Importance of Diazonium Salts in the Synthesis of Aromatic Compounds

NCERT Reference: Chapter 9 – Amines – Page 253

Quick Notes

Diazonium salts are key intermediates for synthesising aromatic compounds.
They allow the introduction of substituents like –F, –Cl, –Br, –I, –CN, –OH, and –H.

These substitutions are often not possible via direct electrophilic substitution.

Can also be used in coupling reactions to form azo compounds (Ar–N=N–Ar′).

Full Notes

Diazonium salts are widely used in organic synthesis because of their ability to undergo substitution and coupling reactions under mild conditions.

The diazonium group (–N₂⁺) acts as an excellent leaving group, making the aromatic ring susceptible to transformation.

Reactions Involving Displacement of Nitrogen

Diazonium salts can be converted into a wide range of functionalised aromatic compounds by replacing the diazonium group (–N₂⁺) with various nucleophiles.

Halogenation:
- Chlorination and Bromination using CuCl and CuBr respectively (Sandmeyer reaction).
- Iodination by simple treatment with KI.
- Fluorination by treating with HBF₄ to form diazonium fluoroborate, followed by heating.
Hydroxylation:
- Warming an aqueous solution of the diazonium salt forms phenol.
Cyanation:
- Treatment with CuCN leads to the formation of benzonitrile.
Reduction:
- Reaction with hypophosphorous acid (H₃PO₂) or ethanol reduces the diazonium salt to benzene.
Nitration:
- Benzenediazonium fluoroborate reacts with sodium nitrite in the presence of Cu to form nitrobenzene.

These transformations allow for the synthesis of substituted benzene derivatives which cannot be prepared by direct substitution methods.

Reactions Involving Retention of Diazo Group (Coupling Reactions)

Diazonium salts can undergo coupling reactions with aromatic compounds having electron-donating groups (like phenols and arylamines) to form brightly coloured azo compounds.

These reactions take place at the para position to the –OH or –NH₂ group if it is free.
Products are intensely coloured dyes, useful in textile and dye industries.

Examples:

Coupling with phenol gives p-hydroxyazobenzene.

Coupling with aniline gives p-aminoazobenzene.

Summary

Diazonium salts enable mild and versatile substitution on aromatic rings.
They give access to halides hydroxyl nitrile nitro and hydrogen derivatives.
Coupling reactions retain the diazo group to form azo dyes used widely in industry.