The Azo Coupling reaction is most common in the synthesis of azo dyes. Reactions are carried out under critical conditions in order to reduce the production of side groups. The condtions are standard or just below Standard ambient Pressure and Temperature (SATP)
There is an optimum pH range for specific coupling reactions. Phenols, commonly written as ArOH react in alkaline conditions for two reasons. Firstly, the anion, ArO- is more water soluble than phenol itself. Secondly, O- is a more powerful electron releasing group than -OH. Very high alkaline conditions should be avoided as it promotes decompostion of diazonium salts. It may also cause cation to anion conversion.
This is less reactive in a coupling reaction. In an azo-coupling reaction the lowest pH is most favourable as it produces the most water soluble species.
A good example of a compound used to synthesise many azo dyes is 1-amino-8-hydroxynaphthalene-3,6-disulfonic acid. Also known as H-acid. Its structure is shown below.
Production of monoazo compounds is relatively straight forward. Only coupling components need be selected along with the reaction conditions. Produciton of compounds containing more than one azo component is however a more difficult situation.
In Textile dyeing there are two types of compound. Those which are bonded to the metal, medially metallised, and those which are not, terminally metallised.
Commercially, medially metallised compounds are the most important. These compounds are formed from a reaction between a transition metal ion and its ligands. Ortho-positions adjacent to the azo group contain groups, such as -OH, -CO2 or -NH-, which are capable of coordination with M+ ion. For example, Cu(II), Co(III), Cr(III).
Metal Complexes are however, duller in appearance than the azo-dye ligand, which limits their uses. The reduction in the colour's brightness is due to the braodening of the visible absorption band. The broadening is likely to be due to the presence of isomers with slightly different absorption bands or an overlap o absorption bands. This is associated with pi-pi* transitions of the ligand arising from metal ion d-d transitions of ligand to metal. For more information on why azo compounds are coloured visit Conjugation and Colour