Pyrolysis kinetics of 2-phenoxycarboxylic acids in the gas phase

✍ Scribed by Gabriel Chuchani; Rosa M. Dominguez; Alexandra Rotinov; Ignacio Martin

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 67 KB
• Volume: 12
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(199908)12:8<612::aid-poc169>3.0.co;2-s

✦ Synopsis

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 À1 ) = (12.08 AE 0.54) À(190.3 AE 6.7)kJ mol À1 (2.303RT) À1 ; for 2-phenoxypropionic acid, log k 1 (s À1 ) = (12.21 AE 0.31) À(172.9 AE 3.6)kJ mol À1 (2.303RT) À1 ; for 2-phenoxybutyric acid, log k 1 (s À1 ) = (12.29 AE 0.38) À(171.7 AE 4.3)kJ mol À1 (2.303RT) À1 ; and for 2-phenoxyisobutyric acid, log k 1 (s À1 ) = (12.84 AE 0.36) À(155.3 AE 3.6)kJ mol À1 (2.303RT) À1 . 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.