19.26:
Diazonium Group Substitution with Halogens and Cyanide: Sandmeyer and Schiemann Reactions
Recall that primary arylamines, upon diazotization, yield arenediazonium salts as synthetically useful intermediates. The diazonium group is a good leaving group and can be used to generate substituted benzenes.
For instance, in the Sandmeyer reaction, arenediazonium salts are treated with copper(I) salts of halide or cyanide to form aryl halides and aryl nitriles, respectively.
Aryl nitriles can be further hydrolyzed to yield carboxylic acids.
However, using Sandmeyer reactions, direct installation of fluorine and iodine on an aromatic ring is not possible as fluorine reacts violently while iodine reacts very slowly.
For fluorination, the arenediazonium salts are treated with HBF4 acid to generate aryl fluoride via Schiemann reactions.
For iodination, treatment with KI results in the formation of aryl iodide.
Overall, the general sequence to synthesize substituted benzenes involves nitration of benzene, followed by reduction to yield amino benzenes that can undergo diazotization and finally substitution.
19.26:
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.