Question

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

Hint:

Unlike chlorination and bromination of diazonium salts, preparation of iodobenzene does not require CuCl or CuBr catalyst. Potassium iodide alone is sufficient.

Answer 1

Concept: Aryl iodides are difficult to prepare directly from benzene because electrophilic iodination is reversible and generally gives poor yields. Benzene diazonium chloride provides a convenient route for the synthesis of iodobenzene. Among all halogen substitution reactions of diazonium salts, the preparation of iodobenzene is particularly simple because no copper catalyst is required. Potassium iodide itself acts as the source of iodide ions that replace the diazonium group.

Step 1: Treatment with potassium iodide. Benzene diazonium chloride is treated with an aqueous solution of potassium iodide. The iodide ion acts as a nucleophile and replaces the diazonium group. \[ C_6H_5N_2^+Cl^- + KI \rightarrow C_6H_5I + KCl + N_2\uparrow \]

Step 2: Evolution of nitrogen gas. During the reaction, molecular nitrogen is liberated. \[ N_2 \uparrow \] Since nitrogen gas is extremely stable, its evolution drives the reaction towards completion.

Step 3: Formation of iodobenzene. The diazonium group attached to the aromatic ring is replaced by iodine. Thus, the final product formed is iodobenzene. \[ \boxed{C_6H_5I} \]

Importance of the reaction. This reaction is extremely useful because aryl iodides are valuable intermediates in organic synthesis. The diazonium salt route provides a high-yield method for their preparation.

Conclusion: Iodobenzene is obtained from benzene diazonium chloride by treatment with potassium iodide solution. \[ \boxed{ C_6H_5N_2^+Cl^- + KI \rightarrow C_6H_5I + KCl + N_2 } \]