Gas phase reactions of unsaturated esters with Cl atoms

Background, aim, and scope Acrylate and methacrylate esters are α,β-unsaturated esters that contain vinyl groups directly attached to the carbonyl carbon (CH₂=CHCOO- and CH₂=CCH₃COO-, respectively) and are widely used in the polymer plastic and resin production. Rate coefficients for Cl reactions for most of the unsaturated esters have not been previously determined, and a good understanding is needed of all the atmospheric oxidation processes of these compounds in order to determine lifetimes in the atmosphere and to evaluate the impact of these reactions on the formation of photo-oxidants and therefore on health and environment. Materials and methods The relative rate technique has been used to obtain rate coefficients for the reactions between the Cl atom and a series of unsaturated esters. The experiments have been carried out in a static Teflon reactor at room temperature and atmospheric pressure (N₂ as bath gas) using gas chromatography with flame ionization detection as detection system. Results The following rate coefficients are obtained (in cubic meter per molecule per second): methyl acrylate + Cl = 1.71 ± 0.13 × 10⁻¹⁰; methyl methacrylate + Cl = 2.30 ± 0.18 × 10⁻¹⁰; ethyl acrylate + Cl = 1.82 ± 0.13 × 10⁻¹⁰; ethyl methacrylate + Cl = 2.71 ± 0.21 × 10⁻¹⁰; butyl acrylate + Cl = 2.94 ± 0.23 × 10⁻¹⁰; butyl methacrylate + Cl = 3.83 ± 0.30 × 10⁻¹⁰; methyl 3-methyl acrylate + Cl = 2.21 ± 0.17 × 10⁻¹⁰; and methyl 3,3-dimethyl acrylate + Cl = 3.58 ± 0.28 × 10⁻¹⁰. Discussion Rate coefficients calculated for Cl reactions are around one order of magnitude higher than OH ones. The effect in the reactivity of increased substitution at the carbon-carbon double bond is analyzed and also the effect of the identity of the alkyl group R in the -C(O)OR. Atmospheric lifetimes of the compounds against the attack by the major oxidants are estimated and the atmospheric implications are discussed. Conclusions The dominant atmospheric loss process for acrylate esters is clearly their daytime reaction with the hydroxyl radical. However, in coastal areas and in the marine boundary layer and in some industrial zones, Cl-atom-initiated degradation of the unsaturated esters considered here can be a significant if not dominant homogeneous loss process. Recommendations and perspectives Product analysis should be necessary in order to evaluate the real environmental impact of these reactions. OH and ozone reactions of most of the considered compounds have already been studied and products determined, but kinetic and products information for NO₃ radical reactions is especially scarce.