Name: Diazonium Salts for Aromatic Synthesis
Uploaded: 2019-07-19
Duration: 5 min 14 s
Description: Benzene diazonium salt is primarily used for the synthesis of substituted aromatic compounds. Are you curious to learn about the synthesis of such compounds?...

Question

How will you synthesise the following: Azobenzene from benzene?

Accepted Answer

A nitro group is introduced into benzene ring when benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture).

$C_{6} H_{6} \to_{H_{2} {SO}_{4}}^{{HNO}_{3}} C_{6} H_{5} - {NO}_{2} .$

When, a nitro group is reduced it is easily converted to amino group. The reduction can be brought about by tin and hydrochloric acid or iron and hydrochloric acid or hydrogen in presence of nickel or platinum or lithium aluminium hydride.

$C_{6} H_{5} - {NO}_{2} + 6 H \overset{Fe + HCl}{\to} C_{6} H_{5} {NH}_{2} + 2 H_{2} O$ .

Aniline is converted into benzene diazonium chloride salt with the help of sodium nitrite and hydrochloric acid kept at a temperature of $0 - 5 ° C$ . This reaction is known as Diazotization reaction.

$C_{6} H_{5} {NH}_{2} + {HNO}_{2} + HCl \overset{0 - 5 ° C}{\to} C_{6} H_{5} {N_{2}}^{+} {Cl}^{-} + 2 H_{2} O$ .

Arene diazonium salts react with electron rich (highly reactive) aromatic compounds such as phenols and amines to form brightly coloured azo compounds, $Ar - N = N - Ar$ . This reaction is known as Coupling reaction. Azo compounds are used as dyes. Coupling takes place almost exclusively at the para position if it is unoccupied. If it is not, then coupling takes place at ortho position. Coupling with phenols occurs in basic medium $(pH 9 - 10)$ whereas, with amines it occurs in faintly acidic medium $(pH 4 - 5)$ at $273 - 278 K$ .

$C_{6} H_{5} OH \to_{{OH}^{-}}^{{ArN}_{2}^{+} {Cl}^{-}} Ar - N = N - C_{6} H_{4} - OH + HCl$ .

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