Ru(III)-catalysed oxidation of some amines by chloramine-T in hydrochloric acid medium: Mechanistic aspects and kinetic modeling

The kinetics of oxidation of five amines viz., ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), aminoethylpiperazine (AEP) and isophoronediamine (IPDA) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in the presence of HCl and Ru(III) chloride was studied at 303 K. The five reactions followed identical kinetics and the experimental rate law is rate=k[CAT]0[amine]0x[H+]y[Ru(III)]z, where x, y and z are fractions. A variation of the ionic strength or dielectric constant of the medium and the addition of halide ions and p-toluenesulfonamide had no significant effect on the rate of the reaction. The solvent isotope effect has been studied in D2O medium. The activation parameters have been evaluated from the Arrhenius plots. Under comparable experimental conditions, the rate of oxidation of amines increases in the order: AEP > TETA > DETA > EDA > IPDA. An isokinetic relationship is observed with β = 377 K, indicating enthalpy as a controlling factor. Oxidation products were identified. CH3C6H4SO2N+H2Cl of the oxidant has been postulated as the reactive oxidizing species. Further, the kinetics of Ru(III)-catalysed oxidation of these amines have been compared with unanalyzed reactions (in the absence of Ru(III) catalyst) and found that the catalysed reactions are 2–3-fold faster. The catalytic constant (KC) was also calculated for each amine at different temperatures from the plots of log KC against 1/T, values of activation parameters with respect to the catalyst have been evaluated. The observed results have also been explained by a plausible mechanism and the related rate law has been deduced.

The following general scheme is proposed for the Ru(III)-catalysed oxidation of amines of cholramine-T in acid medium to explain the observed experimental results:Figure optionsDownload as PowerPoint slideHere, Ts = p-CH3C6H6SO2 and n = 1 for ethylendiamine, aminoethylpiperazine and isophoronediamine, 3 for diethylenetriamine and 7 for triethylenetetramine.The above scheme leads to the following rate law:rate=K1K2K3k4[CAT]t[amine][H+][Ru(III)]1+K1[H+]+K1K2[amine][H+]+K1K2K3[amine][H+][Ru(III)]