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Mechanism of the Reaction of OH with Alkynes in the Presence of Oxygen

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posted on 19.02.2016, 04:34 by James Lockhart, Mark A. Blitz, Dwayne E. Heard, Paul W. Seakins, Robin J. Shannon
Previous work has shown that the branching ratio of the reaction of OH/C2H2/O2 to glyoxal and formic acid is dependent on oxygen fraction, and a significant component of the product yield under atmospheric conditions is formed from reaction of chemically activated OH–C2H2 adduct. In this article, isotopic substitution is used to determine the mechanism of the OH/C2H2/O2 reaction resolving previous contradictory observations in the literature. Using laser flash photolysis and probing OH concentrations via laser induced fluorescence, a rate coefficient of kHO–C2H2+O2 = (6.17 ± 0.68) × 10–12 cm3 molecule–1 s–1 is determined at 298 K from the analysis of biexponential OH decays in the presence of C2H2 and low concentrations of O2. The studies have been extended to propyne and but-2-yne. The reactions of OH with propyne and but-2-yne have been studied as a function of pressure in the absence of oxygen. The reaction of OH with propyne is in the fall off region from 2–25 Torr of nitrogen at room temperature. A pressure independent value of (4.21 ± 0.47) × 10–12 cm3 molecule–1 s–1 was obtained from averaging the eight independent measurements at 25 and 75 Torr. The reaction of OH with but-2-yne at 298 K is pressure independent (5–25 Torr N2) with a value of (1.87 ± 0.19) × 10–11 cm3 molecule–1 s–1. Analysis of biexpontial OH decays in alkyne/low O2 conditions gives the following rate coefficients at 298 K: kHO–C3H4+O2 = (8.00 ± 0.82) × 10–12 cm3 molecule–1 s–1 and kHO–C4H6+O2 = (6.45 ± 0.68) × 10–12 cm3 molecule–1 s–1. The branching ratio of bicarbonyl to organic acid in the presence of excess oxygen also shows an oxygen fraction dependence for propyne and but-2-yne, qualitatively similar to that for acetylene. For an oxygen fraction of 0.2 at 298 K, pressure independent yields of methylglyoxal (0.70 ± 0.03) and biacetyl (0.74 ± 0.03) were determined for the propyne and but-2-yne systems, respectively. The yield of acid increases with temperature from 212–500 K. Master equation calculations show that, under atmospheric conditions, the acetyl cofragment of organic acid production will dissociate, consistent with experimental observations.

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