Ru(III)-catalysed oxidation of some N-heterocycles by chloramine-T in hydrochloric acid medium: A kinetic and mechanistic study

Abstract

The kinetics of the ruthenium(III) chloride (Ru(III))-catalysed oxidation of five N-heterocycles (S) viz. imidazole (IzlH), benzimidazole (BzlH), 2-hydroxybenzimidazole (2-HyBzlH), 2-aminobenzimidazole (2-AmBzlH) and 2-phenylbenzimidazole (2-PhBzlH) by sodium-N-chloro-p-toluenesulfonamide (chloramine-T; CAT) in the presence of HCl has been studied at 313 K. The oxidation reaction follows the identical kinetics for all the five N-heterocycles and obeys the rate law, rate = k CAT0 S 0 x H+y Ru(III)z, where x, y and z are less than unity. Addition of p-toluenesulfonamide (PTS) retards the reaction rate. Variation of ionic strength of the medium and the addition of halide ions show negligible effect on the rate of the reaction. The rate was found to increase in D2O medium and showed positive dielectric effect. The reaction products are identified. The rates are measured at different temperatures for all substrates and the composite activation parameters have been computed from the Arrhenius plots. From enthalpy-entropy relationships and Exner correlations, the calculated isokinetic temperature (β) of 392 K is much higher than the experimental temperature (313 K), indicating that, the rate has been under enthalpy control. Relative reactivity of these substrates are in the order: 2-HyBzlH > 2-AmBzlH > BzlH > IzlH > 2-PhBzlH. This trend may be attributed to resonance and inductive effects. Further, the kinetics of Ru(III)-catalysed oxidation of these N-heterocycles have been compared with uncatalysed reactions (in the absence of Ru(III) catalyst) and found that the catalysed reactions are 16-20 times faster. The catalytic constant (KC) was also calculated for each substrate at different temperatures. From the plots of log KC versus 1/T, values of activation parameters with respect to the catalyst have been evaluated. H2O+Cl has been postulated as the reactive oxidizing species. The reaction mechanism and the derived rate law are consistent with the observed experimental results. © 2004 Elsevier B.V. All rights reserved