Azo coupling

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An azo coupling is an organic reaction between a diazonium compound and another aromatic compound that produces an azo compound. In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the electrophile and the activated arene is a nucleophile.[1] In most cases, including the examples below, the diazonium compound is also aromatic.


The treatment of aniline with nitrous acid, produces a diazonium salt, in a reaction called diazotization. Diazonium salts are important synthetic intermediates that can undergo coupling reactions to form azo dyes and substitution reaction to effect the functional group present on aromatic rings.

Uses of the reaction[edit]

The product will absorb longer wavelengths of light (specifically they absorb in the visible region) than the reactants because of increased conjugation. Consequently, aromatic azo compounds tend to be brightly colored due to the extended conjugated systems. Many are used as dyes (see azo dye).[2] Important azo dyes include methyl red and pigment red 170.

Azo coupling is also used to produce prontosil and other sulfa drugs.

Examples of coupling reactions[edit]

Many procedures have been described.[3][4] Phenol reacts with benzenediazonium chloride to give a yellow-orange azo compound. The reaction is base-catalysed.[2]


The related dye called aniline yellow is produced from the reaction of aniline and the diazonium salt.[2]


Naphthols are popular acceptors. One example is the synthesis of the dye "organol brown" from aniline and 1-naphthol:

Organol brown synthesis

Similarly β-naphthol couples with phenyldiazonium electrophile to produce an intense orange-red dye.


  2. ^ a b c Klaus Hunger, Peter Mischke, Wolfgang Rieper, Roderich Raue, Klaus Kunde, Aloys Engel "Azo Dyes” in Ullmann’s Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim.doi:10.1002/14356007.a03_245.
  3. ^ J. L. Hartwell and Louis F. Fieser, "Coupling of o-tolidine and Chicago acid", Org. Synth. ; Coll. Vol. 2: 145 
  4. ^ H. T. Clarke and W. R. Kirner, "Methyl red", Org. Synth. ; Coll. Vol. 1: 374