Dimensiosolvatic Effects. IV. Topomerization in Alkyl<b><i>α</i></b>-Chlorobenzyl Ethers and Insights into Mechanisms of Their Thermolyses
作者:Michinori Oki、Hiroshi Ikeda、Hiromichi Miyake、Hirohito Mishima、Shinji Toyota
DOI:10.1246/bcsj.71.915
日期:1998.4
Rates of topomerization in α-chlorobenzyl ethyl ether and its p-methyl as well as p-methoxy derivatives were determined in various solvents by the dynamic NMR method. The topomerization process was deduced to be ionic because the rates are enhanced in polar solvents as well as by electron-donating substituents. Concentration dependence study of the topomerization in carbon tetrachloride revealed that the observed process is unimolecular; the rates are not affected by concentration of the substrate if the concentration is lower than 0.2 mol L−1. The rates of topomerization are smaller for solutions in bulky solvents than in small solvents, the effects being clear for compounds with electron-donating substituents. The key feature of the kinetic parameters is that the entropy of activation is large and negative for all the solvents examined. The reaction mechanisms are discussed on the basis of these data. The effects of the molecular size of the solvent, dimensiosolvatic effects, are attributed to the effectiveness of the solvation in the formation of ion pairs. The results show that thermolyses of alkyl α-chlorobenzyl ethers are much slower reactions than the ionization. The nature of the reaction is discussed.
利用动态NMR方法,在不同溶剂中测定了α-氯苄基乙基醚及其对甲基和对甲氧基衍生物的异构化速率。推断异构化过程为离子型,因为在极性溶剂中以及电子供体取代基的存在下,异构化速率得到增强。通过在四氯化碳中对异构化的浓度依赖性研究揭示,观察到的过程是单分子的;如果底物浓度低于0.2 mol L−1,异构化速率不受浓度的影响。在大体积溶剂中溶液的异构化速率比在小体积溶剂中小,对于具有电子供体取代基的化合物,这种效应很明显。动力学参数的关键特征是,对于所有检测的溶剂,活化熵都是大且负值。基于这些数据讨论了反应机理。溶剂分子大小、溶剂维度效应的影响归因于离子对形成中的溶剂化效果。结果表明,烷基α-氯苄基醚的热分解反应比电离反应慢得多。讨论了反应的性质。