Rate Constants for Reaction of OH Radicals with Acetates and Glycols in the Gas Phase

Summary

Rate constants for the gas phase reaction of OH radicals with (1) n-butyl-acetate, (2) 2-ethoxy-ethanol, (3) 2-butoxy-ethanol, and (4) 1-acetoxy-2-ethoxy-ethane between T = 298 - 500 K have been measured using a combined laser photolysis/resonance fluorescence (LPRF) technique. The Results are:

$${k_1}(T) = (3.1 \pm \,\,0.7)\,{10^{ - 11}}\,\exp ( - 594 \pm \,\,126/T)\,c{m^3}/s$$

$${k_2}(T) = (1.8 \pm \,\,0.4)\,{10^{ - 11}}\,\exp \,( - 120 \pm \,\,30/T)\,c{m^3}/s$$

$${K_3}(T) = (1.4 \pm \,\,0.3)\,{10^{ - 11}}c{m^3}/s$$

$${K_4}(T) = (3.6 \pm \,\,0.8)\,{10^{ - 12}}\,\exp (383 \pm \,\,80/T)\,c{m^3}/s$$

The rate constants at 298 K have been analysed in terms of individual C-H bond contributions using estimated thermochemical data and empirical group reactivity correlations. It is found that the primary reaction is dominated in all cases by H atom abstraction from a CH₂ group with the absolute reactivity depending on the α^- substituent and decreasing in the order -O-CH₂- > HO-CH₂- > -CH₂-CH₂- ∿ CH₃-CH₂- > -C(O)O-CH₂-. Based on these findings and on generally accepted oxidation schemes a simplified mechanism is suggested for the atmospheric degradation of glycols. Assuming an average tropospheric OH concentration of 5 x 10⁵ cm^-3 the atmospheric lifetimes are estimated to be 5.5 days and 1.7 days for compounds (1) and (2, 3, 4), respectively.