Ruthenium(III)-catalyzed oxidative cleavage of p-aminoazobenzene by chloramine-B in alkaline medium and uncatalyzed reaction in acid medium:spectrophotometric kinetic and mechanistic study

摘要

p-Aminoazobenzene(PAAB)is one of the important monoazo dyes and its oxidation kinetic study is of much use in understanding the mechanistic profile of PAAB in redox reactions.Consequently,the kinetics of oxidation of PAAB by sodium N-chlorobenzenesulfonamide or chloramine-B(CAB)in HClO4 medium and in NaOH medium catalyzed by ruthenium(III)chloride(Ru~(III))have been investigated at 298 K.U.v.-vis spectrophotometry was used as a basic analytical approach in this study.Under an identical set of experimental conditions,the reactions of PAAB-CAB in HClO4 medium were facile,but the reactions became too slow to be studied in alkaline medium and hence ruthenium(III)chloride has been used as a catalyst in alkaline medium.The stoichiometry(1:2)and oxidation products(nitrosobenzene and p-nitrosoaniline)are the same in both media,but the kinetic and mechanistic patterns were found to be different.The experimental rate laws obtained are:-d[CAB]/dt=k[CAB]_0[PAAB]_0[H~+]in acid medium and-d[CAB]_0/dt=k[CAB]_0[PAAB]_0[OH~-]~x[Ru~(III)]~y in alkaline medium,where x and y are less than unity.The reaction was examined with reference to changes in(a)concentration of benzenesulfonamide,(b)concentration of added neutral salts,(c)ionic strength,(d)dielectric permitivity and(e)solvent isotope effect.The reaction was also studied at different temperatures and the overall activation parameters have been evaluated.The oxidation reaction fails to induce the polymerization of added acrylonitrile.C6H4SO2NHCl and C6H4SO2NCl~-have been postulated as the reactive oxidizing species in acidic and alkaline media,respectively.It was found that the reactions are nearly 20 times faster in acid medium in comparison with alkaline medium.It was also observed that ruthenium(III)was an efficient catalyst for the facile oxidation of PAAB by CAB in alkaline medium by making the reaction go twelve-fold faster than the uncatalyzed reactions.The catalytic constant(K_C)has been calculated at different temperatures and the values of activation parameters with respect to ruthenium(III)have also been evaluated in alkaline medium.The observed results have been explained by plausible mechanisms and the relative rate laws have been deduced.