Kinetics of Oxidation of Thiocyanate Ion by N-Chloro- & N-Bromosuccinimides in Aqueous Methanol & Alkaline Media

Abstract

Kinetics of oxidation of thiocyanate ion (SCN^-) by N-chlorosuccinimide (NCS) and N-bromosuccinimide (NBS) have been studied in 1 : 1 (v/v) aqueous methanol in the presence of perchloric acid, and in aqueous alkaline medium. The rates follow first order kinetics in [oxidant] with both the oxidants in acidic as well as alkaline media, and varying fractional order dependence in [SCN^-] in both the media, except for NBS oxidation in alkaline medium. The rates increase with increase in [HClO₄] for both the oxidants, but decrease with increase in [NaOH] in alkaline medium, with varying fractional orders in [H⁺] and [OH^-]. Variation in ionic strength of the medium or the addition of the reaction products (succinimide, Cl^- or Br^-) has no significant effect on the rates of reactions. Decrease in dielectric constant of the medium increases the rate in all the cases. Oxidation process in 1 : 1 (v<em>/</em>v) water-methanol medium has been shown to follow Michaelis-Menten type mechanism. Mechanisms consistent with observed results under different conditions have also been proposed. The rate-limiting steps have been identified and the corresponding constants have been computed at different temperatures. The activation parameters have been computed from the latter constants.