吡咯在氯消毒中反应机制的理论研究

Reaction Mechanisms of Pyrrole during Chlorination: A Computational Study

摘要: 含氮芳杂环卤代消毒副产物作为一种新兴的饮用水消毒副产物，其毒性显著高于大多数的传统脂肪族消毒副产物，且在水环境中检出率高，已成为饮用水安全领域关注的焦点。然而，关于它们的形成机制和前体却知之甚少。因此，本研究采用量子化学计算方法，探究了吡咯在氯消毒中的反应机制和各种可能的反应产物。考虑到水体中受海水入侵等影响下溴离子的存在，本研究同时关注了前体与次氯酸和次溴酸的反应。在这一过程中，同时探究了取代和加成两种可能的反应机制，并得出产物最可能的形成路径。研究结果表明，吡咯所有的氯/溴化反应中，S_EAr反应是最有利的反应，取代位点的顺序是C2、C5、C3和C4，分别生成2-氯/溴-吡咯、2,5-二氯/溴-吡咯、2,3,5-三氯/溴-吡咯和2,3,4,5-四氯/溴-吡咯，其k_est-rls分别为~10⁴/10⁶、10⁴/10⁶、10/10和10²/10³ M⁻¹ s⁻¹。其中，2,3,5-三溴-吡咯和2,3,4,5-四溴-吡咯和实验检测到的三溴吡咯和四溴吡咯一致。值得注意的是，一氯/溴化和二氯/溴化反应发生的很快，而三氯/溴化和四氯/溴化反应相对慢一些，且溴化反应在动力学上似乎比氯化反应更可行，其k_est-rls值比氯化反应高出1~2个数量级。因此，在氯化和溴化反应中，吡咯C位点是反应的活性位点，并通过S_EAr反应机制形成最终产物2,3,4,5-四氯/溴-吡咯。以上研究结果表明，吡咯很可能是卤代吡咯的潜在前体。

Abstract: Aromatic nitrogenous(N-) heterocyclic halogenated disinfection byproducts (DBPs), as emerging DBPs in drinking water, exhibit significantly higher toxicity than most traditional aliphatic disinfection byproducts and are frequently detected in water environments, thereby becoming the focus in the field of drinking water safety. Therefore, this study employs quantum chemical computational methods to investigate the reaction mechanisms of pyrrole during chlorination and the various potential reaction products. Considering the presence of bromide ions in water bodies due to seawater intrusion and other influences, this study also focuses on the reactions of precursors with hypochlorous acid and hypobromous acid. During this process, both substitution and addition mechanisms were simultaneously investigated, and the most probable formation pathways for the products were determined. In all chlorination/bromination reactions of pyrrole, the S_EAr reaction remains the most favorable, with the order of substitution sites being C2, C5, C3, and C4, producing 2-Cl/Br-pyrrole, 2,5-di-Cl/Br-pyrrole, 2,3,5-tri-Cl/Br-pyrrole, and 2,3,4,5-tetra-Cl/Br-pyrrole, respectively. Their k_est-rls are approximately 10⁴/10⁶, 10⁴/10⁶, 10/10, and 10²/10³ M⁻¹ s⁻¹. Among the products, 2,3,5-tri-Br-pyrrole and 2,3,4,5-tetra-Br-pyrrole are consistent with the tribromopyrrole and tetrabromopyrrole detected in the experiment. It is noteworthy that the mono-chlorination/bromination and di-chlorination/bromination reactions occur rapidly, while the tri-chlorination/bromination and tetra-chlorination/bromination reactions are relatively slower. Moreover, bromination reactions seem to be more feasible kinetically than chlorination reactions, with their k_est-rls values being 1~2 orders of magnitude higher than those of chlorination reactions. Therefore, in chlorination and bromination reactions, the C sites of pyrrole are the active sites for the reaction, and the final products 2,3,4,5-tetra-Cl/Br-pyrrole are formed through the S_EAr reaction mechanism. Given the reported presence of tribromopyrrole and tetrabromopyrrole in actual water bodies, the above results suggest that pyrrole may be a potential precursor of halogenated pyrrole.