Question

Which of the following compound gives benzoic acid when it reacts with hot $KMnO_4$ followed by acidification.

• A. Styrene
• B. Toluene
• C. Benzaldehyde
• D. All of these

Answer 1

Answer: (D)

All of these

Answer 2

The reaction of organic compounds with hot potassium permanganate ($KMnO_4$) followed by acidification is a strong oxidation reaction. We need to determine which of the given compounds yields benzoic acid under these conditions.

Option (1): Styrene (Vinylbenzene). The structure is $\text{C}_6\text{H}_5-\text{CH}=\text{CH}_2$. Hot $KMnO_4$ causes oxidative cleavage of the double bond. The carbon atom of the double bond attached to the phenyl ring is oxidized to a carboxylic acid group, and the terminal carbon atom is oxidized to carbon dioxide.

$\text{C}_6\text{H}_5-\text{CH}=\text{CH}_2 \xrightarrow{KMnO_4, \text{ heat}} \text{C}_6\text{H}_5-\text{COOH} + \text{CO}_2 + \text{H}_2\text{O}$

Thus, styrene gives benzoic acid.

Option (2): Toluene (Methylbenzene). The structure is $\text{C}_6\text{H}_5-\text{CH}_3$. Alkyl groups attached to a benzene ring with at least one benzylic hydrogen are oxidized to a carboxylic acid group by hot $KMnO_4$. The methyl group in toluene has benzylic hydrogens.

$\text{C}_6\text{H}_5-\text{CH}_3 \xrightarrow{KMnO_4, \text{ heat}} \text{C}_6\text{H}_5-\text{COOK} \xrightarrow{H^+} \text{C}_6\text{H}_5-\text{COOH}$

Thus, toluene gives benzoic acid.

Option (3): Benzaldehyde. The structure is $\text{C}_6\text{H}_5-\text{CHO}$. Aldehydes are easily oxidized to carboxylic acids by strong oxidizing agents like $KMnO_4$.

$\text{C}_6\text{H}_5-\text{CHO} \xrightarrow{KMnO_4, \text{ heat}} \text{C}_6\text{H}_5-\text{COOK} \xrightarrow{H^+} \text{C}_6\text{H}_5-\text{COOH}$

Thus, benzaldehyde gives benzoic acid.

Since all three compounds (styrene, toluene, and benzaldehyde) give benzoic acid when reacted with hot $KMnO_4$ followed by acidification, option (4) "All of these" is the correct answer.