Synthesis of Disazo Dyes and Pigments

The situation becomes more complex when two separate diazotisations and azo couplings are required. Four separate strategies leading to disazo colorants may be identified, using the commonly accepted SDC terminology and symbolism, exemplified as follows as strategies (a)-(d). 

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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... 

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. 

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