Diazonium coupling reactions

Although most reactions of diazonium salts involve loss of nitrogen, there are some useful reactions where the nitrogen is retained.  The most important of these is where the diazonium salt acts as an electrophile in an electrophilic aromatic substitution (EAS), forming an “azo compound”.  The reaction only occurs where the aromatic nucleophile is highly activated, and the azo group is attached at the most activated position(s) – usually ortho or para to a hydroxy or amino group.  The resultant azo compounds are brightly colored and used very widely used as dyes, since they provide a conjugated system involving both aromatic rings.  Some common azo dyes are shown in the table,

Methyl Red	Azobenzene	Evans’ Blue	Para Red

The EAS mechanism is shown here, in this synthesis of the dye Methyl Red from 2-aminobenzoic acid and N,N-dimethylaniline:

In another example, Para Red is synthesized from 2-naphthol and 4-nitrobenzenediazonium hydrogen sulfate: