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Oxyhalogen–Sulfur Chemistry: Kinetics and Mechanism of Oxidation of 1,3-Dimethylthiourea by Acidic Bromate
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2017-08-21 00:00:00 , DOI: 10.1021/acs.jpca.7b07587 Olufunke Olagunju 1 , Reuben H. Simoyi 1, 2
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2017-08-21 00:00:00 , DOI: 10.1021/acs.jpca.7b07587 Olufunke Olagunju 1 , Reuben H. Simoyi 1, 2
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The mechanism of oxidation of the well-known radical scavenger dimethylthiourea, DMTU, by acidic bromate was studied. The stoichiometry of the reaction is 4:3: 4BrO3– + 3CS(NHMe)2 + 3H2O → 3SO42– + 3CO(NHMe)2 + 6H+ + 4Br–. In excess acidic bromate, the reaction stoichiometry is 8:5: 8BrO3– + 5CS(NHMe)2 + H2O → 5SO42– + 5CO(NHMe)2 + 4Br2 + 2H+. In excess bromate, the reaction displays well-defined clock reaction characteristics in which initially there is a quiescent period before formation of bromine. The direct reaction of aqueous bromine with DMTU, with a bimolecular rate constant of k = (1.95 ± 0.15) × 105 M–1 s–1, is much faster than reactions that form bromine such that formation of bromine indicates complete consumption of DMTU. ESI spectrometry showed evidence for an oxidation pathway that passes through the sulfenic, sulfinic, and sulfonic acids before formation of sulfate. In contrast to the oxidation of tetramethylthiourea, these oxoacid intermediates are not as abundant or as stable. The final product of oxidation was dimethylurea, the desulfurized DMTU. EPR spectroscopy implicates more than one set of radical species. The absence of the dimeric DMTU species, even in excess reductant indicates negligible formation of thiyl radicals. This also precludes substantial formation of the sulfenic acid intermediate which would form the dimer from a condensation-type reaction with unreacted DMTU. A 20-step reaction mechanism network was modeled which gave a reasonable fit with experimental data.
中文翻译：
氧卤-硫化学：酸性溴酸盐氧化1，3-二甲基硫脲的动力学和机理
研究了酸性溴酸盐对著名的自由基清除剂二甲基硫脲DMTU的氧化机理。反应的化学计量比为4：3：4BrO 3 – + 3CS（NHMe）2 + 3H 2 O→3SO 4 2– + 3CO（NHMe）2 + 6H + + 4Br –。在过量的酸性溴酸盐中，反应化学计量为8：5：8BrO 3 – + 5CS（NHMe）2 + H 2 O→5SO 4 2– + 5CO（NHMe）2 + 4Br 2 + 2H +。在过量的溴酸盐中，反应显示出明确的时钟反应特征，其中在形成溴之前最初有一个静止期。溴水溶液与DMTU的直接反应，双分子速率常数k =（1.95±0.15）×10 5 M –1 s –1比形成溴的反应快得多，因此形成溴表明完全消耗了DMTU。ESI光谱法显示了在形成硫酸盐之前通过亚磺酸，亚磺酸和磺酸的氧化途径的证据。与四甲基硫脲的氧化相反，这些含氧酸中间体不那么丰富或不那么稳定。氧化的最终产物是二甲基脲，即脱硫的DMTU。EPR光谱法涉及一组以上的自由基。即使缺少过量的二聚体DMTU，也表明硫代自由基的形成可忽略不计。这也排除了亚硫酸中间体的大量形成，该亚磺酸中间体将由与未反应的DMTU的缩合型反应形成二聚体。
更新日期：2017-08-22
中文翻译：
氧卤-硫化学：酸性溴酸盐氧化1，3-二甲基硫脲的动力学和机理
研究了酸性溴酸盐对著名的自由基清除剂二甲基硫脲DMTU的氧化机理。反应的化学计量比为4：3：4BrO 3 – + 3CS（NHMe）2 + 3H 2 O→3SO 4 2– + 3CO（NHMe）2 + 6H + + 4Br –。在过量的酸性溴酸盐中，反应化学计量为8：5：8BrO 3 – + 5CS（NHMe）2 + H 2 O→5SO 4 2– + 5CO（NHMe）2 + 4Br 2 + 2H +。在过量的溴酸盐中，反应显示出明确的时钟反应特征，其中在形成溴之前最初有一个静止期。溴水溶液与DMTU的直接反应，双分子速率常数k =（1.95±0.15）×10 5 M –1 s –1比形成溴的反应快得多，因此形成溴表明完全消耗了DMTU。ESI光谱法显示了在形成硫酸盐之前通过亚磺酸，亚磺酸和磺酸的氧化途径的证据。与四甲基硫脲的氧化相反，这些含氧酸中间体不那么丰富或不那么稳定。氧化的最终产物是二甲基脲，即脱硫的DMTU。EPR光谱法涉及一组以上的自由基。即使缺少过量的二聚体DMTU，也表明硫代自由基的形成可忽略不计。这也排除了亚硫酸中间体的大量形成，该亚磺酸中间体将由与未反应的DMTU的缩合型反应形成二聚体。
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