Diazo pigments synthesis

Clariant carried out diazo-coupling for pigment synthesis. Laboratory developments were transferred to pilot scale, reported in the literature, and a further transfer to production scale was announced at conferences [65]. 

One of the first examples of micro reactors used for manufacturing was reported by Cellular Process Chemistry (CPC) [97] and involved a three-step process for the synthesis of a diazo pigment (a) diazotization, (b) coupling, and (c) pigmentation. 

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. 

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). jj-Phenylenediamine is an example of a diamine in which either one or both of the amino groups may be diazotised by careful selection of reaction conditions. The use of dilute hydrochloric acid can result in smooth formation of the monodiazonium salt. The use of nitrosyl sulfuric acid is required to diazotise the second amino group, since the strong electron-withdrawing effect of the diazo group in the monodiazonium salt reduces the basicity of the amino group which remains. 

C.I. Diazo component 9, 37040. Uses as a wet cake in the synthesis of yellow and red azo pigments and as a dried and ground sales product (for azoic combinations). 

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