Kinetics and mechanism of the reaction H + CH3ONO

The reaction of hydrogen atoms with methyl nitrite was studied in a fast-flow system using photoionization mass spectrometry and excess atomic hydrogen. The associated bimolecular rate coefficient can be expressed by in the temperature range of 223-398°K. NO, CH₃OH, CH₄, C₂H₆, CH₂O, and H₂O are the main products; OH and CH₃ radicals were detectable intermediates. The mechanism was deduced from the observed product yields using normal and deuterated reactants. The primary reaction steps were identified as followed by a rapid unimolecular decomposition of CH₂ONO into CH₂O and NO. Since the extent of reaction channel (1b) could not be determined independently, only extreme limits could be obtained for the individual contributions of the two channels of reaction (3) which follows the generation of CH₃O radicals: The most probable values, k_3a/k₃ = 0.31 ± 0.30 and k_3b/k₃ = 0.69 ± 0.30, support the previous results on this reaction, although the range of uncertainties is much greater here.     

Thermal gas-phase decomposition of chloroethylenes. II. Vinyl chloride

The thermal gas-phase decomposition of vinyl chloride has been studied behind shock waves over the temperature range of 1350-1900°K and the density range of 7 × 10^?7-1.5 × 10^?3 mol/cm³ (at 1600°K) in mixtures of C₂H₃Cl highly diluted with argon. The ultraviolet absorption of C₂H₃C was recorded at 230 nm as a function of time. The decomposition proceeds via molecular elimination of HCl. The unimolecular dissociation rate is pressure dependent at all but the highest pressures applied. Application of modified HKRR theory results in the rate expression for the limiting high pressure rate constant, and in a collision efficiency of derived from the limiting low-pressure rate constant.     

The pyrolysis of hexachloroethane

The thermal decomposition of hexachloroethane in the presence of chlorine has been studied over the temperature range 340-400°C and a pressure range of 0.5°2.5 atm. The rate of the unimolecular C? C bond spliting reaction can be described by the Arrhenius equation Comparison of these rate data with thermodynamic data suggests a combination rate for CCl₃ radicals which is consistent with earlier measurements.     

On the thermal decomposition of COS

The thermal dissociation of COS was investigated in shock waves with argon as carrier gas. The concentration was varied between 0.05 and 0.5% COS in argon, the total density from 2.5 × 10^?5 mole/cm³ to 2.5 × 10^?3 mole/cm³. Temperatures between 1500°K and 3100°K were applied. For the reaction the rate constant was found to be in the low pressure range of the unimolecular reaction and in the high pressure range.     

Thermal reaction of HNCO with NO2 at moderate temperatures

The thermal reaction of HNCO with NO₂ has been studied in the temperature range of 623 to 773 K by FTIR spectrometry. Major products measured are CO₂ and NO with a small amount of N₂O. Kinetic modeling of the time resolved concentration profiles of the reactants and products, aided by the thermochemical data of various likely reactive intermediates computed by means of the BAC-MP4 method, allows us to conclude that the reaction is initiated exclusively by a new bimolecular process: with a rate constant, k₁ = 2.5 × 10¹²e^?13,100/T cm³/mols. The well-known bimolecular reaction is the only strong competitive process in this important reactive system throughout the temperature range studied. Kinetic modeling of NO formation and NO₂ decay rates gave rise to values of k₁₀ which were in close agreement with literature data. © 1993 John Wiley & Sons, Inc.     

A shock tube study of the reactions of NH with NO,O2, and O

The reactions of NH(X³Σ⁻) with NO, O₂, and O have been studied in reflected and incident shock wave experiments. The source of NH in all the experiments was the thermal dissociation of isocyanic acid, HNCO. Time-histories of the NH(X³Σ) and OH(X²Π) radicals were measured behind the shock waves using cw, narrow-linewidth laser absorption at 336 nm and 307 nm, respectively. The second-order rate coefficients of the reactions: were determined to be: and cm³ mol⁻¹ s⁻¹, where ƒ and F define the lower and upper uncertainty limits, respectively. The branching fraction of channel defined as k_3b/k_3total, was determined to be 0.19 ± 0.10 over the temperature range of 2940 K to 3040 K.     

A shock tube study of the OH + OH → H2O + O reaction

The rate coefficient for the reaction has been determined in mixtures of nitric acid (HNO₃) and argon in incident shock wave experiments. Quantitative OH time-histories were obtained by cw narrow-linewidth uv laser absorption of the R₁(5) line of the A² σ⁺ ← X² Π_i (0,0) transition at 32606.56 cm^?1 (vacuum). The experiments were conducted over the temperature range 1050–2380 K and the pressure range 0.18–0.60 atm. The second-order rate coefficient was determined to be with overall uncertainties of +11%, ?16% at high temperatures and +25%, ?22% at low temperatures. By incorporating data from previous investigations in the temperature range 298–578 K, the following expression is determined for the temperature range 298–2380 K © 1994 John Wiley & Sons, Inc.     

The thermal unimolecular decompositions of C2H5Cl,i-C3H7Cl,and t-C4H9Cl

The thermal decompositions of ethyl chloride, iso propyl chloride, and tertiary butyl chloride were studied in a static system in the pressure range of 0.1–300 torr. The following Arrhenius equations for the high-pressure limit were obtained: The pressure dependence of the first-order rate constant (falloff) for these three unimolecular dehydrochlorination reactions, starting with approximately equal k_ω values, by proper choice of temperature, is shifted to lower pressures with increasing molecular size:      

Kinetics of the reaction of cyclopentane with trichloroethylene

The kinetics of the thermally and radiation initiated chain reaction between trichloroethylene and cyclopentane to produce 1,1-dichlorovinylcyclopentane and hydrogen chloride have been investigated in the temperature range 250–360°C at high pressure in the gas phase. The rate governing step in the chain is (k₃ = 3.3 × 10⁹ exp ?(4800/RT) cc mole^?1 sec ^?1). The rate of the unimolecular decomposition of trichloroethylene is 1.4 × 10¹⁴ exp ?(61,200/RT) sec^?1.     

A kinetic study of the decomposition of HNO3 and its reaction with NO

The rate of reaction between NO and HNO₃ and the rate of thermal decomposition of HNO₃ have been measured by FTIR spectroscopy. The measurements were made in a teflon lined batch reactor having a surface to volume ratio of 14 m^?1. During the experiments, with initial HNO₃ concentrations between 2 and 12 ppm and NO concentrations between 2 and 30 ppm, a reactant stoichiometry of unity and a first order NO and HNO₃ dependence were confirmed. The observed rate constant for the reaction at 22°C and atmospheric pressure was determined to 1.1 (±0.3) 10^?5 ppm^?1 min^?1. At atmospheric pressure, HNO₃ decomposes into NO₂ and other products with a first order HNO₃ dependence and with a rate constant of 2.0 (±0.2) 10^?3 min^?1. The apparent activation energy for the decomposition is 13 (±4) kJ mol^?1.     

A pyrolysis mechanism for ammonia

The mechanism of NH₃ pyrolysis was investigated over a wide range of conditions behind reflected shock waves. Quantitative time-history measurements of the species NH and NH₂ were made using narrow-linewidth laser absorption. These records were used to establish an improved model mechanism for ammonia pyrolysis. The risetime and peak concentrations of NH and NH₂ in this experimental database have also been summarized graphically. Rate coefficients for several reactions which influence the NH and NH₂ profiles were fitted in the temperature range 2200 K to 2800 K. The reaction and the corresponding best fit rate coefficients are as follows: with a rate coefficient of 4.0 × 10¹³ exp(?3650/RT) cm³ mol^?1 s^?1, with a rate coefficient of 1.5 × 10¹⁵T^?0.5 cm³ mol^?1 s^?1 and with a rate coefficient of 5.0 × 10¹³ exp(?10000/RT) cm³ mol^?1 s^?1. The uncertainty in rate coefficient magnitude in each case is estimated to be ±50%. The temperature dependences of these rate coefficients are based on previous estimates. The experimental data from four earlier measurements of the dissociation reaction were reanalyzed in light of recent data for the rate of NH₃ + H → NH₂1 + H₂, and an improved rate coefficient of 2.2 × 10¹⁶ exp(?93470/RT) cm³ mol^?1 s^?1 in the temperature range 1740 to 3300 K was obtained. The uncertainty in the rate coefficient magnitude is estimated to be ± 15%.     

Kinetics of competitive pathways in the thermal unimolecular decomposition of hex-1-yne

The thermal unimolecular decomposition of hex-1-yne has been investigated over the temperature range of 903–1153 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via the competitive pathways of C₃? C₄ fission and molecular retro-ene decomposition, with the latter being the major pathway under the experimental conditions. RRKM calculations, generalized to take into account two competing pathways, show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters at 1100 K given by and where θ = 2.303 RT kcal/mol and the A factors were assigned from the results of recent shock-tube studies of hex-1-yne and related alkynes. The results for C? C fission are consistent with previous VLPP and shock-tube determinations of the propargyl resonance energy, and the parameters for the molecular pathway are consistent with systematic trends for the retro–ene decomposition of unsaturated hydrocarbons.     

Pyrolysis of 4,4-bisperfluoromethylbutene-1,4 01 in the gas phase

Pyrolysis of (CF₃)₂C(OH)CH₂CH=CH₂, the reverse of the reaction between perfluoroacetone and propene, has been studied in the gas phase between 475° and 598°K. Even at 573°K, the unimolecular reaction rate constant appears to be in its pressure-independent region at 20.0 torr pressure. In a quartz vessel, the decomposition is homogeneous. The specific unimolecular rate constant is where the limits are for one standard deviation. Combining these results with the previously reported results on the reverse reaction, the equilibrium constant for the reaction is It is noteworthy that in the temperature range of the study of the forward reaction (448° to 573°K), the percentage of back reaction in the times of the experiments varies from less than 0.1 to 1.5. Using group additivities and the above ΔH⁰, ΔH of (CF₃)₂CO is calculated to be ?325.2 kcal/mole at 600°K and the average C? C bond is 42.0 kcal/mole.     

Some reactions of the isopropyl radical

The decomposition of ethane sensitized by isopropyl radicals was studied in the temperature range of 496–548°K. The rate of formation of n-butane, isopentane, and 2,3-dimethylbutane were measured. The expression k₁/k₂^½ was found to be where k₁ and k₂ are rate constants of The decomposition of propylene sensitized by isopropyl radicals was studied between 494 and 580°K by determination of the initial rates of formation of the main products. The ratio of k₁₃/k₂^1/2 was evaluated to be where k₁₃ is the rate constant for The isomerization of the isopropyl radical was investigated by studying the decomposition of azoisopropane. The decomposition of the iso-C₃H₇ radical into C₂H₄ and CH₃ was followed by measuring the rate of formation of C₂H₄. On the basis of the experimental data, obtained in the range of 538–666° K, k₁₅/k₂^½ was found: where k₁₅ is the rate constant of      

The reaction of ammonia with nitrogen dioxide in a flow reactor: Implications for the NH2 + NO2 reaction

The NH₃/NO₂ system has been investigated experimentally in an isothermal flow reactor in the temperature range 850–1350 K. The experimental data were interpreted in terms of a detailed reaction mechanism. The flow reactor results, supported by a theoretical analysis of the NH₂? NO₂ complex, suggest that the NH₂ + NO₂ reaction has two major product channels, both proceeding without activation barriers: Our findings indicate that the N₂O + H₂O channel is dominant at low temperatures while H₂NO + NO dominates at high temperatures. The rate constant for reaction (R21) is estimated to be 3.5 · 10¹² cm³/mol-s in the temperature range studied with an uncertainty of a factor of 3. © 1995 John Wiley & Sons, Inc.     

Thermische Zerfallsreaktionen von Nitroverbindungen in Stosswellen. II: Dissoziation von Nitroäthan

A shock wave study of the thermal decomposition of nitroethane in excess Ar at temperatures 900 < T < 1350 K and total concentrations of 4,5 · 10^?6 < [Ar] < 3 · 10^?4 mol cm^?3 showed that the C? N-bond fission is the primary reaction step. This unimolecular reaction could be observed in its transition region near the high pressure limit. The derived rate constants are k_∞ = 10^15.9 exp (–57 kcal mol^?1/RT) s^?1 for the high pressure and k₀/[Ar] = 10^18.0 exp (-36 kcal mol^?1/RT) cm³ mol^?1 s^?1 (at T ? 1100–1200 K) for the low pressure limit. The observed concentration profiles of C₂H₅NO₂ and NO₂ permitted to conclude on the subsequent decomposition of the ethyl radical This reaction was found to be in the fall-off range under the applied conditions.     

Effect of substituents in the gas-phase elimination kinetics of β-substituted ethyl acetates

The kinetics of gas-phase elimination of 3-methyl-1-butyl acetate and 3,3-dimethyl-1-butyl acetate into acetic acid and the corresponding substituted butenes have been measured over the temperature range of 360–420°C and the pressure range of 63–250 Torr. The reactions are homogeneous in both clean and seasoned vessels, obey first-order law, and are unimolecular. The temperature dependence of the rate constants is given by the Arrhenius equation 3-methyl-1-butyl acetate: 3,3-dimethyl-1-butyl acetate: The points in a plot of log (k/k₀) of β-alkyl and several β-substituted ethyl acetates against E_s values appear aligned in an approximate linear relationship. These results may be interpreted as a consequence of steric effects, namely, steric accelerations.     

Chain ion molecular mechanism of ascorbic acid oxidation with hydrogen peroxide. Cu3+ ion as a chain carrier

The kinetics and mechanism of ascorbic acid (DH₂) oxidation have been studied under anaerobic conditions in the presence of Cu²⁺ ions. At 10^?4 ≤ [Cu²⁺]₀ < 10^?3M, 10^?3 ≤ [DH₂]₀ < 10^?2M, 10^?2 ≤ [H₂O₂] ≤ 0.1M, 3 ≤ pH < 4, the following expression for the initial rate of ascorbic acid oxidation was obtained: where χ₂ (25°C) = (6.5 ± 0.6) × 10^?3 sec^?1. The effective activation energy is E₂ = 25 ± 1 kcal/mol. The chain mechanism of the reaction was established by addition of Cu⁺ acceptors (allyl alcohol and acetonitrile). The rate of the catalytic reaction is related to the rate of Cu⁺ initiation in the Cu²⁺ reaction with ascorbic acid by the expression where C is a function of pH and of H₂O₂ concentration. The rate equation where k₁(25°C) = (5.3 ± 1) × 10³M^?1 sec^?1 is true for the steady-state catalytic reaction. The Cu⁺ ion and a species, which undergoes acid–base and unimolecular conversions at the chain propagation step, are involved in quadratic chain termination. Ethanol and terbutanol do not affect the rate of the chain reaction at concentrations up to ≈0.3M. When the Cu²⁺–DH₂–H₂O₂ system is irradiated with UV light (λ = 313 nm), the rate of ascorbic acid oxidation increases by the value of the rate of the photochemical reaction in the absence of the catalyst. Hydroxyl radicals are not formed during the interaction of Cu⁺ with H₂O₂, and the chain mechanism of catalytic oxidation of ascorbic acid is quantitatively described by the following scheme. Initiation: Propagation: Termination:      

Pyrolysis of Propyne/2-Methylbut-1-ene-3-yne mixtures between 350 and 450°C

2-Methylbut-1-ene-3-yne and Propyne mixtures were pyrolyzed at 350–450°C in the absence and presence of O₂ and NO. The major product of the reaction is a polymer, but m-xylene and p-xylene are also produced and were studied as the species of interest. The C₈H₁₀ formation rate is first-order in C₃H₄ and C₅H₆. The rate coefficient is best fitted by though it is not inconsistent with where R is the ideal gas constant in kJ/mol-K. Experiments in the presence of NO show that m-xylene and p-xylene formation occur by two processes: a concerted molecular mechanism (? 41%) and a singlet diradical mechanism (? 59%).     

Effect of Crack Temperature on the Pyrolysis of Ethane in Shock Tubes

Accepting a previously proposed mechanism of ethane pyrolysis in shock tubes, experimental product distributions observed by Burcat et al. have been used, through computer simulation technique, to measure rate constants of following reactions in the temperature range 1206–1386°K. Obtained k₂values are substantially higher than the extrapolation from an Arrhenius equation reported for the same rate constant at low temperatures. Rate constant k₂ is found decreasing with the system temperature at 6.3 atm of argon.