School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
J Phys Chem A. 2012 Aug 9;116(31):8119-29. doi: 10.1021/jp304768n. Epub 2012 Jul 26.
We study how the degree of fluorine substitution for hydrogen atoms in ethene affects its reactivity in the gas phase. The reactions of a series of small fluorocarbon cations (CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+)) with ethene (C(2)H(4)), monofluoroethene (C(2)H(3)F), 1,1-difluoroethene (CH(2)CF(2)), and trifluoroethene (C(2)HF(3)) have been studied in a selected ion flow tube. Rate coefficients and product cations with their branching ratios were determined at 298 K. Because the recombination energy of CF(2)(+) exceeds the ionization energy of all four substituted ethenes, the reactions of this ion produce predominantly the products of nondissociative charge transfer. With their lower recombination energies, charge transfer in the reactions of CF(+), CF(3)(+), and C(2)F(4)(+) is always endothermic, so products can only be produced by reactions in which bonds form and break within a complex. The trends observed in the results of the reactions of CF(+) and CF(3)(+) may partially be explained by the changing value of the dipole moment of the three fluoroethenes, where the cation preferentially attacks the more nucleophilic part of the molecule. Reactions of CF(3)(+) and C(2)F(4)(+) are significantly slower than those of CF(+) and CF(2)(+), with adducts being formed with the former cations. The reactions of C(2)F(4)(+) with the four neutral titled molecules are complex, giving a range of products. All can be characterized by a common first step in the mechanism in which a four-carbon chain intermediate is formed. Thereafter, arrow-pushing mechanisms as used by organic chemists can explain a number of the different products. Using the stationary electron convention, an upper limit for Δ(f)H°(298)(C(3)F(2)H(3)(+), with structure CF(2)═CH-CH(2)(+)) of 628 kJ mol(-1) and a lower limit for Δ(f)H°(298)(C(2)F(2)H(+), with structure CF(2)═CH(+)) of 845 kJ mol(-1) are determined.
我们研究了乙烯中氢原子被氟原子取代的程度如何影响其在气相中的反应活性。在选定的离子流管中，研究了一系列小氟代碳阳离子（CF(+)、CF(2)(+)、CF(3)(+) 和 C(2)F(4)(+)）与乙烯（C(2)H(4)）、单氟乙烯（C(2)H(3)F）、1,1-二氟乙烯（CH(2)CF(2)）和三氟乙烯（C(2)HF(3))的反应。在 298 K 时确定了速率系数和产物阳离子及其分支比。由于 CF(2)(+)的重组能超过所有四种取代乙烯的电离能，因此该离子的反应主要产生非解离电荷转移的产物。由于它们的重组能较低，CF(+)、CF(3)(+) 和 C(2)F(4)(+)反应中的电荷转移总是吸热的，因此只能通过在复合物内形成和断裂键的反应来产生产物。CF(+)和 CF(3)(+)反应结果中观察到的趋势部分可以通过三种氟代乙烯的偶极矩值的变化来解释，其中阳离子优先攻击分子的更亲核部分。CF(3)(+)和 C(2)F(4)(+)的反应明显慢于 CF(+)和 CF(2)(+)的反应，前两种阳离子形成加合物。C(2)F(4)(+)与四种中性标题分子的反应很复杂，生成多种产物。所有产物都可以通过形成四碳链中间产物的共同第一步来描述。此后，正如有机化学家所使用的箭头推机制可以解释许多不同的产物。使用固定电子公约，（CF(2)═CH-CH(2)(+)，结构）的 Δ(f)H°(298)(C(3)F(2)H(3)(+)）的上限为 628 kJ mol(-1)，（CF(2)═CH(+)，结构）的 Δ(f)H°(298)(C(2)F(2)H(+))的下限为 845 kJ mol(-1)。
