18.20:
Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN1
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.
18.20:
Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN1
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.