What is the major product of the following reaction?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) →
Alkyl side chains on benzene containing at least one benzylic hydrogen undergo oxidation with hot alkaline $KMnO_4$ to give benzoic acid, irrespective of chain length.
The given compound is propyl benzene:
$C_{6}H_{5}—CH_{2}CH_{2}CH_{3}$
Benzylic carbon ($—CH_{2}$ attached to ring) contains hydrogen.
Hot $KMnO_{4}$ oxidizes entire side chain to $—COOH$.
Thus:
$C_{6}H_{5}—CH_{2}CH_{2}CH_{3} \rightarrow C_{6}H_{5}—COOH$
Option 1: $C_{6}H_{5}—CH_{2}CH_{2}COOH$
Side chain is not partially oxidized.
Option 2: $C_{6}H_{5}—COOH$
Complete oxidation occurs $\rightarrow$ benzoic acid.
Option 3: $C_{6}H_{5}—CH_{2}COOH$
Incomplete oxidation does not occur.
Option 4: $\text{C}_6\text{H}_5\text{—}\text{CH}_2\text{COCH}_3$
Oxidation gives acid, not ketone.