Reactions of Diazonium Salts Involving Retention of Nitrogen: Coupling Reactions

Instead of losing nitrogen, a diazonium salt can instead RETAIN it, if it reacts

with a sufficiently electron-rich aromatic partner -- a phenol or an aromatic amine --

under mild, typically alkaline (for phenols) or neutral/mildly acidic (for amines)

conditions. In this mode the diazonium ion behaves as a comparatively weak

ELECTROPHILE, and undergoes ordinary electrophilic aromatic substitution on the

activated partner ring, forming a new $-\text{N}=\text{N}-$ azo linkage between the two

rings with no loss of nitrogen at all -- the coupling reaction that builds an azo compound, $\text{Ar}-\text{N}=\text{N}-\text{Ar}'$.

Coupling with phenol. Benzenediazonium chloride, added to phenol dissolved in cold

dilute alkali (which converts phenol to the far more strongly activating phenoxide ion),

attacks predominantly at the position PARA to the $-\text{OH}/-\text{O}^-$ group (the

position most strongly activated), giving $p$-hydroxyazobenzene, an intensely coloured

orange-red solid (see the worked figure below).

Coupling with an aromatic amine. Benzenediazonium chloride similarly couples with

aniline (usually carried out near neutral pH, since strongly acidic conditions would

protonate and deactivate the amine, while strongly alkaline conditions favour the

competing hydrolysis of the diazonium salt to phenol) to give $p$-aminoazobenzene, again

substituting predominantly para to the activating amino group.

Why these products are intensely coloured. Both azo products contain an extended

system of conjugated $\pi$ electrons running across BOTH aromatic rings and the

connecting $-\text{N}=\text{N}-$ bridge. This long conjugated path lowers the energy gap

between the molecule's highest occupied and lowest unoccupied molecular orbitals enough …