Department of Chemistry, University of North Carolina-Asheville, One University Heights, Asheville, North Carolina 28804-8511, USA.
J Phys Chem A. 2010 Sep 30;114(38):10395-402. doi: 10.1021/jp1047166.
The recombination of chloromethyl and t-butyl radicals at room temperature was used to generate neopentyl chloride molecules with 89 kcal mol(-1) of internal energy. The observed unimolecular reactions, which give 2-methyl-2-butene and 2-methyl-1-butene plus HCl, as products, are explained by a mechanism that involves the interchange of a methyl group and the chlorine atom to yield 2-chloro-2-methylbutane, which subsequently eliminates hydrogen chloride by the usual four-centered mechanism to give the observed products. The interchange isomerization process is the rate-limiting step. Similar experiments were done with CD(2)Cl and C(CH(3))(3) radicals to measure the kinetic-isotope effect to help corroborate the proposed mechanism. Density functional theory was employed at the B3PW91/6-31G(d',p') level to verify the Cl/CH(3) interchange mechanism and to characterize the interchange transition state. These calculations, which provide vibrational frequencies and moments of inertia of the molecule and transition state, were used to evaluate the statistical unimolecular rate constants. Matching the calculated and experimental rate constants, gave 62 ± 2 kcal mol(-1) as the threshold energy for interchange of the Cl atom and a methyl group. The calculated models also were used to reinterpret the thermal unimolecular reactions of neopentyl chloride and neopentyl bromide. The previously assumed Wagner-Meerwein rearrangement mechanism for these reactions can be replaced by a mechanism that involves the interchange of the halogen atom and a methyl group followed by HCl or HBr elimination from 2-chloro-2-methylbutane and 2-bromo-2-methylbutane. Electronic structure calculations also were done to find threshold energies for several related molecules, including 2-chloro-3,3-dimethylbutane, 1-chloro-2-methyl-2-phenylpropane, and 1-chloro-2-methyl-2-vinylpropane, to demonstrate the generality of the interchange reaction involving a methyl, or other hydrocarbon groups, and a chlorine atom. The interchange of a halogen atom and a methyl group located on adjacent carbon atoms can be viewed as an extension of the halogen atom interchange mechanisms that is common in 1,2-dihaloalkanes.
室温下氯甲基和叔丁基自由基的复合生成了具有 89 kcal mol(-1) 内能的新戊基氯分子。观察到的单一分子反应生成 2-甲基-2-丁烯和 2-甲基-1-丁烯加 HCl，这一过程通过一个涉及甲基和氯原子交换生成 2-氯-2-甲基丁烷的机制来解释，该化合物随后通过通常的四中心机制消除 HCl 得到观察到的产物。交换异构化过程是速率限制步骤。用 CD(2)Cl 和 C(CH(3))(3) 自由基进行了类似的实验，以测量动力学同位素效应来帮助证实所提出的机制。采用密度泛函理论在 B3PW91/6-31G(d',p')水平上验证了 Cl/CH(3) 交换机制，并对交换过渡态进行了特征化。这些计算提供了分子和过渡态的振动频率和转动惯量，用于评估统计单一分子速率常数。使计算和实验速率常数相匹配，得到 62 ± 2 kcal mol(-1) 作为 Cl 原子和甲基交换的阈能。计算模型还被用于重新解释新戊基氯和新戊基溴的热单一分子反应。可以用涉及卤素原子和甲基交换随后从 2-氯-2-甲基丁烷和 2-溴-2-甲基丁烷中消除 HCl 或 HBr 的机制取代这些反应的 Wagner-Meerwein 重排机制。电子结构计算还用于确定几个相关分子的阈能，包括 2-氯-3,3-二甲基丁烷、1-氯-2-甲基-2-苯基丙烷和 1-氯-2-甲基-2-乙烯基丙烷，以证明涉及甲基或其他烃基和氯原子的交换反应的普遍性。卤素原子和位于相邻碳原子上的甲基的交换可以被视为在 1,2-二卤代烷中常见的卤素原子交换机制的扩展。
