Problem 236 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The product \( \mathrm{C} \) is \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CHO} \), option (d).

Step 1: Oxidation Reaction

The first reaction involves the oxidation of toluene using potassium permanganate \( \mathrm{KMnO}_{4} \). Toluene \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{3} \) is oxidized to benzoic acid \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COOH} \). Therefore, the product \( \mathrm{A} \) is \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COOH} \).

Step 2: Conversion to Acid Chloride

The second step involves reacting benzoic acid \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COOH} \) with thionyl chloride \( \mathrm{SOCl}_{2} \). This reaction converts the carboxylic acid group into an acid chloride. Product \( \mathrm{B} \) is benzoyl chloride \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COCl} \).

Step 3: Reduction to Aldehyde

The final step involves the reduction of benzoyl chloride \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COCl} \) using hydrogen \( \mathrm{H}_{2} \) in the presence of palladium (Pd) and barium sulfate (BaSO\(_4\)). This reaction reduces the acid chloride to an aldehyde. Therefore, the product \( \mathrm{C} \) is benzaldehyde \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CHO} \).

Key Concepts

Oxidation Reaction

In the world of organic chemistry, oxidation reactions play a crucial role in transforming one functional group to another by adding oxygen or removing hydrogen. In the case of our exercise, toluene (\( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{3} \)) is oxidized into benzoic acid (\( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COOH} \)) using potassium permanganate (\( \mathrm{KMnO}_{4} \)). This chemical reaction introduces an oxygen atom into the molecular structure, effectively replacing a methyl group with a carboxylic acid group.

• The oxidizing agent, \( \mathrm{KMnO}_{4} \), is often used in reactions to convert alkyl groups attached to aromatic rings into carboxylic acids.
• This kind of transformation highlights the powerful ability of oxidation reactions to modify hydrocarbons into oxygen-containing compounds.
• It is a common method for preparing carboxylic acids from alcohols, aldehydes, or hydrocarbons.

By understanding oxidation reactions, students can predict and perform conversions between different organic compounds, making it a fundamental concept in organic synthesis.

Acid Chloride Formation

The transformation from a carboxylic acid to an acid chloride is a classic reaction in organic chemistry. Here, the benzoic acid (\( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COOH} \)) from the previous oxidation is converted into benzoyl chloride (\( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COCl} \)) using thionyl chloride (\( \mathrm{SOCl}_{2} \)).

• Thionyl chloride is a reagent that reacts with carboxylic acids to replace the hydroxyl group (\(-OH\)) with a chlorine atom, forming an acid chloride.
• Acid chlorides are highly reactive intermediates, useful for further transformations in organic synthesis due to their ability to react with a variety of nucleophiles.
• This reaction involves the formation of sulfur dioxide (\( \mathrm{SO}_{2} \)) and hydrogen chloride (\( \mathrm{HCl} \)) as by-products, which often escape as gases.

Understanding the conversion to acid chlorides can aid in grasping more advanced synthetic routes in chemistry, where building complexity from simpler compounds is key.

Aldehyde Synthesis

The synthesis of aldehydes is a significant step in organic chemistry, providing a gateway to numerous other compound formations. In the given sequence of reactions, benzoyl chloride (\( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COCl} \)) is reduced to benzaldehyde (\( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CHO} \)). This step utilizes hydrogen (\( \mathrm{H}_{2} \)) gas and palladium on barium sulfate (Pd/BaSO5).

• The use of Pd/BaSO5 specifies a controlled reduction, often referred to as Rosenmund reduction, which is selective for converting acid chlorides to aldehydes without further reduction to alcohols.
• This reaction condition limits the hydrogen source, preventing the aldehyde from being overly reduced into a primary alcohol.
• Aldehydes, once prepared, can participate in a multitude of reactions, including further oxidation, nucleophilic additions, or condensation reactions.

Understanding aldehyde synthesis is crucial for students, as it opens pathways to manipulating carbonyl chemistry, leading to broader chemical explorations.