The reactions of atomic chlorine with acrolein, methacrolein and methyl vinyl ketone

Abstract

An investigation into the reactions between Cl atoms and acrolein (1), methacrolein (2) and methyl vinyl ketone (3)

is presented. Values of the rate constants for the reactions have been determined by an absolute technique for the first time. At a pressure of 1.6 Torr, the rate constants obtained were: k₁ = 1.1 ± 0.2, k₂ = 3.3 ± 0.6 and k₃ = 0.99 ± 0.20 in units of 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. k₁ was also determined at atmospheric pressure using a relative-rate technique. The rate constant obtained was (2.2 ± 0.3) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹; the larger value compared with that for 1.6 Torr is thought to reflect a true dependence of the reaction rate on pressure. The final products of the reactions performed under an atmosphere of synthetic air were investigated using FTIR spectroscopy. The only chlorinated organic species identified as products of the reactions were chloroacetaldehyde in the case of reaction of Cl atoms with acrolein; chloroacetone with methacrolein; and chloroacetaldehyde with methyl vinyl ketone. Branching ratios for abstraction (the fraction of reactions occurring by abstraction relative to the sum of addition and abstraction processes) were found to be 0.22 ± 0.13 for acrolein, 0.18 ± 0.02 for methacrolein and <0.03 for methyl vinyl ketone. The reaction of Cl atoms with methacrolein proceeds ia a mechanism that involves the decomposition of the methyl vinyl radical. The decomposition of this radical in synthetic air, and in the absence of NO, appears to lead to the formation of significantly more CO than previously thought. This observation is in agreement with the work of J. J. Orlando, S. E. Paulson and G. S. Tyndall, Geophys. Res. Lett., 1999, 26, 2191 (ref. 1), who studied the decomposition of the radical under different conditions.