Diazonium Salts (Aromatic Amines)

What are Diazonium Salts?

Diazonium salts are organic compounds containing the functional group –N₂⁺. They are typically formed by the reaction of primary aromatic amines with nitrous acid (HNO₂).

General Reaction (Diazotization)

Ar-NH₂ + HNO₂ + HCl → Ar-N₂⁺Cl⁻ + 2H₂O

Note: Nitrous acid is generated in situ using sodium nitrite (NaNO₂) and HCl. The reaction must be carried out below 5°C.

Key Properties

Colorless crystalline solids.
Stable only below 5°C.
Highly reactive due to N₂ being an excellent leaving group.
Soluble in water and used immediately after preparation.

Reactions of Diazonium Salts

1. Substitution Reactions

Sandmeyer Reaction: Ar-N₂⁺Cl⁻ + CuCl → Ar-Cl
Gattermann Reaction: Ar-N₂⁺Cl⁻ + Cu/HCl → Ar-Cl
Hydrolysis: Ar-N₂⁺Cl⁻ + H₂O → Ar-OH + N₂↑ + HCl

2. Coupling Reactions

Diazonium salts react with phenols or aromatic amines to form brightly colored azo dyes.

Ar-N₂⁺ + Ar′-OH (or Ar′-NH₂) → Ar-N=N-Ar′ (Azo dye)

Applications

Used in synthesis of azo dyes (textiles, leather, inks).
Preparation of halogenated aromatics via Sandmeyer/Gattermann reactions.
Precursors for phenols and aromatic ethers.
Applications in pharmaceuticals 💊, agrochemicals 🌾, organic semiconductors.

Practice & Quiz

Multiple Choice Questions (MCQ)

Diazonium salts are stable at:
- a) 25°C
- b) 0–5°C ✅
- c) 50°C
- d) 100°C
The Sandmeyer reaction produces:
- a) Azo dye
- b) Phenol
- c) Halogenated aromatic ✅
- d) Carboxylic acid

Fill in the Blanks

1. Diazonium salts contain the functional group __________.

2. Coupling of diazonium salts with phenols forms __________ dyes.

True / False

1. Diazonium salts are stable at room temperature. (❌ False)

2. Nitrous acid is generated in situ using NaNO₂ and HCl. (✅ True) 🔍 Precision tools in aromatic chemistry — Diazonium salts are short-lived, but powerful in synthesis.