19.26 : 重氮基团被卤素和氰化物取代：桑德迈尔反应和席曼反应

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.