Atmospheric Chemistry of Two Biodiesel Model Compounds: Methyl Propionate and Ethyl Acetate

Abstract

The atmospheric chemistry of two C(4)H(8)O(2) isomers (methyl propionate and ethyl acetate) was investigated. With relative rate techniques in 980 mbar of air at 293 K the following rate constants were determined: k(C(2)H(5)C(O)OCH(3) + Cl) = (1.57 ± 0.23) × 10(-11), k(C(2)H(5)C(O)OCH(3) + OH) = (9.25 ± 1.27) × 10(-13), k(CH(3)C(O)OC(2)H(5) + Cl) = (1.76 ± 0.22) × 10(-11), and k(CH(3)C(O)OC(2)H(5) + OH) = (1.54 ± 0.22) × 10(-12) cm(3) molecule(-1) s(-1). The chlorine atom initiated oxidation of methyl propionate in 930 mbar of N(2)/O(2) diluent (with, and without, NO(x)) gave methyl pyruvate, propionic acid, acetaldehyde, formic acid, and formaldehyde as products. In experiments conducted in N(2) diluent the formation of CH(3)CHClC(O)OCH(3) and CH(3)CCl(2)C(O)OCH(3) was observed. From the observed product yields we conclude that the branching ratios for reaction of chlorine atoms with the CH(3)-, -CH(2)-, and -OCH(3) groups are <49 ± 9%, 42 ± 7%, and >9 ± 2%, respectively. The chlorine atom initiated oxidation of ethyl acetate in N(2)/O(2) diluent gave acetic acid, acetic acid anhydride, acetic formic anhydride, formaldehyde, and, in the presence of NO(x), PAN. From the yield of these products we conclude that at least 41 ± 6% of the reaction of chlorine atoms with ethyl acetate occurs at the -CH(2)- group. The rate constants and branching ratios for reactions of OH radicals with methyl propionate and ethyl acetate were investigated theoretically using transition state theory. The stationary points along the oxidation pathways were optimized at the CCSD(T)/cc-pVTZ//BHandHLYP/aug-cc-pVTZ level of theory. The reaction of OH radicals with ethyl acetate was computed to occur essentially exclusively (∼99%) at the -CH(2)- group. In contrast, both methyl groups and the -CH(2)- group contribute appreciably in the reaction of OH with methyl propionate. Decomposition via the α-ester rearrangement (to give C(2)H(5)C(O)OH and a HCO radical) and reaction with O(2) (to give CH(3)CH(2)C(O)OC(O)H) are competing atmospheric fates of the alkoxy radical CH(3)CH(2)C(O)OCH(2)O. Chemical activation of CH(3)CH(2)C(O)OCH(2)O radicals formed in the reaction of the corresponding peroxy radical with NO favors the α-ester rearrangement.