Problem 25 - Chemistry in 30 days

Question

Benzamide on reaction with \(\mathrm{POCl}_{3}\) gives (a) Aniline (b) Chlorobenzene (c) Benzyl amine (d) Benzonitrile

Step-by-Step Solution

Answer

Benzamide reacts with
03POClb3 to give benzonitrile (d).

Step 1: Understand the Reaction

Benzamide (
03R-C=O-NHb2) is an amide. The reaction in question involves reacting benzamide with phosphorus oxychloride (
03POClb3), a reagent known for dehydration reactions. In this context, phosphorus oxychloride acts as a dehydrating agent.

Step 2: Identify the Reaction Type

The reaction between benzamide and
03POClb3 typically leads to the formation of a nitrile group (
03-CaN). This is because the reagent
03POClb3 facilitates the dehydration of the amide, removing the elements of water (H2O), and forming a carbon-nitrogen triple bond.

Step 3: Determine the Product

Upon dehydration, the amide functional group (
03-C=O-NHb2) in benzamide is converted into a nitrile (
03-CaN). Thus, benzamide is converted to benzonitrile (
03Cb6Hb5-CaN).

Step 4: Choose the Correct Answer

From the given options, the chemical name for benzonitrile is option (d) - benzonitrile. Thus, the product of the reaction between benzamide and
03POClb3 is benzonitrile.

Key Concepts

Understanding Benzamide

Benzamide is an organic compound categorized as an amide due to its functional group characterized by a carbonyl group attached to a nitrogen atom. The structure of benzamide is derived from benzoic acid, where the hydroxyl group of the acid (-OH) is replaced by an amino group (-NH extsubscript{2}).
Benzamide is structurally expressed as C extsubscript{6}H extsubscript{5}CONH extsubscript{2}, where a benzene ring is attached to the amide group. This makes benzamide an aromatic amide.
Amides like benzamide are known for their relative stability due to resonance in the amide bond, which is typically polar in nature. As a result, benzamide participates in specific chemical reactions, including dehydration when exposed to certain reagents.

Role of Phosphorus Oxychloride

Phosphorus oxychloride, denoted as POCl extsubscript{3}, is a versatile reagent widely used in organic chemistry. It is particularly known for its role as a dehydrating agent. This ability makes it crucial in transforming certain arenes and amines into their nitrile counterparts.
When used in chemical reactions, phosphorus oxychloride facilitates the removal of water elements (two hydrogens and one oxygen), which is essential in converting compounds like amides into nitriles.
For benzamide, POCl extsubscript{3} initiates the dehydration process, as it optimizes conditions such as temperature and solvent, thereby favoring the formation of a carbon-nitrogen triple bond.

Mechanism of Nitrile Formation

The transformation of an amide to a nitrile group involves the introduction of a carbon-nitrogen triple bond. In the context of benzamide reacting with POCl extsubscript{3}, this transformation is accomplished through dehydration.
Here is an overview of the reaction:

• Benzamide (C extsubscript{6}H extsubscript{5}CONH extsubscript{2}) reacts with POCl extsubscript{3}.
• The mechanism involves the removal of a water molecule (H extsubscript{2}O).
• This results in the conversion of the amide group (-CONH extsubscript{2}) to a nitrile group (-CN).

The overall chemical equation follows:\[\text{C}_6\text{H}_5\text{CONH}_2 + \text{POCl}_3 \rightarrow \text{C}_6\text{H}_5\text{C} \equiv \text{N} + \text{H}_2\text{O}\]The product formed is benzonitrile, identified by the presence of the itrile group within the benzene framework.

Understanding Dehydration Reactions in Organic Chemistry

Dehydration reactions play a pivotal role in organic chemistry, transforming molecules by eliminating water components. These reactions are frequently employed to convert specific functional groups into others by removing -OH and -H groups from the involved components.
In the case of benzamide reacting with POCl extsubscript{3}, the dehydration reaction removes water, enabling the formation of a more stable compound, such as a nitrile group. This process is a type of elimination reaction and usually occurs in the presence of an acid or other catalyst like POCl extsubscript{3}.
Through dehydration, complex molecules can be synthesized, offering pathways to various derivatives and compounds like benzonitrile from benzamide. Understanding this core concept makes it easier to predict and carry out similar transformations in organic chemistry.