Azo coupling

An azo coupling is an organic reaction between a diazonium compound and a dialkylaniline (C₆H₅NR₂), phenol or other aromatic compound which produces an azo compound. In this reaction the diazonium salt is an electrophile and the activated arene is a nucleophile in an electrophilic aromatic substitution. In most cases, including the example below, the diazonium compound is also aromatic. The product will absorb longer wavelengths of light than the reactants because of increased conjugation. Aromatic azo compounds tend to be brightly colored due to the extended conjugated systems; many are used as dyes.

Azo couplings are important in the production of dyes and pH indicators such as methyl red and pigment red 170. Prontosil and other sulfa drugs are also produced using this reaction.

An example is the synthesis of the dye organol brown from aniline and 1-naphthol:

Examples of coupling reaction

Reaction with phenol

Phenol is converted into sodium phenoxide, which is more reactive, and is cooled in ice. To this cold benzenediazonium chloride is added. It undergoes coupling reaction and a yellow-orange precipitate of diazo compound is finally obtained.

Reaction with aniline to form aniline yellow

To the cold diazonium compound some liquid aniline is added and is shaken vigorously. Aniline yellow is produced as a yellow solid. The pH of the reaction should be around 4-5 and the remperature not above 5°C.

Reaction with β-naphthol

β-naphthol is dissolved in sodium hydroxide and the solution obtained is cooled and mixed with benzenediazonium chloride solution to produce an intense orange-red precipitate.The pH of the reaction should be around 9-10 and the temperature not above 5°C.