The diazonium salts have the general formula ArN2+X-, where X- may be an anion like Cl-, Br- etc.
The group N2+ (-N≡N+) is called diazonium ion group.
Sandmeyer reaction is a substitution reaction.
In the substistution reactions, nitrogen of the salts is lost as N2 and different groups are introduced in its place.
Various substitution reactions of diazonium salts
1. Replacement by -OH group. By boiling or by steam boiling. Phenol is formed
2. Replacement by hydrogen. By treatment with hypophophorous acid. Benzene is obtained.
3. Replacement by Cl and Br group.When the salt is warmed with cuprous chloride or cuprous bromide the corresponding halide is formed. This reaction is called Sandmeyer reaction