TY - JOUR
T1 - Atmospheric Chemistry of Two Biodiesel Model Compounds: Methyl Propionate and Ethyl Acetate
AU - Andersen, Vibeke Friis
AU - Berhanu, Tesfaye A.
AU - Nilsson, Elna Johanna Kristina
AU - Jørgensen, Solvejg
AU - Nielsen, Ole John
AU - Wallington, Timothy J.
AU - Johnson, Matthew Stanley
PY - 2011/8/18
Y1 - 2011/8/18
N2 - The atmospheric chemistry of two C 4H 8O 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 2H 5C(O)OCH 3 + Cl) = (1.57 ± 0.23) × 10 -11, k(C 2H 5C(O) OCH 3 + OH) = (9.25 ± 1.27) × 10 -13, k(CH 3C(O)OC 2H 5 + Cl) = (1.76 ± 0.22) × 10 -11, and k(CH 3C(O)OC 2H 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 3CHClC(O)OCH 3 and CH 3CCl 2C(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 2H 5C(O)OH and a HCO radical) and reaction with O 2 (to give CH 3CH 2C(O)OC(O)H) are competing atmospheric fates of the alkoxy radical CH 3CH 2C(O)OCH 2O. Chemical activation of CH 3CH 2C(O)OCH 2O radicals formed in the reaction of the corresponding peroxy radical with NO favors the α-ester rearrangement.
AB - The atmospheric chemistry of two C 4H 8O 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 2H 5C(O)OCH 3 + Cl) = (1.57 ± 0.23) × 10 -11, k(C 2H 5C(O) OCH 3 + OH) = (9.25 ± 1.27) × 10 -13, k(CH 3C(O)OC 2H 5 + Cl) = (1.76 ± 0.22) × 10 -11, and k(CH 3C(O)OC 2H 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 3CHClC(O)OCH 3 and CH 3CCl 2C(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 2H 5C(O)OH and a HCO radical) and reaction with O 2 (to give CH 3CH 2C(O)OC(O)H) are competing atmospheric fates of the alkoxy radical CH 3CH 2C(O)OCH 2O. Chemical activation of CH 3CH 2C(O)OCH 2O radicals formed in the reaction of the corresponding peroxy radical with NO favors the α-ester rearrangement.
M3 - Journal article
SN - 1089-5639
VL - 115
SP - 8906
EP - 8919
JO - Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory
JF - Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory
ER -