SOME AZO DYESTUFFS

Azo compounds (ArN=NAr) are prepared bj the interaction of a diazonium salt with a phenol in the presence of sodium hydroxide, for example  

The azo dyes are not of any great practical value owing to their slight solubility in water. Th4 introduction of a sulphonic acid group into the molecide has no effect upon the colour, but renders the dye water-soluble—a fact of great commercial value. The simplest way of achieving this is to employ an amine, e.g., sulphanilic acid, in which the — OjH group is already present. 

This substance is more useful as an indicator than as a dye, for it changes colour at a certain concentration of hydrogen ions (pH 3 -1-4 -4). Treatment 

By condensation of diazotised sulphanilic acid with p-naphthol in the presence of sodium hydroxide, the useful dyestuff Orange II (p-sulphobenzene-azo-3-naphthol) is obtained  

The valuable indicator methyl red (o-carboxybenzene-azo-dimethylaniline) is obtained by coupling diazotised anthranilic acid with dimethylaniline  

The colour change is red (pink) to yellow over the pH range 4-2-6-3. 

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Some azo dyestuffs containing phosphonate groups have been found to be capable of forming exceptionally strong direct dye-fibre bonds, particularly in the case of cellulose. 

An approach to the design of non-mutagenic azo dyes 2. Potential replacements for the benzidine moiety of some mutagenic azo dyestuffs. Dyes Pigments, 8, 431-447. 

The largest user of phenol in the form of thermosetting resins is the plastics industry. Phenol is also used as a solvent and in the manufacture of intermediates for pesticides, pharmaceuticals, and dyestuffs. Styrene is used in the manufacture of synthetic rubber and polystyrene resins. Phthalic anhydride is used in the manufacture of DMT, alkyd resins, and plasticizers such as phthalates. Maleic anhydride is used in the manufacture of polyesters and, to some extent, for alkyd resins. Minor uses include the manufacture of malathion and soil conditioners. Nitrobenzene is used in the manufacture of aniline, benzidine, and dyestuffs and as a solvent in polishes. Aniline is used in the manufacture of dyes, including azo dyes, and rubber chemicals such as vulcanization accelerators and antioxidants. 

Complexes of chromium(III) with multidentate ligands containing azo linkages as well as other donor groups are of interest because of their applications in spectrophotometric analysis and because of their uses as commercial dyestuffs. Some ligands used in the analysis of chromium(III) are listed in Table 65. 

In general, the stability towards acids and alkalies of metal complexes of bidentate diarylazo compounds (20) is inadequate for them to find application as dyestuffs and only a limited number are used as pigments. They are, however, of some historical interest and their relative simplicity compared with metal complexes of tridentate azo compounds makes them useful models for the study of the latter, technologically important class. They are, therefore, considered here in some detail. 

Complexes derived from medially metallizable, tridentate azo compounds represent the single most important class of metal complex dyestuffs in commercial terms. Some indication of this is afforded by the fact that in 1983 the world consumption of premetallized dyes of this type was of the order of 20 000 tons. 

Within the overall category of dyestuffs, azo dyes constitute a significant portion and probably have the least desirable consequences in terms of the surrounding ecosystem. Some of the more common commercially used azo dyes are summarized below. 

Commercial dyestuffs are drawn from the number of different chemical classes. Table 12.4 shows some of these classes. Of these classes, the azos and anthraquinones are the most numerous among the commonly used dyestuffs. A few important dyestuffs exist in each of the other chemical classes. 

Synthetic Dyes.—Ip common usage the term aniline dye is applied to any dyestuff prepared from organic chemical substances. As the first dye made and many of those made at present are derived from aniline the above name is significant. It is not a true name, however, in many cases for though some of the azo and benzidine dyes (p. 573) may be considered as aniline derivatives those derived from naphthalene can not be so considered. Other dyes which we shall study later. 

Apart from its use as a valuable mordant dyestuff for textiles, cochineal (EEC 120) is still used as a colourant in cosmetics, foods, aperitif and beverages (ref. 194) and is one of the several permitted natural colourants which includes for example p-carotene, betanidin from Beta vulgaris, curcumin from Curcuma tonga, certain anthocyanins and chlorophyll complexes to quote a few structures. Commercial interest in natural products such as cochineal and carminic acid has been reactivated by the increasing pressures to avoid synthetic azo colours, their association with potential carcinogenic attributes and the increasing popularity of green issues. Carminic acid is reputed to possess some anticancer activity (ref. 195,196) and is a distant structural relative of the antibiotics, carminomycin and carminomycinone. 

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