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 can benzene diazonium chloride be prepared from nitrobenzene? Starting from a diazonium salt how can you prepare benzoic acid?

Accepted Answer

Benzenediazonium chloride can be formed by first mixing benzene with nitric acid in the presence of sulphuric acid, which forms nitrobenzene. Nitrobenzene can be transformed into aniline. Aniline is dissolved in dilute hydrochloric acid and the solution is cooled to $0 ° - 5 ° C$ . Aqueous solution of sodium nitrite is added to the cooled solution in small portions with stirring. The temperature is maintained at $0 ° - 5 ° C$ . Addition of sodium nitrite solution is discontinued when a drop of the solution turns starch-iodide paper blue indicating the presence of slight excess of nitrous acid. Benzene diazonium chloride is formed in the solution.

$C_{6} H_{6} \to_{H_{2} {SO}_{4}}^{{HNO}_{3}} C_{6} H_{5} {NO}_{2} \to_{Raney Ni / Pt / Pd}^{3 H_{2}} C_{6} H_{5} {NH}_{2} + 2 H_{2} O . C_{6} H_{5} - {NH}_{2} \to_{HCl, 0 - 5 ° C}^{{NaNO}_{2}} C_{6} H_{5} {N_{2}}^{+} {Cl}^{-} + 2 H_{2} O .$

This reaction between a primary arylamine and nitrous acid in presence of a mineral acid to produce aryldiazonium salt is called diazotisation. Nitrous acid is unstable and it is always generated in situ from ${NaNO}_{2}$ and mineral acid like $HCl or H_{2} {SO}_{4}$ . The reaction is carried out at a low temperature $(0 ° - 5 ° C)$ , because both nitrous acid and diazonium salt would decompose at higher temperature.

When diozonium salt solution is treated with cuprous cyanide dissolved in aqueous potassium cyanide, the diazo group is replaced by $- CN$ group and aryl cyanide is formed. This is also called Sandmeyer's reaction. Benzene diazonium chloride with cuprous cyanide in aqueous $KCN$ yields phenyl cyanide or cyanobenzene. This cyanobenzenes can undergo acid hydrolysis produces benzoic acid.

$C_{6} H_{5} {N_{2}}^{+} {Cl}^{-} \overset{CuCN / KCN}{\to} C_{6} H_{5} CN + N_{2} . C_{6} H_{5} CN + 2 H_{2} O \overset{H^{+}}{\to} C_{6} H_{5} COOH + {NH}_{3} .$

How can benzene diazonium chloride be prepared from nitrobenzene? Starting from a diazonium salt how can you prepare benzoic acid?

Important Questions on Aryl Diazonium Salts

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