Treating arylamines with nitrous acid gives aryldiazonium salts that are effective substrates in nucleophilic aromatic substitution reactions. The diazonio group in these salts can be easily displaced by different nucleophiles, yielding a wide variety of substituted benzenes. The leaving group departs as nitrogen gas, and this easy elimination is the driving force for the substitution reaction.
In the Sandmeyer reaction, for example, the diazonio group is replaced by a chloro, bromo, or cyano group. The reagents used are the cuprous salts of these nucleophiles. In the Schiemann reaction, the diazonio group is substituted by a fluoro group under hot acidic conditions. This reaction is useful because the high reactivity of fluorine inhibits the direct fluorination of benzene. Iodobenzene is formed by the reaction of sodium or potassium iodide with the aryldiazonium salt. Hydrolysis of diazonium salt produces phenol.
