Chromic Acid Oxidation of Methylaminopyrazole Formamidine in Sulfuric Acid Medium: A Kinetic and Mechanistic Approach

摘要

The kinetics of chromic acid oxidation of one of aminopyrazole formamidine derivatives, namely iN,N/i-dimethyl-iN’ /i-(5-methyl-1iH/i-pyrazol-3-yl) formamidine (MAPF)in sulfuric acid solutions has been investigated at constant ionic strength and temperature. The progress of the reaction was followed spectrophotometrically. The reaction showed a first order dependence on [chromic acid] and fractional-first order dependences with respect to [MAPF] and [Hsup+/sup]. Increasing ionic strength and solvent polarity of the reaction medium had no significant effect on the oxidation rate. Addition of AgsupI/sup, PdsupII/sup and RusupIII/sup catalysts was found to enhance the reaction rate and the order of catalytic efficiency is: AgsupI/sup RusupIII/sup PdsupII/sup. The final oxidation products of MAPF are identified by spectral and elemental analysis as methylaminopyrazole, dimethylamine and carbon dioxide. A spectral evidence for the formation of chromium(III) product was obtained. A reaction mechanism adequately describing the observed kinetic behavior is proposed, and the reaction constants involved in the different steps of the mechanism have been evaluated. The activation parameters with respect to the rate-determining step of the reaction, along with thermodynamic quantities of the equilibrium constants, are presented and discussed.