Ullmann reaction is coupling organic chemistry reaction which forms biaryl by coupling of two molecules of aryl halide in the presence of copper metal and elevated temperature (200°C). The reaction was discovered by Fritz Ullmann.

Mechanism of this organic reaction is not clear exactly. There are two proposed mechanisms first is the radical mechanism which involves single electron transfer to aryl radical. The aryl radicals they finally react to form biaryl. Another proposed mechanism involves oxidative addition which leads to the formation of organocuprate reagent by the reaction of copper with aryl halide involving single electron transfer. Then there is an occurrence of one more oxidative addition performed by organocuprate on aryl halide and finally, there is the formation of biaryl by reductive elimination.