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

The 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.