19.28: Diazonium Group Substitution with Halogens and Cyanide: Sandmeyer and Schiemann Reactions
Arenediazonium substitution reactions occur when the diazonium group is substituted by various functional groups such as halides, hydroxyl, nitrile, etc. For instance, arenediazonium salts react with copper(I) salts of chloride, bromide, or cyanide to form corresponding aryl chlorides, bromides, and nitriles. These reactions are named Sandmeyer reactions. Although the mechanism of this reaction is complicated, as illustrated in Figure 1, they are believed to progress via an aryl copper intermediate.
Figure 1. The formation of aryl halides and nitriles via the Sandmeyer reaction
The hydrolysis of aryl nitriles into carboxylic acids is very facile. In this context, the Sandmeyer reaction plays a crucial role in converting aryl amines to substituted benzonitrile, as shown in Figure 2.
Figure 2. The conversion of aryl amines to substituted benzonitrile.
Sandmeyer reactions are not used to prepare aryl fluorides and aryl iodides. For aryl fluorides, the arenediazonium salts react with fluoroboric acid to give diazonium fluoroborate in the form of a precipitated salt. As depicted in Figure 3, these residues are isolated, dried, and heated until they decompose to form the corresponding aryl fluorides. Such reactions are named Schiemann reactions. Similarly, arenediazonium salts react with potassium iodide to form aryl iodides.
Figure 3. The formation of aryl fluoride via the Schiemann reaction.