12.6 Reactions of Benzene Diazonium Chloride | Nitrogen Compounds | Chemistry 12

Benzene diazonium chloride ( $C_{6} H_{5} N_{2}^{+} C l^{-}$ ) is a highly reactive intermediate prepared by diazotization of aniline below 10°C. Its reactivity arises from the excellent leaving-group ability of the $- N_{2}^{+}$ group, which departs as stable $N_{2}$ gas.

Summary of Reactions

Reaction	Reagents & Conditions	Product
Hydrolysis	Warm water	Phenol + $N_{2}$ + HCl
Sandmeyer (Cl)	CuCl / HCl	Chlorobenzene + $N_{2}$
Sandmeyer (Br)	CuBr / HBr	Bromobenzene + $N_{2}$
Sandmeyer (CN)	CuCN / KCN	Benzonitrile + $N_{2}$
Balz-Schiemann	$H B F_{4}$ , then heat	Fluorobenzene + $N_{2}$ + $B F_{3}$
Reduction	$H_{3} P O_{2}$ or ethanol	Benzene + $N_{2}$
Azo Coupling	Phenol / NaOH(aq), <10°C	Azo compound (dye) + $N_{2}$

1. Hydrolysis — Formation of Phenol

When an aqueous solution of benzene diazonium chloride is warmed, the $- N_{2}^{+}$ group is replaced by $- O H$ :

$C_{6} H_{5} N_{2}^{+} C l^{-} + H_{2} O Warm C_{6} H_{5} O H + N_{2} (g) + H C l$

This is why diazotization must be kept below 10°C — above this temperature the diazonium salt decomposes to phenol before it can be used.

2. Sandmeyer Reaction

The Sandmeyer reaction replaces the diazonium group with a halogen or cyano group using cuprous (Cu⁺) salts as catalysts:

$C_{6} H_{5} N_{2}^{+} C l^{-} + C u C l H C l C_{6} H_{5} C l + N_{2} (g)$

$C_{6} H_{5} N_{2}^{+} C l^{-} + C u B r H B r C_{6} H_{5} B r + N_{2} (g)$

$C_{6} H_{5} N_{2}^{+} C l^{-} + C u C N K C N C_{6} H_{5} C N + N_{2} (g)$

3. Balz-Schiemann Reaction — Formation of Fluorobenzene

Fluorobenzene cannot be made by the Sandmeyer route. Instead:

React benzene diazonium chloride with fluoroboric acid ( $H B F_{4}$ ) to precipitate benzene diazonium fluoroborate ( $C_{6} H_{5} N_{2}^{+} B F_{4}^{-}$ )
Heat the dry salt:

$C_{6} H_{5} N_{2}^{+} B F_{4}^{-} Δ C_{6} H_{5} F + N_{2} (g) + B F_{3} (g)$

4. Reduction — Formation of Benzene

Hypophosphorous acid ( $H_{3} P O_{2}$ ) or ethanol reduces the diazonium salt, replacing $- N_{2}^{+}$ with $- H$ :

$C_{6} H_{5} N_{2}^{+} C l^{-} + H_{3} P O_{2} + H_{2} O \to C_{6} H_{6} + N_{2} (g) + H_{3} P O_{3} + H C l$

5. Azo Coupling — Formation of Azo Compounds ⭐

This is the most important reaction for FBISE. When benzene diazonium chloride reacts with phenol in alkaline solution (NaOH(aq)), an azo compound is formed:

$C_{6} H_{5} N_{2}^{+} C l^{-} + C_{6} H_{5} O H N a O H (a q), < 10° C C_{6} H_{5} - N = N - C_{6} H_{4} - O H + H C l$

The product is (4-hydroxyphenylazo)benzene (also called para-hydroxyazobenzene), an orange-yellow azo dye.

Mechanism

The diazonium ion ( $C_{6} H_{5} N_{2}^{+}$ ) acts as a weak electrophile
It attacks the para position of the phenoxide ion (formed in NaOH) by electrophilic aromatic substitution
The para position is preferred over ortho due to steric hindrance

The Azo Group

The azo group is $- N = N -$ , a nitrogen-nitrogen double bond linking two aromatic rings. The extended conjugated $π$ system absorbs visible light, giving azo compounds their intense colours.

Key fact: Azo compounds are widely used as synthetic dyes in the textile, food, and cosmetics industries. Other azo dyes can be formed by a similar coupling route using different diazonium salts or coupling components (e.g., naphthol instead of phenol).

6. Why N₂ is Always Evolved

In virtually all displacement reactions of diazonium salts, nitrogen gas ( $N_{2}$ ) is released. The $- N_{2}^{+}$ group is an excellent leaving group because $N_{2}$ is a very stable, non-polar molecule with a strong triple bond.

Summary of Reactions

Reaction	Reagents & Conditions	Product
Hydrolysis	Warm water	Phenol + $N_{2}$ + HCl
Sandmeyer (Cl)	CuCl / HCl	Chlorobenzene + $N_{2}$
Sandmeyer (Br)	CuBr / HBr	Bromobenzene + $N_{2}$
Sandmeyer (CN)	CuCN / KCN	Benzonitrile + $N_{2}$
Balz-Schiemann	$H B F_{4}$ , then heat	Fluorobenzene + $N_{2}$ + $B F_{3}$
Reduction	$H_{3} P O_{2}$ or ethanol	Benzene + $N_{2}$
Azo Coupling	Phenol / NaOH(aq), <10°C	Azo compound (dye) + $N_{2}$

1. Hydrolysis — Formation of Phenol

When an aqueous solution of benzene diazonium chloride is warmed, the

- N_{2}^{+}

group is replaced by

- O H

C_{6} H_{5} N_{2}^{+} C l^{-} + H_{2} O Warm C_{6} H_{5} O H + N_{2} (g) + H C l

This is why diazotization must be kept below 10°C — above this temperature the diazonium salt decomposes to phenol before it can be used.

5. Azo Coupling — Formation of Azo Compounds ⭐

This is the most important reaction for FBISE. When benzene diazonium chloride reacts with phenol in alkaline solution (NaOH(aq)), an azo compound is formed:

C_{6} H_{5} N_{2}^{+} C l^{-} + C_{6} H_{5} O H N a O H (a q), < 10° C C_{6} H_{5} - N = N - C_{6} H_{4} - O H + H C l

The product is (4-hydroxyphenylazo)benzene (also called para-hydroxyazobenzene), an orange-yellow azo dye.

The diazonium ion (

C_{6} H_{5} N_{2}^{+}

) acts as a weak electrophile

It attacks the para position of the phenoxide ion (formed in NaOH) by electrophilic aromatic substitution

The para position is preferred over ortho due to steric hindrance

The azo group is

- N = N -

, a nitrogen-nitrogen double bond linking two aromatic rings. The extended conjugated

π

system absorbs visible light, giving azo compounds their intense colours.