Beijing Key Laboratory of Environmental and Viral Oncology, College of Life Science & Bioengineering, Beijing University of Technology, Beijing 100124, China.
Environ Sci Process Impacts. 2019 Apr 17;21(4):761-770. doi: 10.1039/c8em00581h.
Chloropicrin (TCNM) as one of the most frequently detected nitrogenous disinfection byproducts (N-DBPs) has attracted extensive attention due to its high toxicity. Although much research work on TCNM has been done, its formation mechanism during chlorination has not been known clearly yet. In this study, TCNM formation mechanisms from methylamine (MA) during chlorination, including N-chlorination of MA by hypochlorous acid to generate dichloromethylamine (DCMA) first and then oxidation of DCMA to form nitromethane (NM) and chloronitromethane (CNM), and finally TCNM formation from C-chlorination of NM and CNM, were investigated by using the DFT method. The calculated results show that in N-chlorination of MA, 2-3 water molecules involved in the reaction facilitate Cl+ and proton transfer with the activation free energies (ΔG≠) for the first and second chlorination in the range of 4-7 and 14-17 kcal mol-1, respectively, which are in good agreement with the experimental results. Formation of NM and CNM proceeds through a series of elimination, addition, and oxidation reactions with ΔG≠ of the rate-limiting steps being around 34-37 kcal mol-1, and the subsequent C-chlorination of methyl in NM and CNM by hypochlorous acid is a rapid process with ΔG≠ below 7 kcal mol-1. This infers that the TCNM formation mechanism from DCMA is more likely to undergo first N-oxidation and then C-chlorination. These results can explain the experimental findings that the molar yield of TCNM from MA during chlorination is low (<0.1%) whereas that from NM is rather high (∼45%). This work will be helpful to elucidate formation mechanisms of all the halonitromethanes during chlorination.
三氯硝基甲烷（TCNM）作为最常检测到的含氮消毒副产物（N-DBPs）之一，由于其高毒性而引起了广泛关注。尽管已经对 TCNM 进行了大量研究，但氯化过程中 TCNM 的形成机制仍不清楚。在这项研究中，采用密度泛函理论（DFT）方法研究了氯化过程中甲基胺（MA）生成 TCNM 的机制，包括次氯酸将 MA 进行 N-氯代生成二氯甲基胺（DCMA），然后 DCMA 被氧化生成硝基甲烷（NM）和氯代硝基甲烷（CNM），最后 NM 和 CNM 进行 C-氯代生成 TCNM。计算结果表明，在 MA 的 N-氯代反应中，2-3 个水分子参与反应，有利于 Cl+和质子转移，其第一步和第二步氯代反应的活化自由能（ΔG≠）分别在 4-7 和 14-17 kcal/mol 范围内，这与实验结果吻合较好。NM 和 CNM 的形成通过一系列消除、加成和氧化反应进行，速率限制步骤的ΔG≠约为 34-37 kcal/mol，随后次氯酸对 NM 和 CNM 中甲基的 C-氯代是一个快速过程，ΔG≠低于 7 kcal/mol。这表明，DCMA 生成 TCNM 的机制更有可能先经历 N-氧化，然后再经历 C-氯代。这些结果可以解释实验发现，氯化过程中 MA 生成 TCNM 的摩尔产率较低（<0.1%），而 NM 生成 TCNM 的摩尔产率较高（约 45%）。这项工作有助于阐明氯化过程中所有卤代硝基甲烷的形成机制。
