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

Describe the synthetic applications of aryl diazonium salts.

Accepted Answer

When benzene diazonium salt solution is treated with hypophosphorus acid, the diazo group is replaced by hydrogen and benzene is formed. The reaction is carried out simply by dissolving the amine in hypophosphorus acid and cold ${NaNO}_{2}$ solution is added. The reaction takes place immediately. The net result is removal of ${NH}_{2}$ group from the aromatic ring and hence the process is known as de-amination. Nitro group can also be removed after converting it to $- {NH}_{2}$ group.

$C_{6} H_{5} N_{2} Cl + H_{3} {PO}_{2} + H_{2} O \to C_{6} H_{6} + H_{3} {PO}_{3} + HCl + N_{2}$ .

The nitro compounds are generally prepared by treating the arenediazonium fluoroborate with an aqueous solution of sodium nitrite in presence of copper powder. The reaction is given below,

$C_{6} H_{5} N_{2} Cl \to_{(ii) {NaNO}_{2} / Cu, ∆}^{(i) {HBF}_{4}} C_{6} H_{5} - {NO}_{2} + N_{2} + {NaBF}_{4}$ .

When, benzene diazonium salt solution is boiled with water in presence of dilute sulphuric acid, it undergoes hydrolysis and the diazo group is replaced by hydroxyl group. The phenol is the product. The reaction is given as below,

$C_{6} H_{5} N_{2} Cl + H_{2} O \overset{∆ / [H^{+}]}{\to} C_{6} H_{5} OH + N_{2} + HCl$ .

It is known as Sandmeyer's reaction. When, benzenediazonium chloride solution is added to a solution of cuprous chloride dissolved in $HCl$ or cuprous bromide dissolved in $HBr$ , the diazo group is replaced by chlorine or bromine respectively resulting in the formation of chlorobenzene or bromobenzene.

$C_{6} H_{5} N_{2} Cl \to_{HCl}^{CuCl} C_{6} H_{5} Cl + N_{2}$ .

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 $273 - 278 K$ .

$C_{6} H_{5} N_{2} Cl + {ArNH}_{2} \to_{pH 4 - 5}^{ice cold} C_{6} H_{5} - N = N - C_{6} H_{4} - {NH}_{2} + HCl . C_{6} H_{5} OH \to_{{OH}^{-}}^{{ArN}_{2}^{+} {Cl}^{-}} Ar - N = N - C_{6} H_{4} - OH + HCl .$

Describe the synthetic applications of aryl diazonium salts.

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