Single pulse shock tube studies on the reactions of hydrogen atoms with unsaturated compounds

Tsang

1996

J. Phys. Chem.

Hydrogen atom attack on 1,2-dichlorotetrafluoroethane (DCFE) at temperatures between 970 and 1140 K and pressures of about 200−500 kPa (2−5 bar) has been studied with a single-pulse shock tube reactor. The observed products show that chlorine is readily abstracted, while fluorine is removed in at most trace quantities only. This is despite the greater exothermicity of the fluorine abstraction channel. Relative to the standard reaction of displacement of methyl from 1,3,5-trimethylbenzene, for which k[H + (CH3)3C6H3 → (CH3)2C6H4 + CH3] = 6.7 × 1013 exp(−3255/T) cm3 mol-1 s-1, k(H + DCFE → CF2CF2Cl + HCl) = 2.1 × 1014 exp(−5839/T) cm3 mol-1 s-1 and k(H + DCFE → CFClCF2Cl + HF, 1100 K) ≤ 7 × 109 cm3 mol-1 s-1. Uncertainties are estimated to be factors of 1.1 and 1.4 on relative and absolute bases, respectively. Comparisons with relevant literature data are made. The slow rate of F abstraction implies that reaction of fluorinated compounds via this pathway will be unimportant except in compounds unable to react via other channels. Implications with regard to the destruction of fluorinated compounds in pyrolytic and combustion systems are discussed.

Section: Methodsmentioning

confidence: 84%

“…Experiments were carried out in a heated single-pulse shock tube. Details of the system have been published previously. , Briefly, a gas mixture is shocked, and H atoms are generated from the thermal decomposition of a precursor molecule. The hydrogen atoms react competitively with a reaction standard and the compound to be studied.…”

Section: Methodsmentioning

confidence: 99%

Hydrogen Atom Attack on 1,2-Dichlorotetrafluoroethane: Rates of Halogen Abstraction

Tsang

1996

J. Phys. Chem.

“…Tsang et al 36 have reported k(H + 135TMB → m-xylene + CH 3 ) = 6.7 × 10 13 exp(−3255/T) cm 3 mol −1 s −1 , which in turn is relative to k(H + CH 4 → CH 3 + H 2 ) = 2.4 × 10 11 exp(−7000/T) cm 3 mol −1 s −1 . In recent work, Sheen et al 37 have reanalyzed the data on the H + propene/135TMB system, 17 Kinetics of 1-Butene Decomposition.…”

Section: Resultsmentioning

confidence: 96%

Evaluated Kinetics of Terminal and Non-Terminal Addition of Hydrogen Atoms to 1-Alkenes: A Shock Tube Study of H + 1-Butene

Awan

2015

J. Phys. Chem. A

Single-pulse shock tube methods have been used to thermally generate hydrogen atoms and investigate the kinetics of their addition reactions with 1-butene at temperatures of 880 to 1120 K and pressures of 145 to 245 kPa. Rate parameters for the unimolecular decomposition of 1-butene are also reported. Addition of H atoms to the π bond of 1-butene results in displacement of either methyl or ethyl depending on whether addition occurs at the terminal or nonterminal position. Postshock monitoring of the initial alkene products has been used to determine the relative and absolute reaction rates. Absolute rate constants have been derived relative to the reference reaction of displacement of methyl from 1,3,5-trimethylbenzene (135TMB). With k(H + 135TMB → m-xylene + CH3) = 6.7 × 10(13) exp(-3255/T) cm(3) mol(-1) s(-1), we find the following: k(H + 1-butene → propene + CH3) = k10 = 3.93 × 10(13) exp(-1152 K/T) cm(3) mol(-1) s(-1), [880-1120 K; 145-245 kPa]; k(H + 1-butene → ethene + C2H5) = k11 = 3.44 × 10(13) exp(-1971 K/T) cm(3) mol(-1) s(-1), [971-1120 K; 145-245 kPa]; k10/k11 = 10((0.058±0.059)) exp [(818 ± 141) K/T), 971-1120 K. Uncertainties (2σ) in the absolute rate constants are about a factor of 1.5, while the relative rate constants should be accurate to within ±15%. The displacement rate constants are shown to be very close to the high pressure limiting rate constants for addition of H, and the present measurements are the first direct determination of the branching ratio for 1-olefins at high temperatures. At 1000 K, addition to the terminal site is favored over the nonterminal position by a factor of 2.59 ± 0.39, where the uncertainty is 2σ and includes possible systematic errors. Combining the present results with evaluated data from the literature pertaining to temperatures of <440 K leads us to recommend the following: k∞(H + 1-butene → 2-butyl) = 1.05 × 10(9)T(1.40) exp(-366/T) cm(3) mol(-1) s(-1), [220-2000 K]; k∞(H + 1-butene → 1-butyl) = 9.02 × 10(8)T(1.40) exp(-1162/T) cm(3) mol(-1) s(-1) [220-2000 K]. Analogous rate constants for other unbranched 1-olefins should be very similar. Despite this, a factor of three discrepancy in the branching ratio for terminal and nonterminal addition is noted when comparing the present values with recommendations from a recent model of the important H + propene reaction. This difference is suggested to be well outside of the possible experimental errors of the present study or the expected differences with 1-butene. There thus appear to be inconsistencies in the current model for propene. In particular the addition branching ratio from that model should not be used as a reference value in extrapolations to other systems via rate rules or automated mechanism generation techniques.

“…The rate constants for H + 1,3,5-trimethylbenzene are from Sheen et al Abstraction (R815) is more favorable than substitution (R814) by a ratio of about 2:1. , Uncertainties in the absolute rate constants are expected to be about a factor of 1.5. The absolute rate constants agree with literature values for toluene + H (multiplied by 3 to account for the number of methyl groups on the aromatic ring) to within a factor of 2 for the temperature range of this study. ,− They are also within 30% of the values in JetSurF 2.0 for the reaction of toluene with H and within 30% of previous work by Robaugh and Tsang. , The rate of 1,3,5-trimethylbenzene reaction with CH 3 was found by scaling the JetSurF rate constant for CH 3 + Toluene to the number of labile H’s.…”

Section: Chemical Modeling Resultsmentioning

confidence: 99%

“…The rate constants for H + 1,3,5-trimethylbenzene are from Sheen et al 41 Abstraction (R815) is more favorable than substitution (R814) by a ratio of about 2:1. 53,59 Uncertainties in the absolute rate constants are expected to be about a factor of 1.5. The absolute rate constants agree with literature values for toluene + H (multiplied by 3 to account for the number of methyl groups on the aromatic ring) to within a factor of 2 for the temperature range of this study.…”

Section: Chemical Modeling Resultsmentioning

confidence: 99%

β-Bond Scission and the Yields of H and CH₃ in the Decomposition of Isobutyl Radicals

Mertens

2018

J. Phys. Chem. A

The relative rates of C-C and C-H β-scission reactions of isobutyl radicals (2-methylprop-1-yl, CH) were investigated with shock tube experiments at temperatures of (950 to 1250) K and pressures of (200 to 400) kPa. We produced isobutyl radicals from the decomposition of dilute mixtures of isopentylbenzene and observed the stable decomposition products, propene and isobutene. These alkenes are characteristic of C-C and C-H bond scission, respectively. Propene was the main product, approximately 30 times more abundant than isobutene, indicating that C-C β-scission is the primary pathway. Uncertainty in the ratio of [isobutene]/[propene] from isobutyl decomposition is mainly due to a small amount of side chemistry, which we account for using a kinetics model based on JetSurF 2.0. Our data are well-described after adding chemistry specific to our system and adjusting some rate constants. We compare our data to other commonly used kinetics models: JetSurF 2.0, AramcoMech 2.0, and multiple models from Lawrence Livermore National Laboratory (LLNL). With the kinetics model, we have determined an upper limit of 3.0% on the branching fraction for C-H β-scission in the isobutyl radical for the temperatures and pressures of our experiments. While this agrees with previous high quality experimental results, many combustion kinetics models assume C-H branching values above this upper limit, possibly leading to large systematic inaccuracies in model predictions. Some kinetics models additionally assume contributions from 1,2-H shift reactions-which for isobutyl would produce the same products as C-H β-scission-and our upper limit includes possible involvement of such reactions. We suggest kinetics models should be updated to better reflect current experimental measurements.