Animated Solution for Chemistry - Organic Chemistry: In the above reaction, the reagent 'A' is

Side-Chain Oxidation of Alkylbenzenes

Imagine you are an architect tasked with remodeling a house. You want to completely replace the roof but leave the foundation and the ground floor untouched. In organic chemistry, we often face similar challenges where we need to transform one specific part of a molecule without affecting the rest.

In this problem, we are starting with 4-methoxytoluene and our goal is to transform it into 4-methoxybenzoic acid.

Let's look closely at the structures. The reactant has a benzene ring with a methyl group ($-C{H}_3$) at the top and a methoxy group ($-OC{H}_3$) at the bottom. The product has the exact same benzene ring and methoxy group, but the methyl group at the top has been magically transformed into a carboxyl group ($-COOH$).

What kind of transformation is this? We are adding oxygen atoms and removing hydrogen atoms from the carbon. This is the hallmark of a classic oxidation reaction. To pull this off, we need a chemical sledgehammer—a very strong oxidizing agent.

Let's evaluate the tools in our toolbox (the given options):

$NaB{H}_4$ and $LiAl{H}_4$: These are famous reducing agents. They love to add hydrogen or remove oxygen. They are the exact opposite of what we need. $Zn-Hg/HCl$: This is the reagent for the Clemmensen reduction, used to strip oxygen away from ketones and aldehydes to form alkanes. Again, a reducing agent.

This leaves us with Alkaline $KMn{O}_4$ followed by $H^{+}$. Potassium permanganate ($KMn{O}_4$) is a notoriously powerful oxidizing agent. When applied to an alkylbenzene, it specifically hunts for the benzylic carbon (the carbon directly attached to the benzene ring).

As long as there is at least one hydrogen atom attached to this benzylic carbon, $KMn{O}_4$ will aggressively oxidize it, cleaving off any additional carbon atoms in the chain, and converting that benzylic carbon directly into a carboxylic acid group ($-COOH$).

The reaction is typically done in an alkaline medium, which first produces the carboxylate salt. A final step of acidification ($H^{+}$) is required to protonate the salt and yield the final 4-methoxybenzoic acid.

A Pro-Tip for JEE: This reaction is incredibly robust. Even if you had an ethyl ($-C{H}_{2}C{H}_3$) or a propyl ($-C{H}_{2}C{H}_{2}C{H}_3$) group instead of a methyl group, alkaline $KMn{O}_4$ would still chop off the tail and leave you with the exact same benzoic acid derivative!