Coupling Reaction: The most important reaction of diazonium salts is their ability to produce a large number of colored azo dye. This is called coupling reaction. Thus in alkaline medium benzene diazonium chloride couples with phenol at 0°C to form para-hydroxy azobenzene, an orange dye.
In a coupled reaction energy required by 1 process is supplied by another process. For example: glucose + phosphate become glucose.6.phosphate. This is an endergonic reaction and the energy is supplied to this reaction by another reaction which has to be exergonic reaction i.e. ATP which can become ADP + energy.