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 CrIII 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 CrIV, 2H++[O3CrL]+ L [graphic omitted] CrIV+ 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.