Introduction:

Diazonium salts form an important category of organic compounds that are widely used in organic synthesis, especially in the manufacture of azo dyes, pharmaceuticals, and aromatic derivatives. These compounds are prepared primarily from aromatic amines and are characterized by the presence of the functional group –N₂⁺X⁻. In Class 12 Chemistry, the topic is covered under the chapter “Amines”, and it often carries significant weightage in CBSE Board exams and entrance exams like NEET and JEE.

What are Diazonium Salts?

Diazonium salts are organic compounds that contain a diazonium group (–N₂⁺) attached to an aryl group. The general formula is ArN₂⁺X⁻, where:

- Ar = Aromatic group (usually derived from benzene)

- X⁻ = Anion like Cl⁻, Br⁻, NO₃⁻, HSO₄⁻, etc.

Example: Benzenediazonium chloride (C₆H₅N₂⁺Cl⁻)

General Method of Preparation of Diazonium Salts

Diazonium salts are prepared by a chemical reaction known as diazotization, which involves the reaction of primary aromatic amines with nitrous acid (HNO₂) in an acidic medium under cold conditions (0–5°C).

General Reaction:

ArNH₂ + HNO₂ + HX → ArN₂⁺X⁻ + 2H₂O

Example using Aniline:

C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O

Laboratory Preparation of Diazonium Salts:

In the lab, nitrous acid is prepared in situ:

NaNO₂ + HCl → HNO₂ + NaCl

Step-by-Step Procedure:

1. Prepare a cold solution of aniline in dilute HCl.

2. Cool to 0–5°C.

3. Add NaNO₂ solution slowly while stirring.

Conditions Required for Diazotization Reaction:

Temperature: 0°C to 5°C. Higher temperatures cause decomposition.

Acidic Medium: Usually dilute HCl or H₂SO₄.

Type of Amine: Only primary aromatic amines.

Mechanism of Diazotization Reaction:

Step 1: NaNO₂ + HCl → HNO₂ + NaCl

Step 2: HNO₂ + H⁺ → NO⁺ + H₂O

Step 3: ArNH₂ + NO⁺ → ArN₂⁺ + H₂O

Applications of Diazonium Salts:

1. Azo Dye Formation

2. Sandmeyer Reaction

3. Gattermann Reaction

4. Phenol Formation

5. Synthesis of Aromatic Compounds

6. Medicinal Chemistry Intermediates

Conclusion:

The preparation of diazonium salts via diazotization is a cornerstone topic in Class 12 Organic Chemistry. By understanding the conditions, mechanism, and practical importance of this reaction, students can master one of the most high-scoring sections of their board and entrance exams.