Atmospheric chemistry of ethyl propionate.

Ethyl propionate is a model for fatty acid ethyl esters used as first-generation biodiesel. The atmospheric chemistry of ethyl propionate was investigated at 980 mbar total pressure. Relative rate measurements in 980 mbar N(2) at 293 ± 0.5 K were used to determine rate constants of k(C(2)H(5)C(O)OC(2)H(5) + Cl) = (3.11 ± 0.35) × 10(-11), k(CH(3)CHClC(O)OC(2)H(5) + Cl) = (7.43 ± 0.83) × 10(-12), and k(C(2)H(5)C(O)OC(2)H(5) + OH) = (2.14 ± 0.21) × 10(-12) cm(3) molecule(-1) s(-1). At 273-313 K, a negative Arrhenius activation energy of -3 kJ mol(-1) is observed.. The chlorine atom-initiated oxidation of ethyl propionate in 980 mbar N(2) gave the following products (stoichiometric yields): ClCH(2)CH(2)C(O)OC(2)H(5) (0.204 ± 0.031), CH(3)CHClC(O)OC(2)H(5) (0.251 ± 0.040), and C(2)H(5)C(O)OCHClCH(3) (0.481 ± 0.088). The chlorine atom-initiated oxidation of ethyl propionate in 980 mbar of N(2)/O(2) (with and without NO(x)) gave the following products: ethyl pyruvate (CH(3)C(O)C(O)OC(2)H(5)), propionic acid (C(2)H(5)C(O)OH), formaldehyde (HCHO), and, in the presence of NO(x), PAN (CH(3)C(O)OONO(2)). The lack of acetaldehyde as a product suggests that the CH(3)CH(O)C(O)OC(2)H(5) radical favors isomerization over decomposition. From the observed product yields, we conclude that H-abstraction by chlorine atoms from ethyl propionate occurs 20.4 ± 3.1%, 25.1 ± 4.0%, and 48.1 ± 8.8% from the CH(3)-, -CH(2)-, and -OCH(2)- groups, respectively. The rate constant and branching ratios for the reaction between ethyl propionate and the OH radical were investigated theoretically using quantum mechanical calculations and transition state theory. The stationary points along the reaction path were optimized using the CCSD(T)-F12/VDZ-F12//BH&HLYP/aug-cc-pVTZ level of theory; this model showed that OH radicals abstract hydrogen atoms primarily from the -OCH(2)- group (80%).

Atmospheric chemistry of two biodiesel model compounds: methyl propionate and ethyl acetate.

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.

Atmospheric chemistry of HCF2O(CF2CF2O)xCF2H (x=2-4): kinetics and mechanisms of the chlorine-atom-initiated oxidation.

Smog chamber/FTIR techniques were used to measure k(Cl+HCF(2)O(CF(2)CF(2)O)(x)CF(2)H)=(5.3±1.5)×10(-17) cm(3) molecule(-1) s(-1) in 700 Torr of N(2)/O(2) diluent at 296±1 K. The Cl-initiated atmospheric oxidation of HCF(2)O(CF(2)CF(2)O)(x)CF(2)H, x=2,3 and 4, gave COF(2) in molar yields of (593±41)%, (758±71)% and (939±73)%, respectively, with no other observable carbon-containing products (i.e., essentially complete conversion of the hydrofluoropolyethers into COF(2)). Quantitative infrared spectra for HCF(2)O(CF(2)CF(2)O)(x)CF(2)H (x=2-4) were recorded and used to estimate the radiative efficiencies of the title compounds (1.07, 1.33, and 1.36 W m(-2) ppb(-1)). Global warming potentials (100 year time horizon) of 3870, 4730 and 5060 were estimated for HCF(2)O(CF(2)CF(2)O)(x)CF(2)H, x=2, 3 and 4, respectively. The results are discussed with respect to the atmospheric chemistry and environmental impact of hydrofluoropolyethers.

CHF2OCHF2 (HFE-134): IR spectrum and kinetics and products of the chlorine-atom-initiated oxidation.

Smog chamber/FTIR techniques were used to measure k(Cl + CHF(2)OCHF(2)) = (5.7 +/- 1.5) x 10(-16) cm(3) molecule(-1) s(-1) in 700 Torr of N(2)/O(2) diluent at 296 +/- 1 K. This result is 100 times lower than the previous literature value. The chlorine-atom-initiated atmospheric oxidation of CHF(2)OCHF(2) gives COF(2) in a molar yield of (185 +/- 22) %. The IR spectrum was recorded, and a radiative efficiency of 0.44 W m(-2) ppb(-1) was determined. The results are discussed with respect to the atmospheric chemistry and environmental impact of CHF(2)OCHF(2).