Is the homogeneous thermal dimerization of acetylene a free-radical chain reaction? Kinetic and thermochemical analysis

Abstract

Basic kinetic and thermochemical arguments incorporated into calculations modeling the initial stages of the homogeneous pyrolysis of acetylene reveal that the usual assumption of a free-radical mechanism is flawed. The key findings are that updated thermochemistry for ethynyl, vinyl, and propargyl radicals and the inclusion of falloff corrections in radical reactions lead to (1) exceedingly slow decomposition rates below 1300 K, (2) kinetic inert gas effects, and (3) the formation of benzene rather than vinylacetylene or diacetylene, all predictions at variance with experimental observations. On this basis the involvement of vinylidene is proposed.

Combustion Chemistry of Nitrogen

“…The pressure of the experiments (about 1 mbar) was too low for N2H4 stabilization to be important. Rate coefficients: kl Back and Yokada (1973) and Sarkisov et al (1984); L I microscopic reversibility (MR); k2 via L2 and MR with k-2 = l.Ox 10 14 from H + C2H3 of Duran et al (1988); k3 estimated using Ea = flrlf' + 300 kJ/mol, based upon C2H4 ~ C2H2 + H2 1,2 elimination, A-factor from TST, loss of I rotor, degeneracy = 4; k4 A-factor from Laufer et al (1983) for H2C2 + H2, Ea adjusted to fit data of Stothard et al (1995). Several conclusions emerge from these comparisons.…”

Development and testing of a comprehensive chemical mechanism for the oxidation of methane

“…Even recently, the struc-H · C " C " O, tures of H 2 CCCH and H 2 CCCCH have been debated; these were identified as the propargyl radical and butenyn-2-yl ( · CH CCH 4 CH " C " CH · ) 2 2 respectively [24,25].…”

Crossed-beam reaction of carbon atoms with hydrocarbon molecules. IV. Chemical dynamics of methylpropargyl radical formation, C4H5, from reaction of C(3Pj) with propylene, C3H6 (X 1A)

“…Two less stable radicals, 1,3-butadienyl-2 and 1,3-butadienyl-1 follow with ⌬ f H 0 0 (3)ϭ344Ϯ10 kJ mol Ϫ1 and ⌬ f H 0 0 (4)ϭ357Ϯ10 kJ mol Ϫ1 , respectively. 3,16,17 Additionally, the enthalpy of formation of the bicyclo͓1.1.0͔-butyl, ͑5͒, as well as the butynyl-1 radical, ͑6͒, were determined to be 427Ϯ33 and 485Ϯ17 kJ mol Ϫ1 . 18 Due to their high reactivity, strained C 4 H 5 isomers ͑7͒-͑10͒ could only be identified in solid matrices at 77 K via ESR spectroscopy after irradiating distinct hydrocarbon precursors with Co 60 ␥-rays, but thermodynamical data are missing.…”