Gas-phase oxidation of methyl crotonate and ethyl crotonate. Kinetic study of their reactions toward oh radicals and Cl atoms

Abstract

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with methyl crotonate and ethyl crotonate have been determined at 298 K and atmospheric pressure. The decay of the organics was monitored using gas chromatography with flame ionization detection (GC-FID), and the rate constants were determined using the relative rate method with different reference compounds. Room temperature rate coeficcients were found to be (in cm 3 molecule -1 s -1): k 1(OH + CH 3CH=CHC(O)OCH 3) = (4.65 ± 0.65) × 10 -11, k 2(Cl + CH 3CH=CHC(O)OCH 3) = (2.20 ± 0.55) × 10 -10, k 3(OH + CH 3CH=CHC(O)OCH 2CH 3) = (4.96 ± 0.61) × 10 -11, and k 4(Cl + CH 3CH=CHC(O)OCH 2CH 3) = (2.52 ± 0.62) × 10 -10 with uncertainties representing ±2σ. This is the first determination of k 1, k 3, and k 4 under atmospheric pressure. The rate coefficients are compared with previous determinations for other unsaturated and oxygenated VOCs and reactivity trends are presented. In addition, a comparison between the experimentally determined k OH with k OH predicted from k vs E HOMO relationships is presented. On the other hand, product identification under atmospheric conditions has been performed for the first time for these unsaturated esters by the GC-MS technique in NO x-free conditions. 2-Hydroxypropanal, acetaldehyde, formaldehyde, and formic acid were positively observed as degradation products in agreement with the addition of OH to C2 and C3 of the double bond, followed by decomposition of the 2,3- or 3,2-hydroxyalkoxy radicals formed. Atmospheric lifetimes, based on of the homogeneous sinks of the unsaturated esters studied, are estimated from the kinetic data obtained in the present work.