Theoretical study of the mechanism for the gas-phase pyrolysis kinetics of 2-methylbenzyl chloride

The study of the kinetics and mechanism of dehydrochlorination reaction of 2-methyl benzyl chloride in the gas phase was carried out by means of electronic structure calculations using ab initio Mõller-Plesset MP2/6-31G(d,p), and Density Functional Theory (DFT) methods: B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d,p)], PBE/6-31G(d,p), PBE/6-31++G(d,p). Investigated reaction pathways comprise: Mechanism I, a concerted reaction through a six-centered cyclic transition state (TS) geometry; Mechanism II, a 1,3-chlorine shift followed by beta-elimination and Mechanism III, a single-step elimination with simultaneous HCl and benzocyclobutene formation through a bicyclic type of TS. Calculated parameters ruled out Mechanism III and suggest the elimination reaction may occur by either unimolecular Mechanism I or Mechanism II. However, the TS of the former is 20 kJ/mole more stable than the TS of the latter. Consequently, the Mechanism I seem to be more probable to occur. The rate-determining process is the breaking of C-Cl bond. The involvement of π-electrons of the aromatic system was demonstrated by NBO charges and bond order calculations. The reaction is moderately polar in nature.