Synthesis of Quinacridones by the PPA Process

The major product generated under these conditions is the terephthalic acid 17. However as indicated several competing reactions can occur. Although favoring formation of 12 the initial condensation reaction is reversible. Therefore, prema- 

Alternatively, 12 can be oxidized to the diester 22 by contact with a variety of oxidizing agents, including iodine [19] and dioxygen (air injection) [19A.  

The acids 17 or esters 22 are cyclized in PPA [20] (115-117% H3PO4) which serves both as a reactant and solvent at relative mild temperatures (120-140 °C), affording excellent yields of the corresponding quinacridones. Commercially however the terephthalic acids are the preferred precursors. 

Analytical studies of the PPA reaction processes by real-time infrared spectroscopy [19A] demonstrate that cycUzation to the corresponding quinacridones is very rapid at the typical reaction temperatures and although not confirmed directly, probably proceeds by a two-stage sequential mechanism. Analogous to the thermal process high dilution conditions favor monomolecular reactions and subsequently the quaUty of the ultimate products. The by-products of bimolecular reactions occasionally have been detected in some quinacridone pigments 

Compared to the thermal process, quinacridone production in PPA occurs at significantly lower temperatures (120-140 °C) utilizing the already oxidized intermediates (diarylaminoterephthalic acids, 17) to synthesize the crude products directly. Therefore, excluding the synthesis of DMSS, production of most quin- 

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