18.20 : Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic S_N1

Aryldiazonium salts—prepared from an arylamine and nitrous acid—can undergo substitution reactions involving the displacement of the diazonio group with a nucleophile.

The easy elimination of the nitrogen molecule drives the substitution reaction.

Aryldiazonium salts are versatile intermediates as the diazonio group can be replaced by different nucleophiles to give a wider variety of substituted benzenes.

The Sandmeyer reaction, for example, substitutes the diazonio group with the chloro, bromo, or the cyano group through copper(I) catalysis.

The fluoro group is introduced through the Schiemann reaction which uses hot fluoroboric acid. This is useful considering the high reactivity of fluorine which inhibits direct fluorination of benzene.

Reacting the diazonium salt with potassium iodide introduces the iodo substituent. This is preferred over the rather slow direct iodination method.

Hydrolysis of the diazonium salt introduces the OH group to form phenol.