Ruthenium(III)-catalyzed mechanistic investigation of oxidation of an azo dye by sodium N-haloarenesulfonamidates in acid medium: A comparative spectrophotometric kinetic study

Abstract

The kinetics of ruthenium(III) chloride (Ru(III))-catalyzed oxidation of p-hydroxyazobenzene (PHAB) by sodium N-haloarenesulfonamidates or N-haloamines (RNXNa, where R = CH 3C 6H 4SO 2 or C 6H 5SO 2 and X = Cl or Br): chloramine-T (CAT), chloramine-B (CAB), bromamine-T (BAT) and bromamine-B (BAB) in HCl medium at 298 K has been spectrophotometrically investigated at λ max = 347 nm. UV–vis spectrophotometry was used as a basic analytical approach in this study. Under comparable experimental conditions, the oxidation reaction follows identical kinetics with first-order dependences on [oxidant] 0, on [PHAB] 0 and on [Ru(III)], and less than unity order dependence on [H +]. Additions of halide ion (Cl − or Br −) and reduction product ( p-toluenesulfonamide or benzenesulfonamide) have no significant effect on the rate. The stoichiometry of the title reaction was found to be 1:2 and the oxidation products were identified as p-nitrosophenol and p-nitrosobenzene by GC–MS. The reaction rate was found to decrease with increases in ionic strength of the medium and increases in solvent polarity (dielectric constant). Proton inventory studies made in a mixture of H 2O–D 2O indicated the participation of H + ion in the formation of transition states. The reaction fails to initiate the polymerization of acrylonitrile. The activation parameters for the reaction were deduced from Arrhenius plots. The rates with bromamines were found to be higher than those with chloramines by a factor of 3; the rates follow the sequence: BAB > BAT > CAB > CAT. This effect is attributed to electronic factors. The calculated isokinetic temperature ( β) of 354 K obtained from enthalpy–entropy relationships and Exner correlations was much higher than the experimental temperature of 298 K, indicating that the rate is under enthalpy control; a similar mechanism operates in the oxidation of PHAB by all the four oxidants. Under the identical set of conditions, the kinetics of Ru(III)-catalyzed oxidation of PHAB by N-haloamines has been compared with uncatalyzed reactions, revealing that the catalyzed reactions are seven- to nine-fold faster. The catalytic constant ( K C) has been calculated for each oxidant at different temperatures. The values of activation parameters with respect to the catalyst have been evaluated from the plots of log K C versus 1/ T. Some spectroscopic evidence for the formation of 1:1 complex between PHAB and Ru(III) has been obtained. The observed results have been explained by a plausible mechanism and the related rate law has been deduced.