What is Diazonium chemistry?

A diazonium ion is a cation $\ce{R-N+ #N}$. Diazotisation converts a primary aromatic amine into an aryl diazonium salt with sodium nitrite and a strong acid at 0–$5{}^{\circ}\mathrm{C}$. In a Sandmeyer reaction, a copper(I) salt ($\ce{CuCl}$, $\ce{CuBr}$, $\ce{CuCN}$) replaces the diazonium group by Cl, Br or CN with loss of dinitrogen. In an azo coupling, the diazonium ion, a weak electrophile, attacks an activated aromatic ring to give an azo compound $\mathrm{Ar{-}N{=}N{-}Ar'}$.