Aryldiazonium Salts to Azo Dyes: Diazo Coupling

Arenediazonium salts, when treated with substituted aromatic rings bearing activating groups, undergo a coupling reaction to generate compounds bearing an –N=N– group.

This is an electrophilic aromatic substitution reaction, wherein the electrophilic diazonium ion reacts at the para position of the nucleophilic aromatic compound, resulting in the resonance-stabilized carbocation.

Next, deprotonation of the carbocation leads to the formation of the coupled product.

If the para position of the aromatic ring is unavailable, coupling occurs at the ortho position.

Colored azo compounds are often used as synthetic dyes and pH indicators. For example, 'sunset yellow' is a dye responsible for the yellow color of ice cream.

Methyl orange is used as an acid–base indicator during titrimetric analysis. In the presence of an alkaline solution, it turns into a yellow-colored solution, while in acidic media, upon protonation, the color changes to red.