Synthesis of Azo Pigments

The production of azo pigments relies almost exclusively on the azo coupling reaction [1,2] to afford the azo group. Diazotization of an aromatic amine yields a diazonium compound, which subsequently reacts with a coupling component ( coupling ). 

Generally speaking, the coupling reaction links an aromatic amine to a nucleophilic partner RH (coupling component) the amine is treated with a nitrosyl source XNO to form an azo compound. This sequence is expressed by the following equation  

At aromatic gr heteroarOfrtatiC grCKJp R coupling comppftenl residue X Cl. Br, NOe, HSQ, 

As a result of the stabilization of the coplanar structure, azo groups exhibit cis-trans isomerism. Azo compounds are generally written in the trans configuration, which is the more stable one. However, X-ray structure analyses show azo pigments exclusively in their hydrazone form (see pp. 213,272). 

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Azo pigments, subdivided into the monoazo and disazo pigments, have the azo group (—N=N—) in common. The synthesis of azo pigments is economically attractive, because the standard sequence of diazonium salt formation and subsequent reaction with a wide choice of coupling components allows access to a wide range of products. 

Considering the electrophilic nature of the reaction mechanism, suitable coupling components for the synthesis of azo pigments should carry a nucleophilic center on the aromatic ring system (naphthols) or should be enolizable compounds with reactive methylene functions. Naphthols enter the reaction as naphtholates compounds with reactive methylene groups participate as enolates. 

The synthesis of azo pigments is a typical batch process. The azo coupling is only one of a three-step pigment synthesis (diazotization, azo coupling and pigmentation). WUle et al. [16] described the first successful production of pigments in microfluidic... 

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. 

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. 

The flow chart in Fig. 90 outlines the typical sequence of azo pigment synthesis, including essential equipment ... 

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. 

The synthesis of an aromatic amine generally starts from the corresponding nitro compound. Nitration and subsequent reduction are key reactions in the synthesis of intermediates for azo pigments. 

Reliable measuring techniques [9] and appropriate process control are the basic elements of successful azo pigment synthesis by the continuous process. These are the parameters which are responsible for maintaining constant reaction conditions flow rate, pH, temperature, and the concentrations of the reactants, before and after the point of mixing itself. 

Hansa yellow org chem Group of organic azo pigments with strong tinting power, but poor opacity in paints used where nontoxIcIty Is Important. han-sa yel o ) Hantzsch synthesis org chem The reaction whereby a pyrrole compound is formed when a p-ketoester, chloroacetone, and a primary amine condense. hansh, sin-tha-sas ... 

Benzidine yellow is an example of a large class of organic pigments that contain an azo linkage, -N=N-. Its synthesis relies very heavily on diazonium salt coupling reactions and the benzidine rearrangement. Although benzidine is banned in the U.S. because of suspected... 

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. 

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