Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In the following sequence of reactions : Toluene \(\stackrel{\mathrm{KMnO}_{4}}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{SOCl}_{2}}{\longrightarrow} \mathrm{B} \frac{\mathrm{II}_{2} / \mathrm{Pd}}{\mathrm{BaSO}_{4}} \mathrm{C}\) (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OH}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The final product C is option (b): benzaldehyde (C₆H₅CHO).

Step 1: Understanding the Reaction Pathway

We are given a sequence of reactions starting with toluene: \[\text{Toluene } \stackrel{\text{KMnO}_4}{\rightarrow} \text{ A } \stackrel{\text{SOCl}_2}{\rightarrow} \text{ B } \stackrel{\text{I}_2/\text{Pd}}{\text{BaSO}_4} \text{ C}\]This means toluene is converted to compound A using KMnO4, then A is converted to B using SOCl2, and finally, B is converted to C using \(\text{I}_2/\text{Pd}\) in the presence of \(\text{BaSO}_4\). We need to identify the final compound C from the given options.

Step 2: Oxidation of Toluene

When toluene (C₆H₅CH₃) undergoes oxidation with KMnO₄, the methyl group is oxidized to a carboxylic acid. Therefore, the product A is benzoic acid: \[\text{C}_6\text{H}_5\text{COOH}\]

Step 3: Conversion of Benzoic Acid Using SOCl2

Benzoic acid is then treated with SOCl₂, which converts the carboxylic acid group \(-COOH\) into an acyl chloride group \(-COCl\). Thus, compound B is benzoyl chloride: \[\text{C}_6\text{H}_5\text{COCl}\]

Step 4: Reduction of Benzoyl Chloride

The next reaction involves benzoyl chloride being exposed to \(\text{I}_2/\text{Pd}\) over \(\text{BaSO}_4\), which is a Rosenmund reduction. This reduces the acyl chloride to an aldehyde group. Thus, compound C is benzaldehyde: \[\text{C}_6\text{H}_5\text{CHO}\]

Step 5: Identifying Final Product

Based on the sequence and the transformations, compound C is indeed benzaldehyde \(\text{C}_6\text{H}_5\text{CHO}\). This corresponds to option (b).

Key Concepts

Oxidation Reactions

Oxidation reactions involve the addition of oxygen or the removal of hydrogen from a molecule. They play a crucial role in organic chemistry by altering functional groups and creating new compounds. In the context of the given exercise, toluene undergoes an oxidation reaction with potassium permanganate \(\text{KMnO}_4\). This reagent is a strong oxidizing agent widely used to convert aliphatic side chains attached to aromatic rings into carboxylic acids.

When toluene \(\text{C}_6\text{H}_5\text{CH}_3\) is oxidized by \(\text{KMnO}_4\), the methyl group \(\text{-CH}_3\) is transformed into a carboxylic acid group \(\text{-COOH}\). Thus, toluene is converted into benzoic acid \(\text{C}_6\text{H}_5\text{COOH}\). This transformation is an example of a complete oxidation where the carbon in the methyl group goes to the highest oxidation state possible, effectively forming a new type of bond between carbon and oxygen.

• Oxidation increases the number of bonds from carbon to oxygen.
• It involves breaking C-H bonds and forming C=O bonds.
• In the exercise, benzoic acid is identified as compound A.

Acyl Chloride Formation

Acyl chlorides are highly reactive compounds and key intermediates in organic synthesis. The conversion of a carboxylic acid \(\text{RCOOH}\) into an acyl chloride \(\text{RCOCl}\) is usually achieved using thionyl chloride \(\text{SOCl}_2\). This reaction is a classic example of transforming a less reactive acid into a much more reactive acyl chloride, facilitating further chemical reactions.

In the exercise, benzoic acid \(\text{C}_6\text{H}_5\text{COOH}\) is treated with \(\text{SOCl}_2\), converting it to benzoyl chloride \(\text{C}_6\text{H}_5\text{COCl}\). Here, the \(\text{-OH}\) group of the carboxylic acid is replaced by chlorine \(\text{-Cl}\) from \(\text{SOCl}_2\), releasing gaseous byproducts such as \(\text{SO}_2\) and \(\text{HCl}\). The formation of acyl chloride is referred to as compound B in the solution.

• Acyl chlorides react readily due to the electronegative chlorine atom.
• This conversion prepares compound B for subsequent reactions.
• Acyl chlorides are valuable for creating alcohols, esters, and amides in further reactions.

Rosenmund Reduction

The Rosenmund reduction is a chemical reaction used to reduce acyl chlorides to aldehydes. It employs a catalytic system composed of palladium \(\text{Pd}\) supported on barium sulfate \(\text{BaSO}_4\), often with the presence of a halogen like iodine \(\text{I}_2\). This technology controls the reduction process, selectively transforming acyl chlorides to aldehydes without further reducing to alcohols.

In the scenario outlined, benzoyl chloride \(\text{C}_6\text{H}_5\text{COCl}\) is subjected to the Rosenmund reduction. The catalyst ensures that only one step - the reduction from acyl chloride to aldehyde - takes place, resulting in benzaldehyde \(\text{C}_6\text{H}_5\text{CHO}\). This selectivity is crucial in the synthesis of higher-value chemical products. Compound C, therefore, is identified as benzaldehyde, aligning with option (b).

• Rosenmund reduction is key for specific aldehyde formation.
• The process is mild and controlled, preventing over-reduction.
• It is specifically advantageous for converting acyl chlorides in a targeted manner.