Studies of the reactions of ethynyl radicals with hydrocarbons

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 fork1/k2of 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 ratiok1/k2did not vary over the temperature range 298 to 478 K in the case of 2,2-dimethylpropane but with methane, values forE1—E2and A2/A1of 12.54 ± 1.27 kJ mol-1and 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 fork7/k9,k4/k9,k8/k9andk2/k9respectively. 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 Kk4/k14=0.6 ± 0.1 andk13/k14= 1.1 ± 0.2. Addition of acetylene-d2to bromoacetylene-nitrogen mixtures yields acetylene-d1and butadiyne-d1C2H.+ C2D2→ C2HD +C2D., C2H.+ C2D2→ C4HD + D.. The rate-constant ratiosk12/k11andk2/k12are 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.