Azo Coupling is an electrophilic aromatic substitution reaction where a diazonium cation acts as an electrophile.

• Reaction: $Ar{N}_2^{+}+A{r}^{\prime }-H\to Ar-N=N-A{r}^{\prime }+H^{+}$
• The second aromatic ring ($A{r}^{\prime }$) must be strongly activated by groups like $-OH$ (Phenol) or $-N{H}_2$ (Aniline).
• Conditions: Phenols couple in weakly basic solution (forming orange dyes), whereas amines couple in weakly acidic solution (forming yellow dyes).
• The resulting Azo compounds have an extended conjugated $\pi$ system, making them intensely colored dyes.