■ Background:
Azo dyes make up the largest group of synthetic dyes by number. They are used as food dyes, artists’ colors, basic colors for four-color printing (yellow and magenta), as well as textile dyes on a large scale. Over 350.000 t of azo dyes  are used worldwide only for the dyeing of natural fibers.

Azo dyes are characterized by the diazo group, which couples two molecular parts:

or more general

 

The diazo bridge connects the two components aryl, designated as diazo component, and R, designated as coupling component.

Modern diazo components are, for example:

4B acid and

 

Coupling components are:

       

Chromotropic acid and Schaeffer’s acid

 

To increase solubility and adhesion to fibers such as wool, the sulfo group is introduced. The great variety of azo dyes results from the more or less free variation of substituents in the diazo components as well as in the coupling components:

Example for a modern azo dye:

Start reaction is the diazotation in the presence of hydrochloric acid and sodium nitrite.

The reaction is exothermic and is therefore cooled with ice and it depends on the pH value….