Method of Preparation of Diazonium Salts

NCERT Reference: Chapter 9 – Amines – Page 250–251

Quick Notes

Diazonium salts have the general formula ArN₂⁺X⁻ (Ar = aryl, X⁻ = Cl⁻, Br⁻, etc.).
Prepared by treating aromatic 1° amines with NaNO₂ + HCl at 273–278 K.
HNO₂ is generated in situ from NaNO₂ + HCl.
Benzenediazonium chloride is the most common example.
Salts are stable only at low temperatures and readily decompose to release N₂.

Full Notes

What Are Diazonium Salts?

Diazonium salts are a class of organic compounds with the general formula R–N₂⁺ X⁻, where R is an aryl group and X⁻ is an anion such as Cl⁻, Br⁻, HSO₄⁻, or BF₄⁻.

The positively charged N₂⁺ group is called the diazonium group. These compounds are named by adding the suffix diazonium to the name of the parent hydrocarbon from which they are derived, followed by the name of the counter ion.

For Example: Benzenediazonium chloride

C₆H₅N₂⁺Cl⁻ is called benzenediazonium chloride.

NCERT 12 Chemistry diagram of benzenediazonium chloride showing the C6H5–N2+ Cl− structure.

Primary aliphatic amines form unstable alkyl diazonium salts, which rapidly decompose.

In contrast, primary aromatic amines form arenediazonium salts, which are relatively stable at low temperatures (273–278 K) and are widely used in synthetic organic chemistry.

The relative stability of arenediazonium ions is due to resonance.

NCERT 12 Chemistry resonance structures stabilising the arenediazonium ion with positive charge delocalised onto the ring.

Preparation – Diazotisation Reaction

Diazonium salts are prepared by the diazotisation of primary aromatic amines using nitrous acid (HNO₂), which is generated in situ using NaNO₂ and HCl.

NCERT 12 Chemistry reaction scheme: aniline reacts with NaNO2 and 2HCl at 273–278 K to form benzenediazonium chloride with NaCl and water.

The temperature must be strictly maintained between 273–278 K (0–5 °C)
The reaction must be carried out in cold conditions because diazonium salts are thermally unstable and decompose readily above this range

Key Features of the Reaction

Substrate: Primary aromatic amine (e.g., aniline)
Reagents: Sodium nitrite (NaNO₂) and dilute hydrochloric acid (HCl)
Conditions: Low temperature (0–5 °C or 273–278 K) and freshly prepared nitrous acid (in situ)

Stability and Usage

Diazonium salts are stable for short periods at low temperatures.
They serve as intermediates for preparing:
- Aryl halides
- Phenols
- Nitriles
- Azo dyes

Limitations

Only aromatic primary amines form stable diazonium salts.
Aliphatic diazonium salts are unstable and decompose rapidly.

Summary

Diazonium salts ArN₂⁺X⁻ are made by diazotisation of aromatic 1° amines with NaNO₂ and HCl at 273–278 K.
Resonance stabilises arenediazonium ions and allows their synthetic use at low temperature.
They are key intermediates for aryl halides, phenols, nitriles and azo dyes.
Aliphatic diazonium salts are unstable and decompose readily.