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Pyrolysis kinetics of 2‐phenoxycarboxylic acids in the gas phase
Author(s) -
Chuchani Gabriel,
Dominguez Rosa M.,
Rotinov Alexandra,
Martin Ignacio
Publication year - 1999
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/(sici)1099-1395(199908)12:8<612::aid-poc169>3.0.co;2-s
Subject(s) - chemistry , pyrolysis , phenol , cyclohexene , substituent , medicinal chemistry , kinetics , reaction rate constant , organic chemistry , catalysis , physics , quantum mechanics
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 k 1 (s −1 ) = (12.08 ± 0.54) −(190.3 ± 6.7)kJ mol −1 (2.303 RT ) −1 ; for 2‐phenoxypropionic acid, log k 1 (s −1 ) = (12.21 ± 0.31) −(172.9 ± 3.6)kJ mol −1 (2.303 RT ) −1 ; for 2‐phenoxybutyric acid, log k 1 (s −1 ) =(12.29 ± 0.38) −(171.7 ± 4.3)kJ mol −1 (2.303 RT ) −1 ; and for 2‐phenoxyisobutyric acid, log k 1 (s −1 ) = (12.84 ± 0.36) −(155.3 ± 3.6)kJ mol −1 (2.303 RT ) −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 C 6 H 5 O 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 C 6 H 5 O 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.