Pyrolysis kinetics of 2-phenoxycarboxylic acids in the gas phase

The gas-phase pyrolysis kinetics of primary, secondary and tertiary 2-pheno xycarboxylic acids were studied over the temperature range 240.1 - 409.9 de grees C and pressure range 16.3 - 67.8 Torr. The elimination reactions, car ried out in seasoned vessels and in the presence of the free radical chain inhibitor cyclohexene, are homogeneous, unimolecular and follow a first-ord er rate law. The overall rate coefficients are expressed by the following e quations: for 2-phenoxyacetic acid, log k(1)(s(-1)) = (12.08 +/- 0.54) - (1 90.3 +/- 6.7) kJ mol(-1) (2.303RT)(-1); for 2-phenoxypropionic acid, log k( 1)(s(-1)) = (12.21 +/- 0.31) - (172.9 +/- 3.6) kJ mol(-1) (2.303RT)(-1); fo r 2-phenoxybutyric acid, log k(1)(s(-1)) = (12.29 +/- 0.38) - (171.7 +/- 4. 3) kJ mol(-1) (2.303RT)(-1); and for 2-phenoxyisobutyric acid, log k(1)(s(- 1)) = (12.84 +/- 0.36) - (155.3 +/- 3.6) kJ mol(-1) (2.303RT)(-1). The prod ucts of the phenoxyacids are phenol, the corresponding carbonyl compound an d CO, except for 2-phenoxyisobutyric acid, which undergoes a parallel elimi nation 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 tra nsition state, where the acidic H of the COOH group assists the leaving C6H 5O substituent for phenol formation, followed by the participation of the o xygen carbonyl for lactone formation. Then the unstable lactone intermediat e decomposes into the corresponding carbonyl compound and CO gas. Copyright (C) 1999 John Wiley & Sons, Ltd.