Density functional theory and ab initio study on the reaction mechanisms of the homogeneous, unimolecular elimination kinetics of selected 1‐chloroalkenes in the gas phase

Abstract

AbstractThe mechanisms for the unimolecular elimination kinetics of selected 1‐chloroalkenes in the gas phase were studied at MPW1PW91/6‐31G(d,p), MPW1PW91/6‐31++G(d,p), G3, and G3MP2 levels of theory. Two possible unimolecular mechanisms were considered: mechanism A as a concerted 1,2 elimination process through four‐membered cyclic transition state (TS). mechanism B describing the anchimeric assistance of the double bond in HCl elimination previously suggested in the literature. Calculated parameters suggest that the elimination reactions of 1‐chloroalkenes proceed through mechanism A, in view of the higher energy of activation associated with mechanism B. Density functional method MPW1PW91/6‐31G(d,p) calculated parameters gave a better agreement with the experimental values than G3 and G3MP2. The changes along the reaction path of mechanism A were followed by geometric parameters, natural bond orbital charges, and bond order analysis, suggesting the rate‐determining process is the breaking of CCl bond in the TS. The dehydrochlorination of chloroalkenes occurs in a concerted nonsynchronous fashion with stabilization of the TS by π‐electron delocalization from the neighboring bond. Isomerization reactions for 4‐chloro‐1‐butene, 4‐chloro‐2‐methyl‐1‐butene, and 4‐chloro‐1‐butene are unlikely at the experimental reaction condition because of the higher the enthalpies and energies of activation. © 2012 Wiley Periodicals, Inc.