2—溴羧酸气相热消除反应及其取代基效应的理论研究

用半经验分子轨道方法ＡＭ１对２－溴－丙酸与２－溴－３－甲基－丁酸的气相热消除反应的微观机理进行了细致的讨论，同时研究了不同取代基对α－溴代酸气相热消除反应的影响。研究结果表明：α－溴代酸气相热消除反应的机理均为２步反应，具有五元环过渡态的第一步过程是整个反应的速控步骤，这与实验工作者的预测一致，计算的取代基效应与有机电子论的预测相一致。

THEORETICAL STUDIES ON THE MECHANISM OF 2-BROMOCARBOXYL ACID AND

The gas-phase elimination processes of 2-bromocarboxylic acids and the substituent effects are theoretically studied by using the send-empirical molecular orbital AM1 method. The optimization of all geometric configurations including reactants, transition stated,intermediates and products are carried out by Berny method.Each transition state is further confirmed by vibrational analysis and characterized by unique imaginary vibration frequency.The main conclusions are (1) The elimination process of 2-bromocarboxylic acids is a stepwise reaction. In the first step, a 3-membered ring intermediate is formed via a 5-membered ring transition state, While the product is formed, in the second step, via a 3-membered ring transition state. The first step is the rate-determining one. (2) By comparing the energy barriers, it is realized that the substituents with electronic-drawing effects (e. g. CF2H, CF3, etc.) will be unfavorable for the reaction, at the same time, the substituents with electronic-donating effects (e.g. CH3,, CH(CH3)2,. etc.) will be favorable for the eliminating hydrogen bromide. It should also be pointed out that the large steric effects of the substituting groups will probably have an influence on this elimination reaction.