Problem 34 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Aniline and nitrous acid react to produce (a) nitrobenzene (b) benzene diazonium salt (c) phenol (d) chlorobenzene

Step-by-Step Solution

Answer

The reaction forms benzene diazonium salt (option b).

Step 1: Identify Reactants

The reactants in this chemical reaction are aniline (C6H5NH2) and nitrous acid (HNO2).

Step 2: Determine Reaction Conditions

The reaction typically takes place in an acidic medium, usually in the presence of hydrochloric acid (HCl) and very low temperatures, often around 0-5°C.

Step 3: Recognize Reaction Type

Aniline reacts with nitrous acid under these conditions to form a diazonium salt. This is a classic example of diazotization.

Step 4: Formation of Diazonium Salt

In this reaction, aniline (C6H5NH2) reacts with nitrous acid (HNO2) in the presence of HCl to form benzene diazonium chloride (C6H5N2+Cl−).

Step 5: Choose the Correct Answer

After recognizing the product of the reaction, identify 'benzene diazonium salt' as the correct option.

Key Concepts

Aniline

Aniline is an aromatic amine with the chemical formula \( C_6H_5NH_2 \). It is a derivative of benzene, where one hydrogen atom is replaced by an amino group, -NH\(_2\). This compound is notable for its role in synthesizing dyes, and it is a key starting material in various chemical reactions.
Aniline's aromatic structure contributes significantly to its reactivity. The nitrogen atom in the amino group has a lone pair of electrons which can interact with the aromatic ring, leading to resonance stabilization.
Aniline's reactivity is impacted by different factors, such as acidity and temperature. Due to its basic nature, aniline can react with acids to form anilinium ions. This reaction plays a critical role in processes like diazotization where aniline is transformed into a diazonium compound. Overall, understanding aniline's structure and properties is essential for mastering its role in chemical reactions.

Benzene Diazonium Salt

When aniline undergoes diazotization, it is converted into a benzene diazonium salt. This compound is characterized by having a diazonium group, typically written as \( -N_2^+Cl^- \), replacing the amino group of aniline.
These salts are highly reactive intermediates in organic synthesis. They are used extensively in the preparation of azo dyes and as building blocks in various organic transformations.
The benzene diazonium salt is notably unstable at higher temperatures. This instability can be advantageous, as the diazonium group can be easily displaced by other nucleophiles, allowing for the synthesis of diverse aromatic compounds. The conversion of aniline to benzene diazonium salt is a fundamental reaction in organic chemistry pivotal for producing various derivatives of benzene.

Reaction Conditions

The conversion of aniline to benzene diazonium chloride is sensitive to the reaction conditions. The process known as diazotization typically requires:

• Acidic Medium: Aniline needs an acidic environment to form the intermediate anilinium ion. Commonly, hydrochloric acid (HCl) serves this purpose.
• Temperature Control: The reaction is conducted at low temperatures, roughly between 0-5°C. This prevents the diazonium salt from decomposing, which maintains its stability long enough for further reactions.
• Source of Nitrous Acid: Nitrous acid (HNO\(_2\)) can be generated in situ from sodium nitrite and a strong acid like HCl.

These stringent conditions are essential to ensure the success of diazotization. They stabilize the benzene diazonium salt and allow precise control over its formation and subsequent reactions, making them crucial for various synthetic applications in organic chemistry.