7.4 The Synthetic Value of Diazo-Coupling

7.4.1 Dye-stuffs

Aromatic azo-compounds are coloured. Several of those compounds synthesized by the diazo-coupling are employed as dye-stuffs. These compounds can be classified into three groups.

• • First group of azo-compounds are neutral which are used as azoic combination (or ingrain) dyes. An example is para red which is prepared by coupling of 2-napthol with p -nitrobenzenediazonium salt.



• The second group of azo-compounds posses either a sulfonic acid group or an amino group which are generally adsorbed directly on the fiber from aqueous solution. Examples are orange II (an acidic dye) and Bismarck brown R (a basic dye).





• Azo-compounds that contain chelating groups to bind with a metal ions such as Al(III) are used a mordant dyes. An example is alizarin yellow R, which contains phenolic and carboxyl chelating groups.