Chemical Reactions of Diazonium Salts

Full Notes

Diazonium salts are highly reactive due to the excellent leaving ability of the N₂ group. These reactions are central to synthetic organic chemistry and are broadly classified into:

• Reactions that involve replacement of the diazonium group (N₂ is lost)
• Reactions that involve retention of the diazo group (N₂ is preserved)

Reactions Involving Displacement of Nitrogen

These reactions result in the replacement of the diazonium group (–N₂⁺) with other substituents. They are key steps in aromatic substitution reactions.

Sandmeyer Reaction

Replacement by Cl⁻, Br⁻, or CN⁻ using CuCl, CuBr, or CuCN.

Example: Formation of chlorobenzene and benzonitrile

C₆H₅N₂⁺Cl⁻ + CuCl → C₆H₅Cl + N₂↑

C₆H₅N₂⁺Cl⁻ + CuCN → C₆H₅CN + N₂↑

Gattermann Reaction

Similar to Sandmeyer but uses copper powder and corresponding acid (e.g., HCl or HBr).

Note - product yields in the Sandmeyer reaction are generally higher than those from the Gattermann reaction.

Iodination

When a diazonium salt solution is treated with potassium iodide, iodobenzene is formed.

No catalyst needed. KI alone suffices.

Example: Benzenediazonium to iodobenzene

C₆H₅N₂⁺Cl⁻ + KI → C₆H₅I + N₂↑

Fluorination (Balz–Schiemann Reaction)

Uses fluoroboric acid (HBF₄) to form a stable fluoroborate salt.

• Steps: C₆H₅N₂⁺Cl⁻ + HBF₄ → C₆H₅N₂⁺BF₄⁻
• On heating: C₆H₅N₂⁺BF₄⁻ → C₆H₅F + N₂ + BF₃

Replacement by Hydrogen (H)

Mild reducing agents like hypophosphorous acid (H₃PO₂) or ethanol reduce diazonium salts to arene. These agents themselves get oxidised.

ArN₂⁺Cl⁻ + H₃PO₂ + H₂O → ArH + N₂ + H₃PO₃ + HCl

Example: Reduction by H₃PO₂ and by ethanol

ArN₂⁺Cl⁻ + CH₃CH₂OH → ArH + N₂ + CH₃CHO + HCl

Hydrolysis

On heating diazonium salt solution to ~283 K, the salt undergoes hydrolysis to form phenol.

Reaction: ArN₂⁺Cl⁻ + H₂O → ArOH + N₂ + HCl

Replacement by –NO₂ Group

When diazonium fluoroborate is heated with aqueous sodium nitrite (NaNO₂) in the presence of copper (Cu), the diazonium group is replaced by a nitro group (–NO₂).

Reaction: ArN₂⁺BF₄⁻ + NaNO₂ (aq), Cu, heat → ArNO₂ + N₂ + NaBF₄

Reactions Involving Retention of Diazo Group (Coupling Reactions)

These reactions form azo compounds via coupling of diazonium salts with activated aromatic compounds like phenols and amines. Azo compounds are often used as dyes.

Coupling with Phenols

• Occurs at para position (or ortho if para is blocked).
• Forms yellow/orange azo dyes.

Example: Formation of an azo dye

C₆H₅N₂⁺Cl⁻ + C₆H₅OH → C₆H₅–N=N–C₆H₄OH (Azo dye)

Coupling with Aromatic Amines

• Reacts at the para position of primary aromatic amines.
• Forms brightly coloured azo dyes.

Example: Diazonium coupling with aniline

C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ → C₆H₅–N=N–C₆H₄NH₂

Summary

• Diazonium salts undergo displacement to give halides, phenols, nitriles, hydrogen and nitro derivatives.
• Sandmeyer uses Cu⁺ salts while Gattermann uses copper powder with acid.
• Balz–Schiemann via BF₄⁻ gives aryl fluorides on heating.
• Reductions with H₃PO₂ or ethanol yield arenes with nitrogen loss.
• Hydrolysis forms phenols near 283 K.
• Coupling with phenols or amines forms coloured azo dyes at para or ortho positions.