Rate constants and mechanisms for the reaction of hydroxyl (OD) radicals with acetylene, propyne, and 2-butyne in air at 297 .+-. 2 K

Abstract

OH (OD) radical initiated photooxidation of acetylene, propyne, and 2-butyne was studied at atmospheric pressure at 297 +/- 2 K. The rate constants were determined to be (8.8 +/- 2.0) x 10/sup -13/, (5.71 +/- 0.18) x 10/sup -12/, and (3.01 +/- 0.28) x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/ for acetylene, propyne, and 2-butyne, respectively, by the competitive reaction method using cyclohexane as a reference compound (k/sub (OH + cyclohexane)/ = (7.57 +/- 0.05) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/). The major ultimate products are ..cap alpha..-dicarbonyl compounds, i.e., glyoxal from acetylene, methyglyoxal from propyne, and biacetyl from 2-butyne, as well as formic acid from acetylene and propyne and acetic acid from 2-butyne. On the basis of product analyses the reaction of OH with alkynes was deduced to proceed via addition resulting in the formation of hydroxyvinyl radicals, which further react with O/sub 2/ to give carboxylic acid + RCO or ..cap alpha..-dicarbonyl compounds. 30 references, 8 figures, 2 tables.