Experimental and DFT studies for the kinetics and mechanism of the pyrolysis of 2-(4-substituted-phenoxy)tetrahydro-2H-pyranes in the gas-phase

DFT theoretical calculations were performed to 2-(4-substituted-phenoxy) tetrahydro-2H-pyranes with groups H, OCH3, C(CH3)3, CN and Br. The calculated values for thermodynamic parameters using the WB97XD/6-311 + +G(d,p) level have a good agreement with the experimental values except for the cyano compound. Experimental and theoretical data suggest a mechanism with four-membered cyclic transition state (TS). The results indicate a small increment of rate constants values with raising electron donating ability groups in the phenoxy ring, and a slight decrease effect with electron withdrawing substituents. The effect of resonance structures giving by the phenyl ring seems to be the main factor in the TS stabilization. The assistance of the oxygen atom belonging to the phenoxy group in the pyranyl hydrogen abstraction exerts influence on the rate constants, but the polarization of Cδ+⋯δ−OPh bond is proposed as the rate determining process.