Copper chloride with diazonium ions

In aqueous solution, instead of undergoing reaction (c), the aryl radicals can dimerize to give biaryls (d) or react with diazonium ions to give azo compounds (e), copper(i) chloride... 

Alternatively in aromatic compounds, the chlorine can be introduced by reacting the diazonium ion with copper(I) chloride ... 

The formation of aryl chlorides from arenediazonium ions on reaction with copperfi) chloride is known as the Sandmeyer reaction and also involves the intermediacy of aryl radicals [192]. Copper salts are similarly involved in the reactions of diazonium ions with cyanide, nitrite, and sulfur dioxide. The uses of arenediazonium salts in synthetic reactions forming carbon-carbon, carbon-sulfur, and carbon-boron bonds have been summarized recently [193]. 

In the Sandmeyer reactions involving copper, electron-transfer-mediated radical reactions are certainly involved. In HCI, cuprous chloride (CuCl) is in equifib-rium with the dichlorocuprate ion CuCl2 (Frg. 14.56).The dichlorocuprate ion can transfer an electron to the diazonium ion, which can then lose molecular nitrogen to give a phenyl radical. In turn, the phenyl radical can pluck a chlorine atom from a copper chloride molecule to give the chlorobenzene and a molecule of cuprous chloride. The mechanism for the formation of chlorobenzene in the Sandmeyer reaction is shown in Figure 14.57. 

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