Animated Solution for Chemistry - Organic Chemistry: In the following sequence of reaction, $\text{Toluene}\stackrel{{\text{KMnO}}_4}{\to }A\stackrel{{\text{SOCl}}_2}{\to }B\underset{{\text{BaSO}}_4}{\overset{{\text{H}}_2/\text{Pd}}{\to }}C$ The product $C$ is

Organic chemistry is often like a puzzle where each reagent acts as a specific tool to mold a molecule into a desired shape. In this problem, we are given a starting material, Toluene, and a sequence of three distinct chemical transformations. Our goal is to track the structural changes step-by-step to uncover the final product, $C$.

Let's break down the journey of this molecule.

The Power of Permanganate (Oxidation)

The first step involves treating Toluene (${\text{C}}_6{\text{H}}_5{\text{CH}}_3$) with potassium permanganate (${\text{KMnO}}_4$).

${\text{KMnO}}_4$ is a notoriously strong oxidizing agent. When an alkylbenzene like toluene is subjected to alkaline ${\text{KMnO}}_4$ followed by acidification, the entire alkyl side chain is aggressively oxidized. Regardless of how long the carbon chain is, as long as there is at least one benzylic hydrogen, the chain is cleaved and oxidized down to a single carboxyl group ($\text{-COOH}$).

Thus, the methyl group of toluene is transformed into a carboxylic acid, yielding Benzoic acid as our intermediate product $A$.

The Thionyl Chloride Transformation (Chlorination)

Next, Benzoic acid is reacted with thionyl chloride (${\text{SOCl}}_2$).

This is a classic and highly efficient method for converting carboxylic acids into acid chlorides. The hydroxyl ($\text{-OH}$) part of the carboxyl group is substituted by a chlorine atom. One of the reasons ${\text{SOCl}}_2$ is so popular in the lab is that its byproducts, sulfur dioxide (${\text{SO}}_2$) and hydrogen chloride ($\text{HCl}$), are gases that easily escape the reaction mixture, leaving behind a pure product.

This transformation turns Benzoic acid into Benzoyl chloride, which is our intermediate product $B$.

The Rosenmund Reduction (Selective Hydrogenation)

In the final act, Benzoyl chloride is treated with hydrogen gas (${\text{H}}_2$) in the presence of a palladium ($\text{Pd}$) catalyst supported on barium sulfate (${\text{BaSO}}_4$).

This specific combination of reagents should immediately trigger a memory: it is the famous Rosenmund reduction. The goal here is to reduce the acid chloride to an aldehyde. However, palladium is a very active catalyst and would normally continue to reduce the newly formed aldehyde all the way down to a primary alcohol.

To prevent this over-reduction, barium sulfate (${\text{BaSO}}_4$) is added. It acts as a catalytic "poison," intentionally dampening the reactivity of the palladium. This ensures the reduction stops exactly at the aldehyde stage.

Consequently, Benzoyl chloride is selectively reduced to Benzaldehyde, which is our final product $C$.

Tracing the sequence from Toluene $\to$ Benzoic acid $\to$ Benzoyl chloride $\to$ Benzaldehyde, we find that the final product $C$ is indeed Benzaldehyde, represented by the chemical formula ${\text{C}}_6{\text{H}}_5\text{CHO}$.

Matching this with our given options, the correct choice is (d).