Rate constants of the reactions of CF2ClC(O)OCH3 and CF2ClC(O)OCH2CH3 with OH radicals and Cl atoms at atmospheric pressure

OH-initiated degradation of methyl 2-chloroacetoacetate and ethyl 2-chloroacetoacetate: Kinetics, products and mechanisms at 298 K and atmospheric pressure

Rate coefficients for the gas-phase reactions of OH radicals with CH₃C(O)CHClC(O)OCH₃ (k₁) and CH₃C(O)CHClC(O)OCH₂CH₃ (k₂) were measured using the relative technique with different reference compounds. The experiments were performed at (298 ± 2) K and 750 Torr of nitrogen or synthetic air by in situ FTIR spectroscopy and GC-FID chromatography. The following rate coefficients (in units of cm³molecule^-1 s^-1) were obtained: k_1FTIR= (2.70 ± 0.51) × 10^-11; k_1GC-FID= (2.30 ± 0.71) × 10^-11 and k_2FTIR= (3.37 ± 0.62) × 10^-11; k_2GC-FID= (3.26 ± 0.85) × 10^-11. This work reports the first kinetic study for the reactions of OH radicals with the mentioned chloroacetoacetates. Additionally, product studies are reported in similar conditions of the kinetic experiments. Acetic acid, acetaldehyde, formyl chloride, and methyl 2-chloro-2-oxoacetate were positively identified and quantified as degradation products. According to the identified products, atmospheric chemical mechanisms were proposed. The environmental implications of these reactions were assessed by the tropospheric lifetimes calculations of the title chloroesters. Significant average ozone production of 4.16 ppm for CH₃C(O)CHClC(O)OCH₃ and 5.98 ppm for CH₃C(O)CHClC(O)OCH₂CH_3, respectively were calculated.

Theoretical Investigation on the Mechanism and Kinetics of Atmospheric Reaction of Methyldichloroacetate with Hydroxyl Radical

The atmospheric reaction of methyldichloroacetate (MDCA) with OH radical is studied using electronic structure calculations. Five different pathways were considered for the initial reactions, which results in the formation of alkyl radical of MDCA along with H₂O, HOCl, and CH₃O^•. Among the five pathways studied, the α-carbon atom (-CHCl₂ site) H atom abstraction reaction, which leads to the formation of the alkyl radical intermediate ^•CCl₂C(O)OCH₃ (I1) is found to be more favorable with an energy barrier of 7.3 kcal/mol, and Cl-atom abstraction reaction is having high energy barrier of 21.3 kcal/mol at M06-2X/6-311++G(2df,2p) level. The calculated thermochemical parameters show that except Cl-atom abstraction channel the other initial reaction channels are highly exothermic. The rate constant is calculated for the initial H atom abstraction reactions using canonical variational transition state theory over the temperature range of 278 to 350 K. The Arrhenius plot shows positive temperature dependence for both the reactions. The results from the calculated thermochemical parameters and rate constants show that the formation of the alkyl radical intermediate (I1) is more favorable with the rate constant of 2.07 × 10^-13 cm³ molecule^-1 s^-1 at 298 K. The calculated atmospheric lifetime of MDCA is 28 days at normal atmospheric OH concentration. The results obtained from secondary reactions show that the major product formed from the oxidation chemistry of MDCA is methyl-2-chloro-2-oxoacetate (or) methyl oxalyl chloride.

Atmospheric sink of methyl chlorodifluoroacetate and ethyl chlorodifluoroacetate: temperature dependent rate coefficients, product distribution of their reactions with Cl atoms and CF2ClC(O)OH formation

Rate coefficients as a function of temperature and product distribution studies have been performed for the first time for the gas-phase reactions of chlorine atoms with methyl chlorodifluoracetate (k₁) and ethyl chlorodifluoroacetate (k₂) using the relative rate technique.