Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Chlorobenzene can be prepared by reacting aniline with (a) nitrous acid followed by heating with cuprous chloride (b) chlorine in presence of anhydrous aluminium chloride (c) cuprous chloride (d) hydrochloric acid

Step-by-Step Solution

Answer

(a) nitrous acid followed by heating with cuprous chloride.

Step 1: Understanding the Reaction

To synthesize chlorobenzene from aniline, we need to understand the feasible chemical reactions. Aniline contains an amine group that can undergo a diazotization reaction to replace the amino group with a halogen.

Step 2: Identify the Two-Step Process

The reaction involves two main steps: the conversion of aniline to benzenediazonium chloride using nitrous acid, followed by the reaction with copper(I) chloride to form chlorobenzene.

Step 1: Diazotization

In the first step, aniline reacts with nitrous acid (formed in situ from sodium nitrite and hydrochloric acid) to produce benzenediazonium chloride. \[C_6H_5NH_2 + NaNO_2 + 2HCl \rightarrow C_6H_5N_2^+Cl^- + 2H_2O \]

Step 2: Preparation of Chlorobenzene

The benzenediazonium chloride is then heated with cuprous chloride (CuCl) to yield chlorobenzene. This reaction is known as the Sandmeyer reaction. \[C_6H_5N_2^+Cl^- + CuCl \rightarrow C_6H_5Cl + N_2 \]

Step 5: Conclusion and Final Choice

The two main reactants for the preparation of chlorobenzene from aniline are nitrous acid and cuprous chloride. Therefore, option (a) is the correct choice.

Key Concepts

Diazotization: A Crucial Chemical Transformation

Diazotization is a fundamental chemical transformation, especially valuable in organic synthesis. It involves converting an amine group in an aromatic amine, like aniline, into a diazonium compound. This step is critical since it prepares the molecule for further reactions, such as the Sandmeyer reaction. To achieve diazotization, an aromatic primary amine is treated with nitrous acid. However, nitrous acid itself is generally unstable and is formed in situ, which means directly in the reaction mixture. It is typically prepared by reacting sodium nitrite with hydrochloric acid in cold conditions. Once the nitrous acid is generated, it reacts with aniline to produce benzenediazonium chloride, an important intermediate.

The formation of diazonium salts is temperature-sensitive, often conducted in an ice bath to maintain temperatures close to 0°C. This precaution is necessary because diazonium compounds can decompose easily if the temperature rises too high, releasing nitrogen gas and potentially leading to unwanted reactions. Diazotization is not only limited to forming aryl diazonium salts but extends its utility to various derivatives that facilitate numerous chemical transformations.

Benzenediazonium Chloride: The Versatile Intermediate

Benzenediazonium chloride stands out as a versatile and highly functional intermediate in the field of organic chemistry. After its formation via diazotization, this compound opens the door to many useful transformations. Its structure includes a diazonium group, represented as \(C_6H_5N_2^+Cl^-\). This ion pair explores a balanced and reactive state as the nitrogen group can be easily substituted.

The unique property of benzenediazonium chloride is its ability to partake in substitution reactions. This substitution often involves replacing the diazonium group with another group, such as a halide, hydroxyl group, or other substituents depending on the desired target molecule. Among these, the Sandmeyer reaction is a staple method for converting benzenediazonium salts into aryl halides.

• Halogen introduction: Converts diazonium into halogen like chloride or bromide.
• Stability: Relatively stable in cold solutions, perfect for in-situ reactions.
• Versatility: Forms different derivatives for diversified chemical applications.

Understanding its nature and reactivity helps chemists design efficient synthesis pathways for complex molecules, demonstrating why benzenediazonium chloride is critical in chemical laboratories.

Chemical Synthesis via the Sandmeyer Reaction

The Sandmeyer reaction is a pivotal chemical synthesis technique in organic chemistry, often utilized to transform aromatic amines into aryl halides through diazonium intermediates. Utilizing copper (I) salts, such as cuprous chloride (CuCl), this reaction provides a reliable route to replace the diazonium group with a chlorine atom, as seen in converting benzenediazonium chloride to chlorobenzene.

This transformation is straightforward, requiring only a few conditions to be met:

• A diazonium salt, such as benzenediazonium chloride, as the starting material.
• An appropriate copper (I) salt to catalyze the halogen substitution.
• Controlled conditions to prevent any side reactions.

The Sandmeyer reaction not only allows for the introduction of chlorine but also can be adapted to incorporate other halogens or cyano groups, enhancing versatility. It is a cornerstone procedure in chemical synthesis for creating a wide array of substituted aromatic compounds.

Due to its reactivity and reliability, the Sandmeyer reaction finds extensive application in pharmaceuticals, dyes, and materials science. Its efficiency in transforming complex aromatic compounds highlights its importance in synthetic organic chemistry.