Os(VIII)-catalyzed mechanistic investigation of oxidation of some benzimidazoles by chloramine-T in alkaline medium: A kinetic approach

The kinetics of Os(VIII)-catalyzed oxidation of benzimidazole (BzlH) and 2-substituted benzimidazoles viz., 2-methylbenzimidazole (MeBzlH), 2-hydroxybenzimidazole (2-HyBzlH), 2-aminobenzimidazole (2-AmBzlH) and 2-phenylbenzimidazole (2-PhBzlH) by sodium-N chloro-p-toluenesulfonamide (chloramine-T; CAT) in presence of NaOH has been investigated at 303 K. Under comparable experimental conditions, the oxidation reactions follow identical kinetics for all the five benzimidazoles with first order dependence each on [CAT]0 and [substrate]0 and fractional order dependence each on [OH−] and [Os(VIII)]. The reactions were studied at different temperatures and the composite activation parameters have been computed. The calculated isokinetic temperature (β) of 380 K obtained from enthalpy-entropy relationships and Exner correlations was much higher than the experimental temperature of 303 K, indicating that the rate is under enthalpy control. Relative reactivity of oxidation of these follow the order: 2-MeBzlH > 2-HyBzlH > 2-AmBzlH > BzlH > IzlH > 2-PhBzlH. This trend may be attributed to inductive effects. Further, the kinetics of Os(VIII)-catalyzed oxidation of these benzimidazoles have been compared with those of their uncatalyzed reactions (in the absence of catalyst) and found that the catalyzed reactions are 10–15 fold faster than the uncatalyzed reactions. The observed results have been explained by a plausible mechanism and the related rate law has been deduced.