Animated Solution for Chemistry - Organic Chemistry: The compound formed as a result of oxidation of ethyl benzene by ${\text{KMnO}}_4$ is

When we talk about strong oxidizing agents in organic chemistry, potassium permanganate (${\text{KMnO}}_4$) and potassium dichromate (${\text{K}}_2{\text{Cr}}_2{\text{O}}_7$) are the heavyweights. They are notorious for tearing through carbon-carbon bonds under the right conditions.

In this problem, we are treating ethyl benzene with ${\text{KMnO}}_4$ in an acidic or alkaline medium. To predict the product, we need to understand a very specific and fascinating rule regarding the oxidation of alkyl benzenes.

The benzene ring itself is incredibly stable due to its aromaticity. It acts like an impenetrable fortress against oxidation. However, the carbon atoms directly attached to the ring—known as benzylic carbons—are highly vulnerable, provided they have at least one hydrogen atom attached to them (a benzylic hydrogen).

The golden rule states: Any alkyl group attached to a benzene ring, regardless of its chain length, will be completely oxidized to a carboxyl group ($-\text{COOH}$) as long as it possesses at least one benzylic hydrogen.

Let's look at our reactant, ethyl benzene (${\text{C}}_6{\text{H}}_5-{\text{CH}}_2{\text{CH}}_3$).

The carbon directly attached to the benzene ring is part of a $-{\text{CH}}_2-$ group. This means it has two benzylic hydrogens. Because it meets the requirement, the powerful ${\text{KMnO}}_4$ will attack this position.

The entire ethyl chain is cleaved. The benzylic carbon is oxidized to a $-\text{COOH}$ group, while the remaining carbon atoms in the chain are oxidized away into ${\text{CO}}_2$ and ${\text{H}}_2\text{O}$.

The resulting product is a benzene ring with a carboxyl group attached, which is benzoic acid.

What if we had used tert-butyl benzene instead? In tert-butyl benzene, the carbon attached to the ring is bonded to three other methyl groups and zero hydrogens.

Because it lacks a benzylic hydrogen, ${\text{KMnO}}_4$ cannot initiate the oxidation process. The reaction simply does not occur! This is a classic trap in competitive exams, so always check for that benzylic hydrogen before you start oxidizing.