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-alkoxyp ropionic acids (2-methoxy-, 20ethoxy- and 2-isopropoxypropionic acid) in ga s phase to form acetaldehyde, carbon monoxide, and the corresponding alcoho l has been analyzed by a combination of experimental and theoretical studie s. The kinetics of these systems were determined in a static system over th e temperature and pressure range of 301.2-370.7 degrees C and 61-190 Torr, respectively, in seasoned vessel, with the free-radical inhibitor cyclohexe ne. The experimental data show that these decompositions are homogeneous, u nimolecular and follow a first-order-rate law. A detailed characterization, at MP2/6-31G** computational level, points out that the molecular mechanis m corresponds with a two step process; the first and rate limiting step is an elimination of the alkoxy substituent along a five-membered cyclic trans ition structure associated with an intramolecular hydrogen transfer from th e hydroxylic oxygen atom of the carboxyl group to the oxygen atom of the al koxy fragment, to yield the corresponding alcohol and the a-lactone interme diate; 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.