A reaction of diazonium salts used to prepare chloro- or bromo-substituted aromatic compounds. The method is to diazotize an aromatic amide at low temperature and add an equimolar solution of the halogen acid and copper(I) halide. A complex of the diazonium salt and copper halide forms, which decomposes when the temperature is raised. The copper halide acts as a catalyst in the reaction of the halide ions from the acid, for example C6H5N2+(aq)+Cl−(aq)+CuCl(aq) → C6H5Cl(l)+N2(g)+CuCl(aq) The reaction was discovered in 1884 by the German chemist Traugott Sandmeyer (1854–1922). See also Gattermann reaction.
C6H5N2+(aq)+Cl−(aq)+CuCl(aq) → C6H5Cl(l)+N2(g)+CuCl(aq)