Synthetic Transformations of Aryl Diazonium Salts

An important reaction of aryl diazonium ions is their conversion to phenols by hydrolysis  

This is the most general method for preparing phenols. It is easily performed the aqueous acidic solution in which the diazonium salt is prepared is heated and gives the phenol directly. An aryl cation is probably generated, which is then captured by water acting as a nucleophile. 

Sulfuric acid is normally used instead of hydrochloric acid in the diazotization step so as to minimize the competition with water for capture of the cationic intermediate. Hydrogen sulfate anion (HS04 ) is less nucleophilic than chloride. 

The reaction of an aryl diazonium salt with potassium iodide is the standard method for the preparation of aryl iodides. The diazonium salt is prepared from a primary aromatic amine in the usual way, a solution of potassium iodide is then added, and the reaction mixture is brought to room temperature or heated to accelerate the reaction. 

PROBLEM 22.18 Show by a series of equations how you could prepare m-bromoiodobenzene from benzene. J 

Show how you could prepare m-bromoiodobenzene from benzene. 

---

Replacement of diazonium groups by halide is a very valuable synthetic alternative to direct halogenation for preparation of aryl halides. There are three broad types of procedures decomposition of aryl diazonium-halide adducts with expulsion of nitrogen, copper-mediated redox processes, and thermal processes proceeding via aryl radicals. The first type of process is probably involved in the reaction of aryl diazonium salts with iodide ion. Smooth high-yield transformation takes place in the absence of any metal catalyst. The mechanism of the reaction... 

---