Mechanistic studies on the Ru(III)-catalyzed oxidation of some aromatic primary diamines by chloramine-T in hydrochloric acid medium: A kinetic approach
The kinetics of ruthenium(III) chloride (Ru(III))-catalyzed oxidation of aromatic primary diamines viz., 4,4′-diaminodiphenyl methane (DDM), 4,4′-diaminodiphenyl sulfone (DDS) and 4,4′-diaminodiphenyl ether (DDE), by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in HCl medium have been investigated at 298 K. Under comparable experimental conditions, the three oxidation reactions follow identical kinetics with first-order dependence both on [oxidant]0 and [Ru(III)], and less than unity order dependence on [Amine]0 and on [H+]. The stoichiometry for each reaction was found and the oxidation products were identified through GC–MS analysis. The reactions were subjected to changes in (a) ionic strength, (b) p- toluenesulfonamide, (c) dielectric permittivity, (d) halide ions, (e) solvent isotope and (f) temperature effect. The reaction mixture fails to initiate the polymerization of acrylonitrile. The activation parameters for the overall reaction were deduced from Arrhenius plots. Under comparable experimental conditions, the relative reactivity of these amines towards CAT is in the order: DDS > DDE > DDM. This reactivity trend may be attributed to electronic factors. The isokinetic temperature (β) of 350 K calculated from enthalpy–entropy relationships and the Exner criterion was much higher than the experimental temperature of 298 K employed in the present study, indicating that the reaction is under enthalpy control. Under the identical set of experimental conditions, the kinetics of Ru(III)- catalyzed oxidation of amines by CAT has been compared with those of uncatalyzed reactions; the catalyzed reactions are found to be about 5-fold faster. The catalytic constant (KC) has been calculated for each amine at different temperatures. The values of activation parameters with respect to Ru(III)-catalyst have been evaluated from the plots of log KC versus 1/T. The protonated species CH3C6H4SO2N+H2Cl has been postulated as the reactive oxidizing species of CAT. The observed results have been explained by a plausible mechanism and the related rate law has been deduced.
Graphical abstractThe kinetics of ruthenium(III) chloride (Ru(III))-catalyzed oxidation of aromatic primary diamines viz., 4,4′-diaminodiphenyl methane (DDM), 4,4′-diaminodiphenyl sulfone (DDS) and 4,4′-diaminodiphenyl ether (DDE) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in HCl medium have been investigated at 298 K. Scheme 1 has been proposed for the Ru(III) catalyzed oxidation of aromatic primary diamines by CAT in acid medium to substantiate the observed kinetics.Figure optionsDownload full-size imageDownload as PowerPoint slideHere, n = 2 for DDM and 1 for DDE. In Scheme 1, X and X′ represent the complex intermediate species.Above scheme leads to the following rate law:Rate=K1K2k3[CAT]t[Amine][H+][Ru(III)]1+K1[H+]+K1K2[Amine][H+]The derived rate law and proposed mechanism are consistent with all the experimental kinetic results.
Journal: Applied Catalysis A: General - Volume 326, Issue 2, 15 July 2007, Pages 202–212