A variation of the Sandmeyer reaction for preparing chloro- or bromoarenes by reaction of the diazonium compound. In the Gattermann reaction the aromatic amine is added to sodium nitrite and the halogen acid (10°C), then fresh copper powder (e.g. from Zn+CuSO4) is added and the solution warmed. The diazonium salt then forms the haloarene, e.g.C6H5N2+Cl− → C6H5Cl+N2 The copper acts as a catalyst. The reaction is easier to perform than the Sandmeyer reaction and takes place at lower temperature, but generally gives lower yields. It was discovered in 1890 by the German chemist Ludwig Gattermann (1860–1920). See also Gattermann-Koch reaction.
C6H5N2+Cl− → C6H5Cl+N2