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

The gas-phase pyrolysis kinetics of tetrahydropyranyl phenoxy ethers: 2-(4-cyanophenoxy)tetrahydro-2H-pyran, and 2-(4-bromophenoxy)tetrahydro-2H-pyran were studied in a static system, with allyl bromide as surface-effect deactivator, and in the presence of toluene to inhibit chain reactions. The temperature and pressure were 350-440 degrees C and 25-89 Torr, respectively. The pyrolysis is homogeneous, unimolecular, and consistent with the first-order rate law yielding 3,4-dihydro-2H-pyran (DHP) and the corresponding 4-substituted phenol. The Arrhenius equations were found as follows: 2-(4-cyanophenoxy)tetrahydro-2H-pyran log k(1) (s(-1)) = (14.76 +/- 0.12) - (232.1 +/- 1.3) kJ mol(-1) (2.303 RT)(-1) 2-(4-bromophenoxy)tetrahydro-2H-pyran log k(1) (s(-1)) = (14.08 +/- 0.15) - (209.1 +/- 1.8) kJ mol(-1) (2.303 RT)(-1) 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 WD97XD/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 C-delta+...(delta-) OPh bond is proposed as the rate determining process.