Question

How will you obtain the following from benzene diazonium chloride ? Write chemical equations involved : (a) Phenol

Hint:

Hydrolysis of benzene diazonium chloride is one of the most important laboratory methods for preparing phenol because the diazonium group is readily replaced by a hydroxyl group with evolution of nitrogen gas.

Answer 1

Concept: Benzene diazonium chloride \((C_6H_5N_2^+Cl^-)\) is one of the most important aromatic diazonium salts. It is generally prepared by the diazotisation of aniline using sodium nitrite and hydrochloric acid at a temperature of \(273-278\,K\). The diazonium group \(( -N_2^+ )\) is an excellent leaving group because molecular nitrogen \((N_2)\) is highly stable. Therefore, benzene diazonium chloride undergoes a variety of substitution reactions in which the diazonium group is replaced by other functional groups such as \(-OH\), \(-Cl\), \(-Br\), \(-I\), \(-CN\), etc. Phenol cannot be prepared directly from benzene by simple substitution reactions because the hydroxyl group cannot be introduced easily into the benzene ring. Hence, conversion of benzene diazonium chloride into phenol serves as an important synthetic method for the preparation of phenol.

Step 1: Hydrolysis of benzene diazonium chloride. When benzene diazonium chloride is warmed with water or dilute mineral acid, hydrolysis takes place. The diazonium group is replaced by a hydroxyl group and nitrogen gas is evolved. \[ C_6H_5N_2^+Cl^- + H_2O \xrightarrow{\Delta} C_6H_5OH + N_2\uparrow + HCl \]

Step 2: Reason for the reaction. The reaction occurs because the diazonium ion is unstable on heating in aqueous medium. The highly stable nitrogen molecule is expelled from the aromatic ring, creating conditions for substitution by the hydroxyl group present in water. The liberation of nitrogen gas acts as a driving force for the reaction, making the conversion highly favourable.

Step 3: Product obtained. The product formed is phenol: \[ \boxed{C_6H_5OH} \] which contains a hydroxyl group directly attached to the benzene ring.

Conclusion: Thus, phenol is obtained from benzene diazonium chloride by hydrolysis with water on warming. \[ \boxed{ C_6H_5N_2^+Cl^- + H_2O \xrightarrow{\Delta} C_6H_5OH + N_2 + HCl } \]