Combined experimental and theoretical studies of the elimination kinetic of 2-methoxytetrahydropyran in the gas phase

The products formed in 2-methoxytetrahydropyran elimination reaction in the gas phase are 3, 4-dihydro-2H-pyran and methanol. The kinetic study was carried out in a static system, with the vessels deactivated with allyl bromide, and the presence of the free radical suppressor toluene. Temperature and pressure ranges were 400-450 °C and 25-83 Torr, respectively. The process is homogeneous, unimolecular, and follows a first-order rate law. The observed rate coefficient is represented by the following equation: log k (s^-1) = (13.95 ± 0.15) - (223.1 ± 2.1) (kJ mol^-1) (2.303RT)^-1. The reactant exists mainly in two low energy chair-like conformations, with the 2-methoxy group in axial or equatorial position. However, the transition state (TS) for the elimination of the two conformers is the same. Theoretical calculations of this reaction were carried for two possible mechanisms from these conformations by using DFT functionals B3LYP, MPW1PW91, and PBE with the basis set 6-31G(d,p) and 6-31G++(d,p). The calculation results demonstrate that 2-methoxytetrahydropyran exists mainly in two conformations, with the 2-methoy group in axial or equatorial position, that are thermal in equilibrium. The average thermodynamic and kinetic parameters, taking into account the populations of the conformers in the equilibrium, are in good agreement with experimental values at B3LYP/6-31++(d,p) level of theory.