A diazonium salt will couple with an aliphatic compound containing an activated carbon-hydrogen bond. This discussion is limited to those reactions in which both the nitrogen atoms of the diazonium salts are retained in the resulting molecule. The discussion is further limited by the exclusion of coupling reactions, which occur with the elimination of a group from an activated methinyl compound, the Japp-Klingemann reaction. These reactions are discussed in Chapter 2. Much of the early work with coupling was prompted by the desire to determine whether the products were of the azo or hydrazonestructure. It is difficult to establish the structures in such cases where the tautomeric forms are possible. It is generally considered that the hydrazone is the stable form.