Makogon Oksana, Flyunt Roman, Tobien Thomas, Naumov Sergej, Bonifacić Marija
Institute of Physico-Chemistry, National Academy of Science of the Ukraine, Naukova Street 3a, UA-79053 L'viv, Ukraine.
J Phys Chem A. 2008 Jul 3;112(26):5908-16. doi: 10.1021/jp711894k. Epub 2008 Jun 10.
Using pulse radiolysis and steady-state gamma-radiolysis techniques, it has been established that, in air-saturated aqueous solutions, peroxyl radicals CH 2HalOO () (Hal = halogen) derived from CH 2Cl 2 and CH 2Br 2 react with dimethyl selenide (Me 2Se), with k on the order of 7 x 10 (7) M (-1) s (-1), to form HCO 2H, CH 2O, CO 2, and CO as final products. An overall two-electron oxidation process leads directly to dimethyl selenoxide (Me 2SeO), along with oxyl radical CH 2HalO (). The latter subsequently oxidizes another Me 2Se molecule by a much faster one-electron transfer mechanism, leading to the formation of equal yields of CH 2O and the dimer radical cation (Me 2Se) 2 (+). In absolute terms, these yields amount to 18% and 28% of the CH 2ClO () and CH 2BrO () yields, respectively, at 1 mM Me 2Se. In competition, CH 2HalO () rearranges into ()CH(OH)Hal. These C-centered radicals react further via two pathways: (a) Addition of an oxygen molecule leads to the corresponding peroxyl radicals, that is, species prone to decomposition into H (+)/O 2 (-) and formylhalide, HC(O)Hal, which further degrades mostly to H (+)/Hal (-) and CO. (b) Elimination of HHal yields the formyl radical H-C()=O with a rate constant of about 6 x 10 (5) s (-1) for Hal = Cl. In an air-saturated solution, the predominant reaction pathway of the H-C()=O radical is addition of oxygen. The formylperoxyl radical HC(O)OO () thus formed reacts with Me 2Se via an overall two-electron transfer mechanism, giving additional Me 2SeO and formyloxyl radicals HC(O)O(). The latter rearrange via a 1,2 H-atom shift into ()C(O)OH, which reacts with O2 to give CO2 and O2()(-). The minor fraction of H-C()=O undergoes hydration, with an estimated rate constant of k approximately 2 x 10(5) s(-1). The resulting HC()(OH)2 radical, upon reaction with O2, yields HCO 2H and H (+)/O2(*-). Some of the conclusions about the reactions of halogenated alkoxyl radicals are supported by quantum chemical calculations [B3LYP/6-31G(d,p)] taking into account the influence of water as a dielectric continuum [by the self-consistent reaction field polarized continuum model (SCRF=PCM) technique]. Based on detailed product studies, mechanisms are proposed for the free-radical degradation of CH 2Cl 2 and CH 2Br 2 in the presence of oxygen and an electron donor (namely, Me 2Se in this study), and properties of the reactive intermediates are discussed.
使用脉冲辐解和稳态伽马辐解技术已证实，在空气饱和的水溶液中，由二氯甲烷（CH₂Cl₂）和二溴甲烷（CH₂Br₂）衍生的过氧自由基CH₂HalOO⁎（Hal = 卤素）与二甲基硒（Me₂Se）反应，反应速率常数k约为7×10⁷ M⁻¹ s⁻¹，最终产物为甲酸（HCO₂H）、甲醛（CH₂O）、二氧化碳（CO₂）和一氧化碳（CO）。一个总的双电子氧化过程直接导致生成二甲基亚硒醚（Me₂SeO）以及氧基自由基CH₂HalO⁎。后者随后通过快得多的单电子转移机制氧化另一个Me₂Se分子，导致生成等量产率的CH₂O和二聚自由基阳离子（Me₂Se）₂⁎⁺。绝对而言，在1 mM Me₂Se时，这些产率分别相当于CH₂ClO⁎和CH₂BrO⁎产率的18%和28%。在竞争反应中，CH₂HalO⁎重排为⁎CH(OH)Hal。这些以碳为中心的自由基通过两条途径进一步反应：（a）与氧分子加成生成相应的过氧自由基，即易于分解为H⁺/O₂⁎⁻和甲酰卤HC(O)Hal的物种，甲酰卤大多进一步降解为H⁺/Hal⁻和CO。（b）消除HHal生成甲酰基自由基H - C⁎=O，对于Hal = Cl，其速率常数约为6×10⁵ s⁻¹。在空气饱和溶液中，H - C⁎=O自由基的主要反应途径是与氧加成。由此形成的甲酰过氧自由基HC(O)OO⁎通过总的双电子转移机制与Me₂Se反应，生成额外的Me₂SeO和甲酰氧基自由基HC(O)O⁎。后者通过1,2 - H原子迁移重排为⁎C(O)OH，其与O₂反应生成CO₂和O₂⁎⁻。一小部分H - C⁎=O发生水合反应，并估计速率常数k约为2×10⁵ s⁻¹。生成的HC⁎(OH)₂自由基与O₂反应，生成HCO₂H和H⁺/O₂⁎⁻。关于卤代烷氧基自由基反应的一些结论得到了量子化学计算[B3LYP/6 - 31G(d,p)]的支持，该计算考虑了水作为介电连续介质的影响[通过自洽反应场极化连续介质模型（SCRF = PCM）技术]。基于详细的产物研究，提出了在有氧和电子供体（即本研究中的Me₂Se）存在下CH₂Cl₂和CH₂Br₂自由基降解的机制，并讨论了反应中间体的性质。
