Azo dye

As a consequence of π-delocalization, aryl azo compounds have vivid colors, especially reds, oranges, and yellows.

The anionic character of most dyes arises from the presence of 1-3 sulfonic acid groups, which are fully ionized at the pH of the dyed article: Most proteins are cationic, thus dyeing of leather and wool corresponds to an ion exchange reaction.

Generally, coupling partners are other aromatic compounds with electron-donating groups:[7] In practice, acetoacetanilides are widely used as coupling partners: Azo dyes are also prepared by the condensation of nitrated aromatic compounds with anilines followed by reduction of the resulting azoxy intermediate: For textile dying, a typical nitro coupling partner would be disodium 4,4′-dinitrostilbene-2,2′-disulfonate.

The lightfastness depends not only on the properties of the organic azo compound, but also on the way they have been absorbed on the pigment carrier.

In order to prolong the lifetime of products dyed with azo dyes, it is essential to ensure stability against microbial attack, and tests have shown that azo dyes biodegrade negligibly in short term tests under aerobic conditions.

[10][11] Azo dyes derived from benzidine are carcinogens; exposure to them has classically been associated with bladder cancer.

[5] Certain azo dyes degrade under reductive conditions to release any of a group of defined aromatic amines.

Since September 2003, the European Union has banned the manufacture or sale of consumer goods which contain the listed amines.