Problem 70 - A Complete Resource Book in Chemistry for JEE Main

Question

Benzamide on reaction with \(\mathrm{POCl}_{3}\) gives (a) aniline (b) chlorobenzene (c) benzyl amine (d) benzonitrile

Step-by-Step Solution

Answer

The reaction of benzamide with \(\mathrm{POCl}_3\) forms benzonitrile.

Step 1: Understanding the Reaction

Benzamide reacts with phosphorus oxychloride ( POCl_3 ) to undergo a dehydration reaction.

Step 2: Dehydration Mechanism

In this reaction, the amide group in benzamide loses a molecule of water (H2O). This typically involves the removal of the oxygen from the carbonyl group (\(C=O\)) and a hydrogen atom from the amide \(\text{-NH}_2\) group.

Step 3: Formation of Intermediate

The removal of water from benzamide creates an intermediate that facilitates the formation of a \(\text{-C=N}\) triple bond.

Step 4: Identify the Product

The product formed is a nitrile. For benzamide, this reaction results in the formation of benzonitrile.

Key Concepts

Benzamide Reaction

The benzamide reaction involves benzamide, a common organic compound, reacting with phosphorus oxychloride (\( \text{POCl}_3 \)). This process is an example of how amides can be transformed into other compounds using dehydration.

• Benzamide serves as an amide that can be converted into nitriles.
• In the context of this reaction, benzamide acts as a substrate.
• \( \text{POCl}_3 \) is a strong dehydrating agent that facilitates the transformation.

To sum up, this reaction is used in organic chemistry to modify amides, turning them into more reactive molecules like nitriles, which can be further utilized in various chemical syntheses.

Dehydration Mechanism

Dehydration in chemistry is the process of removing water (\( \text{H}_2\text{O} \)) from a molecule. In the case of benzamide with \( \text{POCl}_3 \), dehydration leads to significant structural changes.

• The process begins with the loss of the oxygen from the carbonyl group (\( \text{C=O} \)).
• Next, the molecule sheds a hydrogen atom from the \( \text{-NH}_2 \) group.

As a result of these steps, a water molecule is removed, and the compound transitions towards becoming a nitrile. Understanding this mechanism is crucial for predicting the outcomes of similar chemical reactions involving amides and dehydrating agents.

Benzonitrile Formation

The transformation of benzamide into benzonitrile is a direct result of the dehydration mechanism.

• This chemical change signifies the establishment of a \( \text{-C≡N} \)
• Nitriles like benzonitrile are important in the production of various pharmaceuticals and other organic materials due to their reactivity.
• The triple bond between carbon and nitrogen provides unique properties that are leveraged in synthetic chemistry.

Through this reaction, benzamide is efficiently converted to benzonitrile, demonstrating the transformative nature of dehydration reactions and the role they play in expanding the toolkit of organic chemists.