Kinetics and mechanism of the oxidation of thiourea by chlorine dioxide

Abstract

The stoichiometry and kinetics of the oxidation of thiourea (SC(NH2)2) by chlorine dioxide (ClO2) have been studied by uv‐vis spectrophotometry using conventional and stopped‐flow mixing techniques at 25.0 ± 0.1°C, pH 0.3–4.8. In high acid and initial 10:1 molar ratio of thiourea to chlorine dioxide, thiourea is oxidized relatively rapidly to dithiobisformamidine ion ((NH2)2CSSC(NH2)22+), which slowly decomposes to thiourea, sulfur, and cyanamide (NCNH2). In high acid and excess ClO2, thiourea is oxidized to relatively stable formamidine sulfinic acid ((NH) (NH2)CSO2H). In high acid and molar ratios of ClO2 to thiourea of 5:1 and higher, some oxidation to formamidine sulfonic acid ((NH) (NH2)CSO3H) occurs. At lower acidity, along with Cl−, the major ClO2 reduction product, byproduct sulfate is detected and, at pH < 3, ClO2−, also, appears. Kinetics data were collected for high excess thiourea with varying pH. The [ClO2]‐time curves are straight lines with negative slopes that increase in magnitude with increasing [thiourea]. The dependence on [thiourea] is first‐order; the dependence on [ClO2] is zero‐order for 90% of reaction. With decreasing pH, the rate increases and the disappearance of ClO2 becomes autocatalytic. Studies of the effects of reaction products on the rate of reaction lead to the conclusion that autocatalysis at low pH is due to the greater reactivity of HClO2 compared with ClO2−. A 10‐step mechanism incorporating a slow one‐electron transfer from thiourea to ClO2 to generate the (NH) (NH2)CS · radical and subsequent more rapid reactions has been constructed and implemented in a computer simulation which provides a reasonably accurate fit to the observed kinetics curves. © 1993 John Wiley & Sons, Inc. Copyright © 1993 John Wiley & Sons, Inc.