Studies of the reactions of ethynyl radicals with hydrocarbons

Abstract

Abstract It has been shown that ethynyl radicals may be satisfactorily generated by the photolysis, at 253.7 nm, of bromoacetylene in the presence of nitric oxide. Acetylene and butadiyne are primary products, being formed exclusively by the reactions C2H. + C2HBr→C2H2 + C2Br., C2H. + C2HBr→C4H2 + Br.. Nitric oxide decreases the rates of formation of both products, indicating the effective scavenging of ethynyl radicals by this compound. Addition of an inert gas (nitrogen or carbon dioxide) increases the ratio [C4H2]/[C2H2] from 3.5 (no inert gas) to 7 (total pressure 80 kPa (1 Pa = 1 N m-2)), the ratio thereafter remaining constant. The most obvious explanation for this behaviour is that, during photolysis, ethynyl radicals produced in the absence of inert gas have excess translational energy and, probably, enhanced reactivity. With increasing inert gas pressure, fewer ‘hot’ radicals react and the change in the ratio [C4H2]/[C2H2] reflects the change in selectivity of ‘thermalized’ ethynyl radicals. On account of this, investigations of the reactions of C2H.with added hydrocarbons were carried out with a standard 1:1:100 bromoacetylene-nitric oxide-nitrogen mixture. Results obtained with added alkanes (methane, ethane, 2,2 dimethylpropane) showed that ethynyl radicals abstract hydrogen atoms to form acetylene: C2H. + RH→C2H2 + R., The relative importance of reactions (1) and (2) has been estimated and values for k1/k2 of 0.016 ± 0.005, 0.54 ± 0.04 and 0 .91 ± 0.04 have been obtained for methane, and ethane 2,2-dimethylpropane respectively. The ratio k1/k2 did not vary over the temperature range 298 to 478 K in the case of 2,2-dimethylpropane but with methane, values for E1 — E2 and A2/A1 of 12.54 ± 1.27 kJ mol-1 and 0.54 ± 0.25, respectively, were obtained. Studies of the reactions of ethynyl radicals with alkynes (acetylene, butadiyne and propyne) have shown that the radicals abstract hydrogen atoms (to form acetylene), displace hydrogen atoms (to form a di- or triyne) and, in the case of propyne, displace a methyl radical. For propyne, the relevant reactions are C2H. + C3H4→C2H2 + C3H3., C2H. + C3H4→C4H2 + CH3., C2H. + C3H4→C5H4 + H., and Values of 25 ± 3, 5 ± 2, 9.9 ± 1 and 23 ± 3 at 298 K have been obtained for k7/k9, k4/k9, k8/k9 and k2/k9 respectively. In the presence of butadiyne, acetylene and hexatriyne are formed as primary products. Acetylene is formed by reactions (4) and (13), C2H. +C4H2 → C2H2 + C4H., whilst hexatriyne is formed by the displacement reaction (14) C2H. + C4H2→C6H2+H.. Kinetic measurements have shown that at 298 K k4/k14 =0.6 ± 0.1 and k13/k14 = 1.1 ± 0.2. Addition of acetylene-d2 to bromoacetylene-nitrogen mixtures yields acetylene-d1 and butadiyne-d1 C2H. + C2D2→ C2HD +C2D., C2H. + C2D2→ C4HD + D.. The rate-constant ratios k12/k11 and k2/k12 are 2 .8 ± 2.5 and 1.5 ± 0.3 respectively. This work thus indicates that ethynyl radical addition-elimination reactions, leading to polyalkynes, occur to a comparable extent to hydrogen-abstraction reactions in acetylene-containing systems. These results are shown to be of significance in regard to the formation and subsequent reactions of polyalkynes in both the pyrolysis and flames of acetylene and other hydrocarbons.