Question

Show how each of the following compounds can be converted to benzoic acid:
(I) Acetophenone
(II) Ethyl benzene

Hint:

In side-chain oxidation of alkylbenzenes, the entire alkyl group is oxidized to a single \(-COOH\) group, no matter how many carbons are in the chain.

Answer 1

Step 1: Understanding the Concept:
Aromatic side chains or acyl groups attached to a benzene ring can be oxidized to a carboxylic acid group (\(-COOH\)) using strong oxidizing agents, regardless of the length of the side chain, provided there is at least one benzylic hydrogen or it's an acyl group.
Step 2: Detailed Explanation:
(I) Acetophenone to Benzoic acid:
Acetophenone is treated with alkaline \(KMnO_4\) and heated. The methyl keto side chain is oxidized to a carboxylate salt. Subsequent treatment with dilute acid (hydrolysis) yields benzoic acid.
\[ C_6H_5COCH_3 \xrightarrow{KMnO_4 / KOH, \Delta} C_6H_5COOK \xrightarrow{H_3O^+} C_6H_5COOH \]
(II) Ethyl benzene to Benzoic acid:
Ethyl benzene contains benzylic hydrogens. Strong oxidation with alkaline \(KMnO_4\) cleaves the alkyl chain at the benzylic position, converting the entire side chain into a carboxylate group. Acidification yields benzoic acid.
\[ C_6H_5CH_2CH_3 \xrightarrow{KMnO_4 / KOH, \Delta} C_6H_5COOK \xrightarrow{H_3O^+} C_6H_5COOH \]
Step 3: Final Answer:
Both compounds are converted to benzoic acid by heating with alkaline \(KMnO_4\) followed by acidification.