Synthesis of Azo Dyes and Pigments

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 

Diazotisation is always carried out under strongly acidic conditions, 

It is especially important to control the acidity when aromatic diamines are treated with nitrous acid to form either the mono or bis diazonium salts, a process of some importance in the synthesis of disazo dyes and pigments (see later). p-Phenylenediamine is an example of a 

The quantity of sodium nitrite used in diazotisation is usually the equimolar amount required by reaction stoichiometry or in very slight excess. A large excess of nitrite is avoided because of the instability of 

The diazotisation reaction provides a classical example of the application of physical chemistry in the elucidation of the detail of organic 

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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... 

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. 

Uses. (9-Nitrochlorobenzene is used in the synthesis of azo dye intermediates such as o-chloroaniline (Fast YeUow G Base), i9-nitroani1ine (Fast Orange GR Base), o-anisidine (Fast Red BB Base), o-phenetidine, and (9-aminophenol (see Azo dyes). It also is used in corrosion inhibitors, pigments, and agriculture chemicals. -Nitrochlorobenzene is used principally in the production of intermediates for azo and sulfur dyes. Other uses include pharmaceuticals (qv), photochemicals, mbber chemicals (qv), and insecticides (see Insectcontroltechnology). Typical intermediates manufactured from the para isomer are -lutioaruline (Fast Red GC Base), anisidine, -aminophenol, -nitrophenol, -phenylenediamine, 2-chloro-/)-anisidine (Fast Red R Base), 2,4-dinitrochlorobenzene, and l,2-dichloro-4-nitrobenzene. 

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-... 

Makedonski P., Brandesa M., Grahnb W., Kowalsky W., Wichem J., Wiese S., Johannes H., Synthesis of new kinds of reactive azo dyes and their application for fibre-optical pH-measurements, Dyes and Pigments 2004 61 109-119. 

Synthesis of Dyes and Pigments eontaining more than two Azo Groups... 

D. H. Song, H. Y. Yoo, and J. P. Kim, Synthesis of stilbene-based azo dyes and application for dichroic materials in poly(vinyl alcohol) polarizing films. Dyes and Pigments, 75, p. 727 (2007). 

The development of modern organic pigments started with the synthesis of dyestuffs for the textile industry. The period up to 1900 was characterized by the discovery and development of many dyes derived from coal-tar intermediates. Rapid advances in color chemistry were initiated after the discovery of diazo compounds and azo derivatives (shown to be largely hydrazone derivatives). The wide color potential of this class of pigments and their relative ease of preparation led to the development of azo colors, which represent the largest fraction of manufactured organic pigments. 

Related product properties are improved, for example, optical properties such as the glossiness and transparency (see Table 5.1). There are no negative consequences for dye manufacturing with the new microreactor made crystals, as the tinctorial power is the same as for conventional synthesis. Tinctorial power is a measure for the adhesion of the pigment to wool stuff or similar material. The intensification in coloration properties means that the same amount of material can be treated now with less amounts of the Yellow 12 azo pigment that reduces materials costs and increases the profitability of the pigment manufacture. 

Monochlorobenzene. The largest use for monochlorobenzene, accounting for about 40 percent of the consumption, is in the production of chloronitrobenzenes. p-Nitro-chlorobenzene (NCR) is converted into p-phenylenediamines for use as antioxidants in rubber processing. A smaller use for NCR is in the synthesis the pain reliever acetaminophen. ort/io-Nitrochlorobenzene is a raw material for producing insecticides and several azo pigments. A large number of dyes also can be derived from either chlorobenzene or nitro-chlorobenzene. About 25 percent of the monochlorobenzene is used as a solvent for pesticide formulation and in MDI processing. The manufacture of diphenyl oxide, for heat transfer fluids, and phenylphenols, for disinfectants, consumes another 22 percent. About 8 percent is used to make dichloro-diphenylsulfone, an intermediate in the manufacture of sulfone polymers. 

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