A combined experimental and theoretical study of the unimolecular elimination kinetics of 2-alkoxypropionic acids in the gas phase

The reaction mechanism associated with the decomposition of three 2-alkoxypropionic acids (2-methoxy-, 20ethoxy- and 2-isopropoxypropionic acid) in gas phase to form acetaldehyde, carbon monoxide, and the corresponding alcohol has been analyzed by a combination of experimental and theoretical studies. The kinetics of these systems were determined in a static system over the temperature and pressure range of 301.2-370.7 degrees C and 61-190 Torr, respectively, in seasoned vessel, with the free-radical inhibitor cyclohexene. The experimental data show that these decompositions are homogeneous, unimolecular and follow a first-order-rate law. A detailed characterization, at MP2/6-31G** computational level, points out that the molecular mechanism corresponds with a two step process; the first and rate limiting step is an elimination of the alkoxy substituent along a five-membered cyclic transition structure associated with an intramolecular hydrogen transfer from the hydroxylic oxygen atom of the carboxyl group to the oxygen atom of the alkoxy fragment, to yield the corresponding alcohol and the a-lactone intermediate; the second step is the ring opening process of this intermediate to yield acetaldehyde and carbon monoxide. The rate coefficients obtained from experimental data and theoretical calculations are in good agreement. (C) 1999 Elsevier Science B.V. All rights reserved.