Metal-ion oxidations in solution. Part XII. Oxidation of thiourea and NN′-ethylenethiourea by chromium(VI) in perchlorate media

Abstract

The oxidation of thiourea and its NN′-ethylene derivative (L) proceeds via the formation of 1 : 1 complexes, [O₃CrL], which are considered to be sulphur bonded. These intermediates, formed within the time of mixing in the stopped-flow apparatus, have been characterised spectroscopically and thermodynamic parameters for reactions (i) have been measured. At 25 °C for thiourea. K= 380 ± 25 l^–2 mol^–2, ΔH=–9.8 ± 0.6 kcal mol^–1, H⁺+[(HO)CrO₃]^–+ L [graphic omitted] [O₃CrL]+ H₂O (i), and ΔS=–21 ± 3 cal K^–1 mol^–1, whilst for NN′-ethylenethiourea K= 211 ± 20 I^–2 mol^–2, ΔH=–8.8 ± 0.9 kcal mol^–1, and ΔS=–19 ± 4 cal K^–1 mol^–1. The kinetics of the electron-transfer reactions to yield Cr^III have been investigated. The reaction products in both cases are [Cr(H₂O)₆]³⁺ and [Chromium(III)–L]³⁺ complexes. In both systems, the principal path involves a reaction of overall stoicheiometry (ii) with the formation of a Cr^IV, 2H⁺+[O₃CrL]+ L [graphic omitted] Cr^IV+ L – L (ii), intermediate which undergoes further reduction. For L = thiourea at 25 °C, k= 33 I³ mol^–3 s^–1, ΔH^‡=⩽1 kcal mol^–1, and ΔS^‡=–45 ± 6 cal K^–1 mol^–1, whilst for NN′-ethylenethiourea k= 47.5 I³ mol^–3 s^–1, ΔH⩽1.5 kcal mol^–1, and ΔS^‡=–45 ± 5 cal K^–1 mol^–1. Details of other hydrogen ion-catalysed reactions are presented and the data are compared with those for other systems of this type.