Chigwada Tabitha R, Chikwana Edward, Simoyi Reuben H
Department of Chemistry, Portland State University, Portland, Oregon 97207-0751, USA.
J Phys Chem A. 2005 Feb 17;109(6):1081-93. doi: 10.1021/jp0458654.
The oxidation of 1-phenyl-2-thiourea (PTU) by chlorite was studied in aqueous acidic media. The reaction is extremely complex with reaction dynamics strongly influenced by the pH of reaction medium. In excess chlorite concentrations the reaction stoichiometry involves the complete desulfurization of PTU to yield a urea residue and sulfate: 2ClO2- + PhN(H)CSNH2 + H2O --> SO4(2-) + PhN(H)CONH2 + 2Cl- + 2H+. In excess PTU, mixtures of sulfinic and sulfonic acids are formed. The reaction was followed spectrophotometrically by observing the formation of chlorine dioxide which is formed from the reaction of the reactive intermediate HOCl and chlorite: 2ClO2- + HOCl + H+ --> 2ClO2(aq) + Cl- + H2O. The complexity of the ClO2- - PTU reaction arises from the fact that the reaction of ClO2 with PTU is slow enough to allow the accumulation of ClO2 in the presence of PTU. Hence the formation of ClO2 was observed to be oligooscillatory with transient formation of ClO2 even in conditions of excess oxidant. The reaction showed complex acid dependence with acid catalysis in pH conditions higher than pKa of HClO2 and acid retardation in pH conditions of less than 2.0. The rate of oxidation of PTU was given by -d[PTU]/dt = k1[ClO2-][PTU] + k2[HClO2][PTU] with the rate law: -d[PTU]/dt = Cl(III)[PTU]0/K(a1) + [H+] [k1K(a1) + k2[H+]]; where [Cl(III)]T is the sum of chlorite and chlorous acid and K(a1) is the acid dissociation constant for chlorous acid. The following bimolecular rate constants were evaluated; k1 = 31.5+/-2.3 M(-1) s(-1) and k2 = 114+/-7 M(-1) s(-1). The direct reaction of ClO2 with PTU was autocatalytic in low acid concentrations with a stoichiometric ratio of 8:5; 8ClO2 + 5PhN(H)CSNH2 + 9H2O --> 5SO4(2-) + 5PhN(H)CONH2 + 8Cl- + 18H+. The proposed mechanism implicates HOCl as a major intermediate whose autocatalytic production determined the observed global dynamics of the reaction. A comprehensive 29-reaction scheme is evoked to describe the complex reaction dynamics.
在酸性水介质中研究了亚氯酸盐对1-苯基-2-硫脲（PTU）的氧化反应。该反应极其复杂，反应动力学受反应介质pH值的强烈影响。在亚氯酸盐浓度过量时，反应化学计量涉及PTU完全脱硫生成尿素残基和硫酸盐：2ClO₂⁻ + PhN(H)CSNH₂ + H₂O → SO₄²⁻ + PhN(H)CONH₂ + 2Cl⁻ + 2H⁺。在PTU过量时，会形成亚磺酸和磺酸的混合物。通过分光光度法监测反应过程中二氧化氯的生成来跟踪反应，二氧化氯由活性中间体HOCl与亚氯酸盐反应生成：2ClO₂⁻ + HOCl + H⁺ → 2ClO₂(aq) + Cl⁻ + H₂O。ClO₂⁻ - PTU反应的复杂性源于ClO₂与PTU的反应足够缓慢，以至于在PTU存在时ClO₂会积累。因此，即使在氧化剂过量的条件下，也观察到二氧化氯的生成是少次振荡的，伴有二氧化氯的瞬态生成。该反应表现出复杂的酸依赖性，在高于HClO₂ pKa的pH条件下有酸催化作用，在pH小于2.0的条件下有酸抑制作用。PTU的氧化速率由 -d[PTU]/dt = k₁[ClO₂⁻][PTU] + k₂[HClO₂][PTU]给出，速率方程为：-d[PTU]/dt = [Cl(III)]ₜ[PTU]₀/Kₐ₁ + [H⁺][k₁Kₐ₁ + k₂[H⁺]]；其中[Cl(III)]ₜ是亚氯酸盐和亚氯酸的总和，Kₐ₁是亚氯酸的酸解离常数。评估得到以下双分子速率常数：k₁ = 31.5 ± 2.3 M⁻¹ s⁻¹，k₂ = 114 ± 7 M⁻¹ s⁻¹。在低酸浓度下，ClO₂与PTU的直接反应是自催化的，化学计量比为8:5；8ClO₂ + 5PhN(H)CSNH₂ + 9H₂O → 5SO₄²⁻ + 5PhN(H)CONH₂ + 8Cl⁻ + 18H⁺。所提出的反应机理表明HOCl是主要中间体，其自催化生成决定了观察到的反应整体动力学。提出了一个包含29个反应的综合反应方案来描述复杂的反应动力学。
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