Theoretical study on the formation mechanism of polychlorinated dibenzothiophenes/thianthrenes from 2-chlorothiophenol molecules
Theoretical study on the formation mechanism of polychlorinated dibenzothiophenes/thianthrenes from 2-chlorothiophenol molecules
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2-氯苯硫酚分子形成多氯二苯并噻吩/噻蒽机理的理论研究
DOI：
10.1016/j.jes.2017.05.007
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发表时间：
2018
影响因子：
6.9
通讯作者：
Zhang Aiqian
中科院分区：
文献类型：
--
作者：
Yu Xiaoqing;Chang Jiamin;Liu Xian;Pan Wenxiao;Zhang Aiqian
多氯代二苯并噻吩/噻吩（PCDT/TAs）是一种类似于多氯代二苯并对二恶英/二苯并呋喃（PCDD/Fs）的硫化化合物，已有充分的证据表明，氯代噻吩前体可发生自由基-自由基偶联反应。然而，目前对PCDT/TAs形成机制的理解仅限于氯噻吩氧自由基作为PCDT/TAs唯一必需的中间体这一固有观点。本研究探讨了PCDT/TAs形成的反应途径，涉及两种新型自由基，即。取代苯基自由基和取代噻吩氧基二自由基。以2-氯噻吩酚（2-CTP）作为氯噻吩酚的模型化合物，我们发现2-CTP与H自由基反应时，除了讨论较多的氯噻吩氧基自由基外，还容易形成取代苯基自由基和取代噻吩氧基二自由基。此外，这些自由基的直接自偶联和交叉偶联可导致PCDT/TAs的形成，包括1-一氯噻吩（1- mcta）、1,6-二氯噻吩（1,6- dcta）、4,6-二氯二苯并噻吩（4,6- dcdt）和1,6-二氯二苯并噻吩（1,6- dcdt）。在这项工作中提出的途径被证明是热力学和动力学有利的。特别地，从能量的角度比较了硫化二恶英和氧化二恶英体系的形成机制，表明与PCDD/Fs相比，用硫原子取代氧大大降低了PCDT/TA形成过程中速率控制步骤的激活障碍。本工作的计算结果可以提高我们对氯噻吩前体PCDT/TAs形成机制的理解，并为环境科学家提供信息。
Homogeneous formation of polychlorinated dibenzothiophenes/thianthrenes (PCDT/TAs), sulfurated compounds analogous to polychlorinated dibenzo-p-dioxin/dibenzofurans (PCDD/Fs), has been well-documented to occurviaradical–radical coupling reactions from chlorinated thiophenol precursors. However, the current understanding of the formation mechanism of PCDT/TAs is exclusively limited to the inherent point of view that chlorothiophenoxy radicals act as the only required intermediates for PCDT/TAs. This study investigates reaction pathways for the formation of PCDT/TAs involving two new types of radical species,i.e., substituted phenyl radicals and substituted thiophenoxyl diradicals. Taking 2-chlorothiophenol (2-CTP) as a model compound for chlorothiophenols, we found that apart from the mostly discussed chlorothiophenoxy radicals, substituted phenyl radicals and substituted thiophenoxyl diradicals could also be readily formedviathe reaction of 2-CTP with H radicals. Furthermore, direct self- and cross-coupling of these radicals can result in the formation of PCDT/TAs, including 1-monochlorothianthrene (1-MCTA), 1,6-dichlorothianthrene (1,6-DCTA), 4,6-dichlorodibenzothiophene (4,6-DCDT) and 1,6-dichlorodibenzothiophene (1,6-DCDT). The pathways proposed in this work are proven to be both thermodynamically and kinetically favorable. Particularly, comparisons were made between the formation mechanisms of sulfurated and oxygenated dioxin systems from an energetic point view, showing that replacing oxygen with sulfur atoms greatly reduces the activation barriers of the rate-controlling steps involved in the PCDT/TA formation processes compared with those involved for PCDD/Fs. The calculated results in this work may improve our understanding of the formation mechanism of PCDT/TAs from chlorothiophenol precursors and should be informative to environmental scientists.