The very-high-temperature pyrolysis of ethane: Evidence against high rates for dissociative recombination reactions of methyl radicals

The pyrolysis of 1 and 2% ethane in krypton has been studied in shock waves by the laser-schlieren technique over 1700–4800 K. For 2400–2800 K an effective zero density gradient is seen following the rapid dissociation of the ethane. Through simulation with various mechanisms it is evident that the high rates for the dissociative recombination reactions of methyl radicals obtained in recent shock-tube studies, are incompatible with this observation; these rates must be reduced at least an order of magnitude. On the basis of theory and previous low-temperature (T) measurements, k = 7.8 × 10¹¹ (-6562/T) (cm³/mol s) is recommended for the second of these reactions.     

High temperature pyrolysis of formaldehyde in shock waves

Thermal decomposition of formaldehyde diluted with Ar was studied behind reflected shock waves in the temperature range of 1200–2000 K at total pressures between 1.3 and 3.0 atm. The study was carried out for compositions from the concentrated mixture, 4% CH₂O, to the highly dilute mixture, 0.01% CH₂O by using time-resolve IR-laser absorption and IR-emission, and a single-pulse technique. From a computer-simulation study, the mechanism and the rate-constant expressions that could explain all of our data and previously reported ARAS data were discussed. This data obtained over a wide concentration range from 50 ppm CH₂O to 4% CH₂O were satisfactorily modeled by a five-reaction mechanism. © 1993 John Wiley & Sons, Inc.     

High temperature pyrolysis of 1,5-cyclooctadiene and 4-vinylcyclohexene in shock waves

1,5-cyclooctadiene or 4-vinylcyclohexene mixture diluted with argon was heated to temperatures in the range 880–1230 K behind reflected shock waves. Profiles of IR-laser absorption were measured at 3.39 μm. From these profiles, rate constants k₁ and k₂ for the decyclization reactions 1,5-cyclooctadiene → biradical and 4-vinylcyclohexene → biradical were evaluated as k₁ = 5.2 × 10¹⁴ exp(?48.3 kcal/RT) s^?1 and k₂ = 3.5 × 10¹⁴ exp(?55.3 kcal/RT) s^?1, respectively. © 1993 John Wiley & Sons, Inc.     

Thermal decomposition of propyne and allene in shock waves

Propyne (p-C₃H₄) or allene (a-C₃H₄) mixtures, highly diluted with Ar, were heated to the temperature range 1200–1570 K at pressures of 1.7–2.6 atm behind reflected shock waves. The thermal decompositions of propyne and allene were studied by both measuring the profiles of the IR emission at 3.48 μm or 5.18 μm and analyzing the concentrations of reacted gas mixtures. The mechanism and the rate constant expressions were discussed from both the profiles and the concentrations of reactant and products obtained. The rate constant expressions for reactions, (1) p-C₃H₄ → a-C₃H₄, (?1) a-C₃H₄ → p-C₃H₄, and (5) p-C₃H₄ + H → CH₃ + C₂H₂ were evaluated.     

The high temperature pyrolysis of ethylbenzene: Evidence for dissociation to benzyl and methyl radicals

The pyrolysis of ethylbenzene has been investigated in shock waves with the laser schlieren technique. Mixtures of 1 and 2% ethylbenzene in krypton were studied for reaction conditions of 1300-1800 K, 70-550 torr. At high temperatures, the initial rapid endothermic dissociation is followed by a region of net exothermic reaction, which is readily understood as arising mainly from methyl radical recombination after dissociation to methyl and benzyl radicals. The initial unimolecular dissociation rates show no detectable dependence on pressure; with ΔH⁰₂₉₈ = 75.7 kcal mol^?1 these rates are An RRKM extrapolation suggests in excellent agreement with previous lower temperature data.     

A spectroscopic determination of the methyl radical recombination rate constant in shock waves

The reactions CH₃ +

and CD₃ +

have been studied in shoch waves at 1200–1500 K and densities of 2 × 10^?6 ?2 × 10^?4 mol cm^?3 using UV absorption near 216 nm. The rate constants at the highest densities: k_H = (1.7 ± 0.6) × 10^?11 cm³ s^?1 and k_D = (2.2 ± 0.9) × 10^?11 cm³ s^?1 are close to the second order limit. At the lowest densities the rates are lower by a factor of 5. The experimental results agree well with theoretical predictions based on the statistical adiabatic channel model but differ from those of conventional RRKM calculations. A direct observation of the equilibrium C₂H₆ ? 2CH₃ favours the “high” value for ΔH⁰₀ (87.76 kcal/mol).     

Soot formation in the pyrolysis of benzene,methylbenzene, and ethylbenzene in shock waves

Soot formation in the pyrolysis of benzene, methylbenzene, and ethylbenzene and in the oxidative pyrolysis of benzene in shock waves has been investigated using an absorption-emission technique. Even in the presence of small amounts of oxygen, soot formation in the pyrolysis of these hydrocarbons is accompanied by a decrease in the temperature of the reacting mixture. The soot yield has been measured as a function of temperature over wide initial reactant concentration ranges. A new, larger value was obtained for the coefficient of light absorption by soot particles at a wavelength of 632.8 nm. A revised, substantially modified kinetic model is suggested for soot formation. This model has been verified against experimental data available from the literature on the time profiles of the concentrations of some key components at the early stages of pyrolysis and oxidation of various hydrocarbons in a wide range of process conditions. The model reproduces fairly well the time dependences of the soot yield and soot particle temperature measured in this study for benzene, methylbenzene, and ethylbenzene pyrolysis.     

Nonisothermal effects in the process of soot formation in ethylene pyrolysis behind shock waves

The nonisothermal nature of hydrocarbon pyrolysis explains the differences in the critical temperatures of soot formation in the experimental studies of these processes. When reaction heats are taken into account, the critical temperatures become close to 1600 K for all the systems studied. The estimated standard enthalpy of carbon atom formation in the composition of soot particles is δH_{f, z}. ≈ 11 ±6 kJ/mol. A kinetic model is proposed for soot formation in ethylene pyrolysis that describes the experimental data. The calculated temperature of soot particles may differ substantially depending on the choice of a model for energy exchange in collisions.     

The very low-pressure pyrolysis of phenyl methyl sulfide and benzyl methyl sulfide. The enthalpy of formation of the methylthio and phenylthio radicals

The kinetics and mechanisms of the unimolecular decompositions of phenyl methyl sulfide (PhSCH₃) and benzyl methyl sulfide (PhCH₂SCH₃) have been studied at very low pressures (VLPP). Both reactions essentially proceed by simple carbon-sulfur bond fission into the stabilized phenylthio (PhS·) and benzyl (PhCH₂·) radicals, respectively. The bond dissociation energies BDE(PhS-CH₃) = 67.5 ± 2.0 kcal/mol and BDE(PhCH₂-SCH₃) = 59.4 ± 2 kcal/mol, and the enthalpies of formation of the phenylthio and methylthio radicals ΔH° _,298K(PhS·, g) = 56.8 ± 2.0 kcal/mol and ΔH°_{f, 298K}(CH₃S·, g) = 34.2 ± 2.0 kcal/mol have been derived from the kinetic data, and the results are compared with earlier work on the same systems. The present values reveal that the stabilization energy of the phenylthio radical (9.6 kcal/mol) is considerably smaller than that observed for the related benzyl (13.2 kcal/mol) and phenoxy (17.5 kcal/mol) radicals.     

Unified kinetic model of soot formation in the pyrolysis and oxidation of aliphatic and aromatic hydrocarbons in shock waves

The formation of soot particles in the pyrolysis and oxidation of various aromatic and aliphatic hydrocarbons in argon behind reflected shock waves has been investigated by computational and theoretical methods. The hydrocarbons examined include methane, ethane, propane (aliphatic hydrocarbons with ordinary bonds), acetylene, ethylene, propylene (aliphatic hydrocarbons with multiple bonds), benzene, toluene, and ethylbenzene (simplest aromatic hydrocarbons). Soot formation in the pyrolysis and oxidation of both aromatic and aliphatic hydrocarbons can be simulated in detail within a unified kinetic model. The predictive power of the unified kinetic model has been verified by directly comparing the calculated kinetic data for the formation of products and reactive radicals in the pyrolysis and oxidation of various hydrocarbons to the corresponding experimental data. In all calculations, all the kinetic parameters of the unified kinetic model were strictly fixed. A good quantitative fit between the data calculated via the unified kinetic model and experimental data has been attained.     

Excimer laser perturbations of methane flames: High temperature reactions of OH and CH

An ArF excimer laser was used to perturb radical concentrations and a tunable dye laser was used to follow the rise and subsequent decay of OH and CH in rich (? = 1.6–1.8) atmospheric pressure methane flames. The excimer beam is only slightly focussed to minimize temperature excursions and the influence of diffusion and convection on the decay rates. The observed OH decay is consistent with that predicted using a detailed kinetic mechanism. The observed CH decay is much faster than predicted. The effects of equivalence ratio and height above burner suggests that a major CH decay channel involving an intermediate with higher concentration in rich flames is not properly treated in the mechanism.     

A Quantitative study of alkyl radical reactions by kinetic spectroscopy. Part I. Mutual combination of methyl radicals and combination of methyl radicals with nitric oxide

Combination reactions of the methyl radical have been studied by following the decay of the absorbance of the methyl radical during the course of the reaction by means of kinetic spectroscopy. The limiting values of the second-order rate constants at high pressure were determined for two reactions at room temperature: The extinction coefficient of the methyl radical was found to have a maximum value of (1.02 ± 0.06) X 10⁴ 1 mole^?1 cm^?1 at 216.4 nm. Integration of the extinction coefficient over the absorption band of the methyl radical gave an oscillator strength of 1.0 X 10^?2.     

Inter- and intramolecular pathways for the formation of tetrahydrofurans from beta-(Phosphatoxy)alkyl radicals. Evidence for a dissociative mechanism

beta-(Phosphatoxy)alkyl radicals generated by photolysis of Barton PTOC esters in the presence of allyl alcohol and tert-butyl mercaptan undergo nucleophilic substitution followed by 5-exo-trig radical ring closure leading to tetrahydrofurans in good yield and with high trans selectivity. beta-(Phosphatoxy)alkyl radicals obtained by intramolecular hydrogen 1,5-abstraction with an alkoxyl radical undergo nucleophilic displacement providing tetrahydrofurans. The ensemble of results, including the effects of leaving groups and substituents, strongly support a dissociative mechanism for these radical nucleophilic displacement reactions.     

Hydrogen abstraction reactions from organosilicon compounds. The reactions of methyl,trifluoromethyl, and ethyl radicals with trichlorosilane

Arrhenius parameters have been determined for the hydrogen-abstraction reactions: R + SiHCl₃ + RH + SiCl₃

Quantum-mechanical calculations on pressure and temperature dependence of three-body recombination reactions: application to ozone formation rates

A quantum-mechanical model is designed for the calculation of termolecular association reaction rate coefficients in the low-pressure fall-off regime. The dynamics is set up within the energy transfer mechanism and the kinetic scheme is the steady-state approximation. We applied this model to the formation of ozone O + O2 + M --> O3 + M for M = Ar, making use of semiquantitative potential energy surfaces. The stabilization process is treated by means of the vibrational close-coupling infinite order sudden scattering theory. Major approximations include the neglect of the O3 vibrational bending mode and rovibrational couplings. We calculated individual isotope-specific rate constants and rate constant ratios over the temperature range 10-1000 K and the pressure fall-off region 10(-7)-10(2) bar. The present results show a qualitative and semiquantitative agreement with available experiments, particularly in the temperature region of atmospheric interest.     

Kinetics and rate constants for the reaction of tri-n-butylgermanium hydride with methyl iodide and carbon tetrachloride. The combination of methyl and trichloromethyl radicals in solution.

The kinetics of the photoinitiated reductions of methyl iodide and carbon tetrachloride by tri-n-butylgermanium hydride in cyclohexane at 25°C have been studied and absolute rate constants have been measured. Rate constants for the combination of CH₃? and CCl₃? radicals are equal within experimental error and are also equal to the values found for the self-reactions of most non-polymeric radicals in low viscosity solvents, i.e. ～1–3 × 10⁹ M^?1 sec^?1. Rate constants for hydrogen atom abstraction by CH₃? and CCl₃? radicals are both ～1?2 × 10⁵ M^?1 sec^?1. Tri-n-butyltin hydride is about 10–20 times as good a hydrogen donor to alkyl radicals as is tri-n-butylgermanium hydride. The strength of the germanium–hydrogen bond, D(n-Bu₃Ge–H) is estimated to be approximately 84 kcal/mole.     

Flash photolysis of biacetyl in gas phase. Rate constants of reactions between methyl and acetyl radicals

The flash photolysis of biacetyl produces CO, C₂H₆, and CH₃COCH₃ as main products, and in small amounts CO₂, C₂H₄, and CH₃CHO. The rate constants of reactions (2) and (3) of thermally equilibrated radicals were calculated from the amounts of products: .