Electronic Properties of N-Heteroaromatics. XVI. Charge Transfer Properties of Pyrazolone Antipyreties. On the Complex Formation of Aminopyrine with Benzoic Acid and Salicylic Acid

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The anomalous phenomena with respect to solubility and coloration, which are observed when aminopyrine is combined with either benzoic acid or salicylic acid, have been studied by solubility method, refractometry, and spectrophotometry, and the following findings have been obtained from the experimental data : (1) a formation of chargetransfer complexes of 1 : 1 molar ratio between aminopyrine and either benzoic acid or salicylic acid were presumed, (2) from the comparative study of pyrazolone derivatives, it was concluded that 4-amino or 4-substituted amino moiety on the pyrazolone portions was found to be indispensable for the complex formation, and (3) as there were considerable differences between stability constants obtained by solubility method and those obtained from Benesi-Hildebrand plots, it was presumed that charge-transfer force might not be a sole driving force of the interaction.