Alkyne oxidation I. Acetylene oxidation

Abstract

The slow combustion of acetylene has been studied analytically and kinetically over a wide range of experimental conditions by continually sampling the reactants and products through a pinhole leak in a Pyrex reaction vessel into the ion source of a quadrupole mass spectrometer. The mass spectra were interpreted and the reaction products identified with the help of labelled acetylene (C 2 D 2 ) and oxygen ( 18 O 2 ). There is no incubation period in the disappearance of the reactants and the formation of glyoxal in the early stages of reaction gives the initial pressure drop. The addition of glyoxal has no effect on the reaction. The products of the reaction and their kinetic orders with respect to acetylene are, carbon monoxide (2.0), carbon dioxide (2.0), water (2.3), hydrogen (1.7), glyoxal (2.5-shown by labelling experiments to be produced from one acetylene and two oxygen molecules), form aldehyde (1.9), acetaldehyde (shown to come from wall catalysed rearrangement of vinyl alcohol), acrolein, keten, formic acid (2.25), acetic acid (2.25-shown by isotopic experiments to come from acetaldehyde), methanol and hydrogen peroxide. The consumption of acetylene and oxygen are dependent on the second power of the acetylene concentration and the reaction is largely independent of the concentration of oxygen and incrt gases. Isotopic mixing occurs in mixtures of acetylene and dideuterio-acetylene by way of ethynyl radical reactions. The reaction is suggested to be initiated by reaction between two acetylene molecules and terminated by the interaction of two hydroperoxyl radicals. The main chain produces the intermediate products glyoxal, form aldehyde and keten, the first being produced to a higher order and the second two to a lower order dependence on acetylene concentration than the rate of removal of acetylene. Two mechanisms are considered to account for the kinetics, the first involving conversion of hydroperoxyl to hydroxyl which acts as the main chain carrier and the second involving both hydroxyl and hydroperoxyl as chain carriers.