Gattermann Reaction: If benzene diazonium salt is heated with Cu and halogen acids, diazo group is substituted by halogen. This reaction is known as Gattermann reaction. It is named for the German chemist Ludwig Gattermann and is similar to the Friedel-Crafts reaction.
This reaction is useful for the preparation of aromatic halides and cyanides. Chlorine or bromine can be introduced in the benzene ring by treating the benzene diazonium salt solution with corresponding halogen acid in the presence of copper powder.