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The kinetics and mechanism of the homogeneous, unimolecular gas-phase elimination of 2-(4-substituted-phenoxy)tetrahydro-2H-pyranes

Abstract:

The gas-phase elimination kinetics of tetrahydropyranyl phenoxy ethers: 2-phenoxytetrahydro-2H-pyran, 2-(4-methoxyphenoxy)tetrahydro-2H-pyran, and 2-(4-tert-butylphenoxy)tetrahydro-2H-pyran were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor toluene. The working temperature and pressure were 330 to 390°C and 25 to 89 Torr, respectively. The reactions yielded DHP and the corresponding 4-substituted phenol. The eliminations are homogeneous, unimolecular, and satisfy a first-order rate law. The Arrhenius equations for decompositions were found as follows: 2-phenoxytetrahydro-2H-pyran log k1 (s−1) = (14.18 ± 0.21) − (211.6 ± 0.4) kJ mol−1 (2.303 RT)−1 2-(4-methoxyphenoxy)tetrahydro-2H-pyran log k1 (s−1) = (14.11 ± 0.18) − (203.6 ± 0.3) kJ mol−1 (2.303 RT)−1 2-(4-tert-butylphenoxy)tetrahydro-2H-pyran log k1 (s−1) = (14.08 ± 0.08) − (205.9 ± 1.0) kJ mol−1 (2.303 RT)−1. The analysis of kinetic and thermodynamic parameters for thermal elimination of 2-(4-substituted-phenoxy)tetrahydro-2H-pyranes suggests that the reaction proceeds via 4-member cyclic transition state. The results obtained confirm a slight increase of rate constant with increasing electron donating ability groups in the phenoxy ring. The pyran hydrogen abstraction by the oxygen of the phenoxy group appears to be the determinant factor in the reaction rate.