Electrophilic aromatic reactivities via pyrolysis of 1-arylethyl acetates. Part 15. Non-additivity of chloro-substituent effects: mechanism of the elimination
作者:Ernest Glyde、Roger Taylor
DOI:10.1039/p29770001541
日期:——
gas-phase elimination of acetic acid from polychloro-substituted 1-arylethyl acetates, measured between 642.0 and 692.6 K, show that the effects of the chloro-substituents are not additive in this reaction. The reactivities of the esters are greater than calculated on the basis of additivity of the substituent effects, the deviation being greater the less reactive the ester. This result, which parallels
从多氯取代的1-芳基乙酸乙酯的气相消除乙酸的速率(在642.0和692.6 K之间测得)表明,氯取代基的作用在该反应中没有加和作用。酯的反应性大于根据取代基作用的加和性计算的反应性,该酯的反应性越小,偏差越大。该结果与在聚甲基取代的酯上观察到的结果相似,归因于过渡态极性的变化,使得过渡态极性随着酯反应性的降低而降低。具有邻位邻氯取代基的化合物显示出特别增强的反应性,这可能是由于邻位之间的直接对位轨道重叠增强了-氯原子和初期的侧链α-碳化。因为在硝化反应中,五氯取代比2,3,5,6-四氯取代产生的失活少,并且前者的支护作用增加可能是负责任的:在硝化作用中,该作用表现为亲电试剂促进配位作用的增加。作为替代解释,目前不能排除由于空间拥挤而导致的C–Cσ键重叠减少(从而导致–I效应减小)。