Question

Write down the product compound for the following reaction:

• A. Benzoic acid salt (Potassium benzoate)
• B. Isophthalic acid salt
• C. Phenyl acetone
• D. Benzaldehyde

Hint:

It doesn't matter if the side chain is Methyl, Ethyl, or Isobutyl; if it has a benzylic hydrogen, $KMnO_4$ turns it into a $-COOK$ (or $-COOH$ after acidification) group. Note: Tertiary butyl groups ($no$ benzylic H) will $not$ react this way!

Answer 1

The Correct Option is A

Concept: The oxidation of alkyl side chains on an aromatic ring using strong oxidizing agents like alkaline $KMnO_4$ is a standard method to produce carboxylic acids. Regardless of the length of the alkyl chain, it is oxidized down to a single $-COOH$ group, provided there is at least one benzylic hydrogen present.

Step 1: Analyze the starting material.
The reactant is an alkylbenzene (specifically, isobutylbenzene). The carbon atom directly attached to the benzene ring (the benzylic carbon) is attached to one hydrogen atom.

Step 2: Apply the oxidation rule.
When treated with hot alkaline $KMnO_4$, any alkyl group with at least one benzylic hydrogen is oxidized completely to a carboxyl group.
• The entire isobutyl side chain is cleaved.
• The benzylic carbon is converted to a carboxylate group ($-COOK$ in the presence of $KOH$).

Step 3: Identify the final product.
The aromatic ring remains intact, and the side chain becomes a $-COOK$ group. The resulting product is potassium benzoate.