12.5 Phenylamine and Azo Compounds | Nitrogen Compounds | Chemistry 12

Phenylamine (Aniline)

Phenylamine ( $C_{6} H_{5} N H_{2}$ ), also called aniline, is the simplest aromatic amine. It consists of an amino ( $- N H_{2}$ ) group attached directly to a benzene ring.

Phenylamine is a much weaker base than aliphatic amines or ammonia. The lone pair on the nitrogen atom is delocalised into the $π$ system of the benzene ring through resonance. This reduces the availability of the lone pair for donation to a proton ( $H^{+}$ ).

$C_{6} H_{5} N H_{2} + H^{+} ⇌ C_{6} H_{5} N H_{3}^{+}$

Order of basicity: Ethylamine > Ammonia > Phenylamine

Diazotization of Phenylamine

Phenylamine can be converted to benzene diazonium chloride ( $C_{6} H_{5} N_{2}^{+} C l^{-}$ ) by a reaction called diazotization.

Reagents and Conditions

Reagents: $N a N O_{2}$ + $H C l$ (which generate nitrous acid, $H N O_{2}$ , in situ)
Temperature: $0-5° C$ (below $10° C$ )

$C_{6} H_{5} N H_{2} + N a N O_{2} + 2 H C l 0 - 5° C C_{6} H_{5} N_{2}^{+} C l^{-} + N a C l + 2 H_{2} O$

⚠️ Why low temperature? Diazonium salts are unstable and decompose rapidly above $10° C$ . Keeping the temperature below $10° C$ prevents decomposition into phenol and allows the diazonium salt to be used in subsequent reactions.

Reactions of Benzene Diazonium Chloride

1. Hydrolysis to Phenol

When an aqueous solution of benzene diazonium chloride is warmed, it hydrolyses to give phenol:

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

The $- N_{2}^{+}$ group leaves as stable $N_{2}$ gas (a good leaving group), and the hydroxyl group from water replaces it.

This is the reaction described in SLO C-12-E-26: phenylamine → diazonium salt → phenol.

Azo Coupling Reaction

Benzene diazonium chloride reacts with phenol in alkaline conditions (NaOH) to form an azo compound. This is called azo coupling.

Reaction with Phenol (SLO C-12-E-27c)

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

The diazonium ion acts as an electrophile.
It attacks the para position of the phenol ring (activated by the $- O H$ group in alkaline conditions).
The product is 4-(phenylazo)phenol, an orange-red azo dye.

Why Are Azo Compounds Coloured?

Azo compounds contain an extended conjugated system: two aromatic rings connected by the $- N = N -$ (azo) group. This large conjugated $π$ system absorbs light in the visible region of the electromagnetic spectrum, giving the compounds their characteristic intense colours (orange, red, or yellow).

Summary Table

Reaction	Reagents/Conditions	Product
Phenylamine → Diazonium salt	$N a N O_{2}$ , $H C l$ , $0-5° C$	$C_{6} H_{5} N_{2}^{+} C l^{-}$
Diazonium salt → Phenol	Warm water	$C_{6} H_{5} O H + N_{2}$
Diazonium salt + Phenol	Alkaline phenol ( $N a O H$ )	Azo dye ( $- N = N -$ )

Phenylamine (Aniline)

Phenylamine ( $C_{6} H_{5} N H_{2}$ ), also called aniline, is the simplest aromatic amine. It consists of an amino ( $- N H_{2}$ ) group attached directly to a benzene ring.

Basicity of Phenylamine

$C_{6} H_{5} N H_{2} + H^{+} ⇌ C_{6} H_{5} N H_{3}^{+}$

Order of basicity: Ethylamine > Ammonia > Phenylamine

Diazotization of Phenylamine

Phenylamine can be converted to benzene diazonium chloride ( $C_{6} H_{5} N_{2}^{+} C l^{-}$ ) by a reaction called diazotization.

Reagents and Conditions

Reagents: $N a N O_{2}$ + $H C l$ (which generate nitrous acid, $H N O_{2}$ , in situ)
Temperature: $0-5° C$ (below $10° C$ )

$C_{6} H_{5} N H_{2} + N a N O_{2} + 2 H C l 0 - 5° C C_{6} H_{5} N_{2}^{+} C l^{-} + N a C l + 2 H_{2} O$

Reactions of Benzene Diazonium Chloride

1. Hydrolysis to Phenol

When an aqueous solution of benzene diazonium chloride is warmed, it hydrolyses to give phenol:

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

The $- N_{2}^{+}$ group leaves as stable $N_{2}$ gas (a good leaving group), and the hydroxyl group from water replaces it.

This is the reaction described in SLO C-12-E-26: phenylamine → diazonium salt → phenol.

Azo Coupling Reaction

Benzene diazonium chloride reacts with phenol in alkaline conditions (NaOH) to form an azo compound. This is called azo coupling.

Reaction with Phenol (SLO C-12-E-27c)

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

The diazonium ion acts as an electrophile.
It attacks the para position of the phenol ring (activated by the $- O H$ group in alkaline conditions).
The product is 4-(phenylazo)phenol, an orange-red azo dye.

Why Are Azo Compounds Coloured?

Summary Table

Reaction	Reagents/Conditions	Product
Phenylamine → Diazonium salt	$N a N O_{2}$ , $H C l$ , $0-5° C$	$C_{6} H_{5} N_{2}^{+} C l^{-}$
Diazonium salt → Phenol	Warm water	$C_{6} H_{5} O H + N_{2}$
Diazonium salt + Phenol	Alkaline phenol ( $N a O H$ )	Azo dye ( $- N = N -$ )