The reaction mechanism of the gas-phase thermal decomposition kinetics of neopentyl halides: A DFT study

The kinetics and mechanisms of the gas-phase elimination reactions of neopentyl chloride and neopentyl bromide have been studied by means of electronic structure calculations using density functional methods: B3LYP/6-31G(d,p), B3LYP/ 6-31++G(d,p), MPW1PW91/6-31G(d,p), MPW1PW91/6-31++G(d, p), PBEPBE/6-31G(d,p), PBEPBE /6-31++G(d,p). The reaction channels that account in products formation have a common first step involving a Wagner-Meerwein rearrangement. The migration of the halide from the terminal carbon to the more substituted carbon is followed by beta-elimination of HCl or HBr to give two olefins: the Sayzeff and Hoffmann products. Theoretical calculations demonstrated that these eliminations proceed through concerted asynchronous process. The transition state (TS) located for the rate-determining step shows the halide detached and bridging between the terminal carbon and the quaternary carbon, while the methyl group is also migrating in a concerted fashion. The TS is described as an intimate ion-pair with a large negative charge at the halide atom. The concerted migration of methyl group provides stabilization of the TS by delocalizing the electron density between the terminal carbon and the quaternary carbon. The B3LYP/6-31++G(d,p) allows to obtain reasonable energies and enthalpies of activation. The nature of these reactions is examined in terms of geometrical parameters, electron distribution, and bond order analysis.