survive this brutal oxidation. Key Points:

• Alkaline $\ce{KMnO4}$ is the standard reagent to oxidize $\ce{-CH3}$ on aromatic ring to $\ce{-COOH}$
• $\ce{-OCH3}$ (methoxy ether) survives oxidation by $\ce{KMnO4}$
• All alkyl groups on benzene ring ultimately give $\ce{-COOH}$ under this oxidation