Abstract
The gas-phase pyrolysis kinetics of primary, secondary and tertiary 2-phenoxycarboxylic acids were studied over the temperature range 240.1–409.9 °C and pressure range 16.3–67.8 Torr. The elimination reactions, carried out in seasoned vessels and in the presence of the free radical chain inhibitor cyclohexene, are homogeneous, unimolecular and follow a first-order rate law. The overall rate coefficients are expressed by the following equations: for 2-phenoxyacetic acid, log k1(s−1) = (12.08 ± 0.54) −(190.3 ± 6.7)kJ mol−1(2.303RT)−1; for 2-phenoxypropionic acid, log k1 (s−1) = (12.21 ± 0.31) −(172.9 ± 3.6)kJ mol−1(2.303RT)−1; for 2-phenoxybutyric acid, log k1(s−1) =(12.29 ± 0.38) −(171.7 ± 4.3)kJ mol−1(2.303RT)−1; and for 2-phenoxyisobutyric acid, log k1(s−1) = (12.84 ± 0.36) −(155.3 ± 3.6)kJ mol−1(2.303RT)−1. The products of the phenoxyacids are phenol, the corresponding carbonyl compound and CO, except for 2-phenoxyisobutyric acid, which undergoes a parallel elimination into phenol and methylacrylic acid. The reaction rates increase from primary to tertiary carbon bearing the C6H5O group. The mechanism of these reactions appears to proceed through a semi-polar five-membered cyclic transition state, where the acidic H of the COOH group assists the leaving C6H5O substituent for phenol formation, followed by the participation of the oxygen carbonyl for lactone formation. Then the unstable lactone intermediate decomposes into the corresponding carbonyl compound and CO gas. Copyright © 1999 John Wiley & Sons, Ltd.
Published Version
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