Kinetics and products study of the reaction of Cl atoms with methyl dichloroacetate: reactivity, mechanism, and environmental implications

Abstract

The relative rate coefficient of the gas-phase reaction of methyl dichloroacetate (CHCl₂C(O)OCH₃) with Cl atoms (k_Cl) was obtained at 298 K and atmospheric pressure. All the experiments were performed in a 480 L Pyrex glass atmospheric simulation reactor coupled to an “in situ” Fourier transform infrared (FTIR) spectrometer. The rate coefficient obtained from the average of different experiments was: k_Cl = (3.31 ± 0.88) × 10⁻¹³ cm³ per molecule per s. In addition, the product studies were performed in under similar conditions to those of the kinetic experiments in two different photoreactors by in situ FTIR spectroscopy and GC-MS/SPME. Dichloroacetic acid, phosgene, methyl trichloroacetate, and carbon monoxide were the main products identified and quantified. The obtained product yields for the reaction with Cl atoms were as follows: (24 ± 2), (19 ± 3), (16 ± 1), and (44 ± 2)% for Cl₂CHCOOH, COCl₂, CO, and CCl₃C(O)OCH₃, respectively. The initial pathway for the degradation of methyl dichloroacetate in the reaction with Cl atoms occurs via H-atom abstraction at the alkyl groups. The atmospheric implications of the reactions were assessed by the estimation of the tropospheric lifetime of τ_Cl = 3 years. In addition, an acidification potential of 0.45 was estimated, suggesting a possible impact of the emission of methyl dichloroacetate on the rainfall acidification. On the other hand, significant global warming potentials of 8.2, 2.2, and 0.6 were calculated for the studied chloroester for the time horizons of 20, 100, and 500 years, respectively. Chlorinated persistent products, such as dichloroacetic acid, could have an impact on the atmosphere and other environmental matrixes as well as on human health and the biota.