Problem 139 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

In arder to convert aniline into chlorobenzene the reagent used is: [Manipal 2004] (a) \(\mathrm{NaNO}_{2} / \mathrm{HCl}, \mathrm{CuCl}\) (b) \(\mathrm{Cl}_{2} / \mathrm{CCl}_{4}\) (c) \(\mathrm{Cl}_{2} / \mathrm{AlCl}_{3}\) (d) \(\mathrm{CuCl}_{2}\)

Step-by-Step Solution

Answer

The correct option is (a) NaNO_{2} / HCl, CuCl.

Step 1: Understand the Reaction

To convert aniline to chlorobenzene, aniline ( H_2) needs to undergo diazotization, forming a diazonium salt, which is then substituted with Cl.

Step 2: Diazotization Reagents

The reagents required for diazotization of aniline to form a diazonium salt includes NO_{2} and HCl.

Step 3: Sandmeyer Reaction

The diazonium salt reacts with CuCl (in the presence of chloride ion) in the Sandmeyer reaction to form chlorobenzene.

Step 4: Identify the Correct Reagent Option

Based on the steps needed, the correct reagent combination for this reaction is NaNO_{2} / HCl to form the diazonium salt, followed by CuCl to replace the diazo group with a chlorine.

Key Concepts

Diazotization

Diazotization is a chemical process that involves converting an amine group into a diazonium salt. This is a crucial step in transforming aniline into other aromatic compounds, like chlorobenzene. The process begins with aniline, which is an aromatic amine, reacting with nitrous acid. This acid is generally prepared in situ through the reaction of sodium nitrite (\(\text{NaNO}_2\) and hydrochloric acid (HCl).

• Aniline reacts with nitrous acid to form a diazonium cation (\(\text{N}_2^+\)
• The reaction typically occurs at low temperatures to maintain the stability of the diazonium salt.

This diazonium salt is an essential intermediate for further substitution reactions, especially in processes like the Sandmeyer reaction.

Reagent Selection

Choosing the proper reagents is critical for successful chemical transformations. In the conversion of aniline to chlorobenzene, the reagents selected must effectively facilitate both diazotization and the subsequent nucleophilic substitution. In this exercise:

• The reagents \(\text{NaNO}_2\) and HCl are employed for diazotization, which forms the highly reactive diazonium salt.
• For the nucleophilic substitution leading to chlorobenzene, copper(I) chloride (\(\mathrm{CuCl}\)) is used. This reagent catalyzes the replacement of the diazonium group with a chlorine atom.

Reagent selection is essential because each chemical plays a unique role in ensuring the chemical reaction progresses as desired.

Aniline to Chlorobenzene Conversion

The conversion of aniline to chlorobenzene is a classic example of electrophilic aromatic substitution, proceeding through the intermediate formation of a diazonium salt and subsequent Sandmeyer reaction. The sequence of reaction steps is as follows:1. **Diazotization:** - Aniline reacts with sodium nitrite (\(\text{NaNO}_2\)) and hydrochloric acid (HCl) to yield a diazonium salt. - This creates the aromatic diazonium ion, which is unstable and requires cold conditions to maintain.2. **Sandmeyer Reaction:** - The diazonium salt reacts with copper(I) chloride (\(\text{CuCl}\)) in the presence of excess chloride ions. - This facilitates the replacement of the diazonium group by a chlorine atom, yielding chlorobenzene as the final product.This transformation highlights the precision required in organic synthesis to ensure selective and high-yield chemical modifications.