Kinetics of methoxy-NNO-Azoxymethane hydrolysis in strong acids

Abstract

The kinetics of methoxy-NNO-azoxymethane (I) hydrolysis in concentrated solutions of strong acids (HBr, HCl, HClO4, and H2SO4) has been investigated by a manometric method. The gas evolution rate is described by the equation corresponding to two consecutive first-order reactions, with the rate constant of the second reaction considerably exceeding the rate constant of the first reaction, i.e., k 2 {ie17-1} k 1. The temperature dependences of k 1 (s−1) in 47.59% HBr in the temperature range from 60 to 90°C and in 64.16% H2SO4 between 80 and 130°C are described by Arrhenius equations with IogA= 12.7 ± 1.5 and 13.6 ± 1.4 and E a = 115 ± 10 and 137 ± 10 kJ/mol, respectively. The parameters of the Arrhenius equation for the rate constant k 2 for the reaction in 64.16% H2SO4 between 80 and 130°C are IogA= 9.1 ± 2.5 and E a = 91 ± 18 kJ/mol. An analysis of the UV spectra of compound I in concentrated H2SO4 shows that I is a weak base \( (pK_{BH^ + } \approx - 6) \). The rate-determining step of the hydrolysis of I is the attack of the nucleophile on the carbon atom of the MeO group of the protonated molecule of I. The resulting methyldiazene dioxide decomposes via a complicated mechanism to evolve N2, NO, and N2O. The pseudo-first-order rate constant k 1 of the reaction at 80°C depends strongly on the acid concentration and on the type of nucleophile (Br−, Cl−, or H2O). The relationship between k 1 and the rate constant k of the bimolecular nucleophilic substitution reaction (SN2) is given by the linear equation log\( [k_1 /(C_H + C_{Nu} )] = m^ \ne m*X_0 + \log (k/K_{BH^ + } ) \), where \( C_{H^ + } \) and C Nu are the concentrations of H+ and nucleophile, respectively; X 0 is the excess acidity; and m ≠ and m* are coefficients. The Swain-Scott equation log\( (k_{Nu} /k_{H_2 O} ) = ns \), where n is the nucleophilicity factor and s is the substrate constant (s = 0.72), is applicable to the rate constants k of the SN2 reactions of the protonated molecule of I with Br−, Cl−, and H2O.