German chemist, Ludwig Gattermann
The gattermann reaction can be defined as a method of formylation of aromatic ring compounds. The reaction is also known by the names of Gattermann salicylaldehyde synthesis as well as gattermann formylation. The gattermann reaction is similar to the to the Friedel-Crafts reaction.
Gattermann reaction is used for obtaining chlorobenzene or bromobenzene from benzenediazonium chloride by treating it with Cu/HCl or Cu/HBr respectively.
An example of a gattermann reaction is shown below