Sandmeyer's reaction

The method most commonly used to prepare aryl halides in practice is not direct electrophilic halogenation of the ring (section 10.3.4) but Sandmeyer's reaction, which starts from an aromatic diazonium salt (Ar-N2+) rather than from benzene itself. In Sandmeyer's reaction the -N2+ group of the diazonium salt is replaced by a halogen atom (typically Cl or Br, delivered via a copper(I) halide catalyst) or by other groups such as -CN, giving the corresponding aryl halide and releasing nitrogen gas. This route is valuable because the position of the diazonium group on the ring (and hence of the halogen in the product) is fixed by whatever aromatic substitution reaction was used earlier to build the diazonium salt's precursor amine, giving much more reliable control over substitution pattern than direct ring halogenation does. The full method, including the diazotisation step that produces the diazonium salt in the first place and the range of gro …