Azo dyes and pigments

The structural chemistry of azo compounds is complicated by the possibilities of isomerism. There are two types of isomerism, which may commonly be encountered with certain azo compounds geometrical isomerism and tautomerism. 

The compound exists normally as the trans or ( )-isomer 21a. This molecule is essentially planar both in the solid state and in solution, although in the gas phase there is evidence that it deviates from planarity. When irradiated with UY light, the ( )-isomer undergoes conversion substantially into the cis or (Z)-isomer 21b which may be isolated as a pure compound. In darkness, the (Z)-isomer reverts thermally to the (F)-isomer which is thermodynamically more stable because of reduced steric congestion. Some early disperse dyes, which were relatively simple azobenzene derivatives introduced commercially initially for application to cellulose acetate fibres, were found to be prone to photochromism (formerly referred to as phototropy), a reversible light-induced colour change. C. I. Disperse Red 1 (22) is an example of a dye which has been observed, under certain circumstances, to give rise to this phenomenon. 

---

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

Undoubtedly the most significant discovery in colour chemistry in the post-Mauveine period was due to the work of Peter Griess, which provided the foundation for the development of the chemistry of azo dyes and pigments. In 1858, Griess demonstrated that the treatment of an aromatic amine with nitrous acid gave rise to an unstable salt (a dia-zonium salt) which could be used to prepare highly coloured compounds. 

Textbooks in general organic chemistry will illustrate that there are many ways of synthesising azo compounds. However, almost without exception, azo dyes and pigments are made on an industrial scale by the same... 

The synthesis of carbonyl colorants uses a wide diversity of chemical methods, in which each individual product essentially has its own characteristic route. This is in complete contrast to the synthesis of azo dyes and pigments (Chapter 3) where a common reaction sequence is universally used. The subject is too vast to attempt to be comprehensive in a text of this type. The following section, therefore, presents an overview of some of the fundamental synthetic strategies which may be used to prepare some of the more important types of carbonyl colorants. 

Azo dyes and azo pigments are the most important class of both non-metallized and metallized colorants. Therefore, this section is devoted entirely to metal complex azo dyes and pigments. 

Uses Intermediate in the manufacture of azo dyes and pigments for printing inks, textiles, paints, plastics, and crayons curing agent for isocyanate-terminated polymers and resins rubber compounding ingredient analytical determination of gold formerly used as chemical intermediate for direct red 61 dye. 

In metabolism studies of azo dyes and pigments in the hamster, in vivo cleavage of the benzidine-based dye, Direet Black 38, to benzidine was shown by analysis of the urine. However, studies of the 3,3 -diehlorobenzidine-based pigment. Pigment Yellow 12, showed no evidenee for in vivo cleavage to release 3,3 -diehlorobenzidine (Nony et al. 1980). 

Nony CR, Bowman MC. 1980. Trace analysis of potentially carcinogenic metabolites of an azo dye and pigment in hamster and human urine as determined by two chromatographic procedures. J Chromatogr Sci 18 64-74. 

Nony CR, Bowman MC, Cairns T, et al. 1980. Metabolism studies of an azo dye and pigment in the hamster based on analysis of urine for potentially carcinogenic aromatic amine metabolites. Anal Toxicol 4 132-140. 

Azo dyes and pigments have been an important commercial product for the past century. Following the discovery of diazonium compounds in 1858, and the first azcbenzene in 1861, this class of compounds was rapidly exploited for use as colorants. One of the reasons for the success iof this class is their relative ease of synthesis. Thus, large numbers of compounds can be readily prepared and their absorption characteristics systematically controlled by structural modifica-... 

---