分子结构对羧酸酯亲核取代反应速率的定量影响规律

The quantitative influence of molecular structure on the nucleophilic substitution reaction rate of carboxylic esters

羧酸酯的亲核取代反应速率受羧酸酯的离去基团(LG)、非离去基团(NLG)和亲核试剂(Nu)结构的影响.从文献整理了大量结构多样的羧酸酯与各种亲核试剂发生亲核取代反应的表观二级速率常数kN,基于亲核取代反应机理,用非离去基团的极化效应指数PEI(NLG)和基团体积参数GVI(NLG),亲核试剂与离去基团的体积参数之比GVI(Nu)/GVI(LG),以及亲核试剂与离去基团共轭酸的p Ka之差(p Ka(Nu)-p Ka(LG))分别表征非离去基团、离去基团和亲核试剂的结构特征及亲核试剂与离去基团的相互竞争,并用上述参数对73组亲核取代反应的logkN建立多元线性回归模型,得到较好的结果.该模型所用参数简便,物理意义明确,为从分子结构特征定量估算羧酸酯亲核取代反应的速率提供了理论依据.

The nucleophilic substitution reaction rate of carboxylic ester is influenced by the structures of leaving group （LG）,non-leaving group （NLG）of carboxylic ester and the nucleophile （Nu）.A large number of apparent second -order rate constants （kN）were colletcd and analyzed,which of nucleophilic substitution reactions between structurally diverse carboxylic esters and a variety of nucleophiles.Based on the mechanism of nucleophilic substitution reaction,the polarizability effect index PEI（NLG）and group volume index GVI（NLG） of non-leaving group,the ratio of group volume index of nucleophile to that of the leaving group GVI（Nu）/GVI （LG）,the difference between pKa of the conjugate acid of nucleophile and that of leaving group （pKa （Nu）-pKa（LG））,were employed to characterize the structures of non-leaving group,leaving group and nucleophile. logkN of 73 nucleophilic substitution reactions were correlated to the structural descriptors mentioned above,and multiple linear regression model was established with good performances.The parameters used in the model were simple and had clear physical meanings.This study provided a theoretical basis for quantitatively estimating the nucleophilic substitution reaction rate of carboxylic esters from molecular structures.