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A high-resolution IR diode laser in conjunction with a Herriot multiple reflection flow-cell has been used to directly determine the rate coefficients for simple alkanes with Cl atoms at room temperature (298 K). The following results were obtained: k(Cl + n-butane) = (1.91 ′ 0.10) × 10 - 1 0 cm 3 molecule - 1 s - 1 , k(Cl + n-pentane) = (2.46 ′ 0.12) × 10 - 1 0 cm 3 molecule - 1 s - 1 , k(Cl + iso-pentane) = (1.94 ′ 0.10) × 10 - 1 0 cm 3 molecule - 1 s - 1 , k(Cl + neopentane) = (1.01 ′ 0.05) × 10 - 1 0 cm 3 molecule - 1 s - 1 , k(Cl + n-hexane) = (3.44 ′ 0.17) × 10 - 1 0 cm 3 molecule - 1 s - 1 where the error limits are ′1σ. These values have been used in conjunction with our own previous measurements on Cl + ethane and literature values on Cl + propane and Cl + iso-butane to generate a structure activity relationship (SAR) for Cl atom ion reactions based on direct measurements. The resulting best fit parameters are kp = (2.61 ′ 0.12) × 10 - 1 1 cm 3 molecule - 1 s - 1 , k s = (8.40 ′ 0.60) × 10 - 1 1 cm 3 molecule - 1 s - 1 . k t = (5.90 ′ 0.30) × 10 - 1 1 cm 3 molecule - 1 s - 1 , with f(-CH 2 -) = f(>CH 2 -) = f(>C

The kinetics of C6H5 reactions with C2H6 (1) and neo-C5H12 (2) have been studied by the pulsed laser photolysis/mass spectrometric method using C6H5COCH3 as the phenyl precursor at temperatures between 565 and 1000 K. The rate constants were determined by kinetic modeling of the absolute yields of C6H6 at each temperature. Another major product, C6H5CH3, formed by the recombination of C6H5 and CH3, could also be quantitatively modeled using the known rate constant for the reaction. A weighted least-squares analysis of the two sets of data gave k1 1011.32 0.05 exp[ (2236 91)/T] cm3 mol 1 s 1 and k2 1011.37 0.03 exp[ (1925 48)/T] cm3 mol 1 s 1 for the temperature range studied. The result of our sensitivity analysis clearly supports that the yields of C6H6 and C6H5CH3 depend primarily on the abstraction reactions and C6H5 CH3, respectively. From the absolute rate constants for the two reactions we obtained the value for the H-abstraction from a primary C-H bond, kp-CH 1010.40 0.06 exp( 1790 102/T) cm3 mol 1 s 1. This result is utilized for analysis of other kinetic data measured for C6H5 reactions with alkanes in solution as well as in the gas phase. 2000 John Wiley & Sons, Inc. Int J Chem Kinet 33: 64–69, 2001

The reaction of atomic chlorine with neopentane was studied in the gas phase with the Very Low Pressure Reactor (VLPR) technique over the temperature range 273–333 K. The absolute reaction rate was found to be temperature-independent, and the average rate constant was k1 = (1.11± 0.13) × 10−10 cm3 molecule−1 s−1 within experimental error. The reaction proceeds via metathesis of a hydrogen atom with no activation energy, and leads to the formation of HCl and neopentyl radical. © 1995 John Wiley & Sons, Inc.

The mechanism for neopentane (NpH) pyrolysis in the absence and presence of additives isobutene, HCl and HBr, in the temperature range 750–800 K, has been reinvestigated with the aid of computer simulation and sensitivity analysis techniques. With best values assigned to all rate constants in the kinetic chain, a basic mechanism comprising 18 reversible reactions involving 19 atomic, radical, and molecular species has been used to simulate pure neopentane pyrolysis data. Predictions of major and minor product yields provided quantitative agreement with experimental data against which the model was tested. The mechanism was supplemented by additional species and reactions in order to simulate experimental neopentane pyrolysis data in the presence of HCl and HBr additives. An apparent discrepancy between a recent direct measurement of k5, the rate constant for thermal decomposition of the neopentyl radical [1], and that reported from studies of neopentane pyrolysis in the presence and absence of HCl [2], has been identified as being due to the use of an incomplete mechanism in the latter determination. Simulations of hydrogen halide catalyzed pyrolyses exhibit a high sensitivity to the thermochemical parameters associated with the neopentyl radical (Np). The influence of uncertainties in ΔH(Np) and S(Np) are evaluated and lead to suggested values ΔH(Np) = 8.7 ± 0.8 kcal mol−1 and S(Np) = 78.8 ± 1.0 cal mol−1 K−1. © 1993 John Wiley & Sons, Inc.

The yields of C5 and C6 alkyl nitrates from neopentane, 2-methylbutane, 2-methylpentane, 3-methylpentane, and cyclohexane have been measured in irradiated CH3ONONO-alkane-air mixtures at 298 ± 2 K and 735-torr total pressure. Additionally, OH radical rate constants for neopentyl nitrate, 3-nitro-2-methylbutane, 2-nitro-2-methylpentane, 2-nitro-3-methylpentane, and cyclohexyl nitrate, relative to that for n-butane, have been determined at 298 ± 2 K. Using a rate constant for the reaction of OH radicals with n-butane of 2.58 × 10−12 cm3 molecule−1 s−1, these OH radical rate constants are (in units of 10−12 cm3 molecule−1 s−1): neopentyl nitrate, 0.87 ± 0.21; cyclohexyl nitrate, 3.35 ± 0.36; 3-nitro-2-methylbutane, 1.75 ± 0.06; 2-nitro-2-methylpentane, 1.75 ± 0.22; and 2-nitro-3-methylpentane, 3.07 ± 0.08. After accounting for consumption of the alkyl nitrates by OH radical reaction and for the yields of the individual alkyl peroxy radicals formed in the reaction of OH radicals with the alkanes studied, the alkyl nitrate yields (which reflect the fraction of the individual RO2 radicals reacting with NO to form RONO2) determined were: neopentyl nitrate, 0.0513 ± 0.0053; cyclohexyl nitrate, 0.160 ± 0.015; 3-nitro-2-methylbutane, 0.109 ± 0.003; 2-nitro-2methylbutane, 0.0533 ± 0.0022; 2-nitro-2-methylpentane, 0.0350 ± 0.0096; 3- + 4-nitro-2-methylpentane, 0.165 ± 0.016; and 2-nitro-3-methylpentane, 0.140 ± 0.014. These results are discussed and compared with previous literature values for the alkyl nitrates formed from primary and secondary alkyl peroxy radicals generated from a series of n-alkanes.

The experimental data for the reactions of oxygen atoms with methane, ethane, and neopentane at temperatures below ca. 600 K have been reexamined. In the case of CH4 and C2H6 reactions, detailed computer models have been assembled to test the assumptions regarding stoichiometries that were made in the original studies in order to derive elementary rate coefficients from the experimentally observed reaction rates. It was found in both cases that the measurements are especially sensitive to secondary reactions not taken into account and impurities in the reagent alkane. Because the original reports did not include sufficient experimental details, it is not now possible to correct their results quantitatively. However, it appears, qualitatively, that the values for the O + CH4 and O + C2H6 rate coefficients were overestimated by factors of approximately 2 to 3 in the 250–400 K temperature range, with the error increasing as T decreases. Although the experimental results for the O + neopentane reaction are not as sensitive to the same kinds of complications, a comparison of the low-temperature measurements with those for the O + ethane reaction suggests that the previously recommended rate coefficients, based on the data of Herron and Huie, are probably also too high by a factor of 2 to 3.

Using methyl nitrite photolysis in air as a source of hydroxyl radicals, relative rate constants for the reaction of OH radicals with a series of alkanes and alkenes have been determined at 299 ± 2 K. The rate constant ratios obtained are: relative to n-hexane = 1.00, neopentane 0.135 ± 0.007, n-butane 0.453 ± 0.007, cyclohexane 1.32 ± 0.04; relative to cyclohexane = 1.00, n-butane 0.341 ± 0.002, cyclopentane 0.704 ± 0.007, 2,3-dimethylbutane 0.827 ± 0.004, ethene 1.12 ± 0.05; relative to propene = 1.00, 2-methyl-2-butene 3.43 ± 0.13, isoprene 3.81 ± 0.17, 2,3-dimethyl-2-butene 4.28 ± 0.21. These relative rate constants are placed on an absolute basis using previous absolute rate constant data and are compared and discussed with literature data.

Pyrolysis of a dilute mixture of neopentane (2,2-dimethylpropane) has been studied behind incident shock waves at an average pressure of 0.35 atm; the reaction was followed by absorption spectroscopy for H atoms. In the temperature range 1230–1455 K, the rate constant for dissociation of neopentane to t-butyl and methyl radicals is 1.1 E 13 exp(−62 kcal/RT) s−1. These data and some of the literature results, between 1000 and 1450 K, can be fitted by an RRKM model of the hindered Gorin type, with five active internal rotors in the complex. To match our data with other literature data down to 800 K, a vibrational model was more satisfactory, but this did not fit very low pressure pyrolysis data in the 1000–1100 K range. Apparently, the VLPP data are too high because of heterogeneous processes or chain reactions.

The pyrolysis of 2,2,3,3-tetramethylbutane (TMB) has been studied using the wall-less reactor. Under homogeneous conditions up to 670°C, the activation energy is 43.2 kcal/mol and log A is 10.70. The products are 2-methylpropene, hydrogen, propene, 2-methyl-2-butene, ethane, ethene, methane, neopentane, and 2,3-dimethyl-2-butene in order of abundance. Addition of toluene, cyclopentadiene, or nitrous oxide to the system increases both the activation energy and the A factor; whereas oxygen decreases both the activation energy and the A factor. Incorporation of stainless-steel rods to provide surface has only a modest effect upon the rate. In view of this evidence, it appears that the homogeneous pyrolysis of TMB at higher pressures is a chain reaction under the described conditions.

The kinetics of acetylene polymerization initiated by neopentane (Np) or acetone (Ac) decompositions has been investigated in a static reactor dynamically coupled to a modulated beam mass spectrometer between 850–950 K. Overall rates follow the expression: R = −d[C_2H_2]/dt = k_s[X]^(1/2)[C_2H_2] + k_u[C_2H_2]_2 (I), where X represents Np or Ac and k_s, k_u the rate constants of the sensitized and unsensitized reactions, respectively. The rate law of the sensitized reaction clearly suggests a chain polymerization mechanism with k_s = k_p(k_i/k_t)^(1/2) (i, t, and p stand for initiation, termination, and propagation, respectively). Remarkably, the derived values of k_p are nearly independent of the sensitizer, although Ac acts as a source of methyl radicals whereas Np also produces hydrogen atoms, and fall in the expected range for the addition of vinylic radicals to acetylene. It is shown that a chain transfer process involving the fast [1,5] intramolecular hydrogen atom shift in 4-methyl-buta-1,3-dien-1-yl radicals (CH_3-CH=CH-CH=ĊH) followed by further addition to C_2H_2 and aromatization, transforms methyl radicals into hydrogen atoms and is able to account for the presence of toluene among the products of the sensitized reactions. Based on current thermochemical data for the but-1-en-3-yn-2-yl radical (CH_2=Ċ-C≡CH) and present rates of propagation it is argued that if the unsensitized polymerization of acetylene also proceeded by a vinyl radical chain, then even the most favorable self-initiation reaction: 2C_2H_2 = C_4H_3 + H (a), would be far too slow. Finally, present results also show that acetone at impurity levels (⩽ 0.1%) can not provide fast enough spurious initiation rates in chain mechanisms for the “unsensitized” acetylene pyrolysis at pressures above 10 torr.

The initial rates of formation of the major products in the thermal reactions of ethylene at temperatures in the neighborhood of 800 K have been measured in the presence and absence of the additives neopentane and ethane. It has been shown that in the absence of the additive the main initiation process is (1) while in the presence of neopentane and ethane the following additional initiation processes occur: (2) From the ratios of the rates of formation of the major products in the presence and absence of the additive the ratios kN/k1 and kE/k1 were measured over the temperature range of 750–820 K. Taking values from the literature for kN and kE, the following value was obtained for k1: Previous results using butene-1 as additive were rexamined and shown to be consistent with this measurement. From this measurement the following values were derived: ΔHf(C2H3) = 63.4 ± 2 kcal/mol and D(C2H3H) = 103 kcal/mol.

Using a relative rate technique, rate constants have been determined for the gas phase reactions of Cl atoms with a series of organics at 296 ± 2 K and atmospheric pressure of air. Using a rate constant of 1.97 × 10−10 cm3 molecule−1 s−1 for the reaction of Cl atoms with n-butane, the following rate constants (in units of 10−11 cm3 molecule−1 s−1) were obtained: ethane, 6.38 ± 0.18; propane, 13.4 ± 0.5; isobutane, 13.7 ± 0.2; n-pentane, 25.2 ± 1.2; isopentane, 20.3 ± 0.8; neopentane, 11.0 ± 0.3; n-hexane, 30.3 ± 0.6; cyclohexane, 31.1 ± 1.4; 2,3-dimethylbutane, 20.7 ± 0.6; n-heptane, 34.1 ± 1.2; acetylene, 6.28 ± 0.18; ethene, 10.6 ± 0.3; propene, 24.4 ± 0.8; benzene, 1.5 ± 0.9; and toluene, 5.89 ± 0.36. These data are compared and discussed with the available literature values.

A symbolic mechanism “μH, YH” has been proposed to account for the homogeneous chain pyrolysis of an organic compound μH in the presence of a hydrogenated additive YH at small extents of reaction. An analysis of this mechanism leads to two limiting cases: the thermal decomposition of neopentane corresponds to the first one (A), that of ethane to the second one (B). Previous experimental work has shown that this mechanism seems to account for a number of experimental observations, especially the inhibition of alkane pyrolyses by alkenes. Experimental investigations were extended by examining the influences oftwo hydrogen halides (ClH and BrH) upon the pyrolyses of neopentane (at 480°C) and ethane (around 540°C). The experiments have been performed in a conventional static Pyrex apparatus and reaction products have been analyzed by gas-liquid chromatography. The study shows that ClH and BrH accelerate the pyrolysis of neopentane (into i-C4H8 + CH4). The experimental results are interpreted by reaction schemes which appear as examples of the mechanism “μH, YH” in the first limiting case (A). The proposed schemes enable one to understand why the accelerating influence of ClH is lower or higher than that of BrH, depending on the concentration of the additive. An evaluation of the rate constant of the elementary steps neo-C5H11 · i-C4H8 + CH3 · is discussed. In the case of ethane pyrolysis, BrH inhibits the formation of the majorproducts (C2H4 + H2) and, even more, that of n-butane traces. The experimental results are interpreted by a reaction scheme which appears as an example of the mechanism “μH, YH” in the second limiting case (B). On the contrary, ClH has no noticeable influence on the reaction kinetics. This result inessentially due to the fact that the bond dissociation energy of ClH(⋍103 kcal/mol) is higher than that of C2H5—H (⋍98 kcal/mol), whereas that of Br—H (⋍88 kcal/mol) is lower.

Published data show that in its early stages (up to 3% decomposition), the pyrolysis of n-hexane in the ranges 723–823 K and 10–100 Torr is not inhibited by the olefin products, in contrast with neopentane pyrolysis which is very strongly inhibited in similar conditions. Detailed consideration of the chain mechanisms for the two pyrolyses shows that the reactivity of the chain terminating radical towards hydrogen abstraction from an allylic CH bond in product olefin is the factor which determines whether or not observable self-inhibition occurs. Thus, n-hexane pyrolysis, whose chain decomposition is terminated by recombination and disproportionation of ethyl, is not significantly self-inhibited, whereas that of neopentane which is terminated by recombination of methyl is very strongly inhibited because methyl is 14× more reactive than ethyl. The implications for other alkanes are briefly discussed.

The kinetics of the reaction have been studied, using the technique of flash photolysis and kinetic spectroscopy to follow the methyl radical concentration. The order of the reaction lies between 2 and 3 throughout the range of pressure from 25 to 380 torr at 22°C, and the results are consistent with a single reaction sequence: The limiting values of the third-order rate coefficients at low pressures are (3.6±0.3) × 1011 1.2 mole−2 sec−1 when M is neopentane, and (0.94 ± 0.03) × 1011 1.2 mole−2 sec−1 when M is nitrogen. The limiting value of the second-order rate coefficient at high pressures is (3.1 ± 0.3) × 108 1. mole−1 sec−1. The rate constant for the independent second-order reaction is shown to be not much greater than 2 × 105 1. mole−1 sec−1, so that this reaction does not complete significantly with the combination reaction. This new interpretation is contrary to currently accepted views.

The Arrhenius parameters have been determined for the SO2(3B1) quenching reaction (9), SO2(3B1) + M → (SO2 M), for 21 different molecules as quenching partner M. The rate constants were calculated from phosphorescence lifetime measurements made over a range of reactant pressures and temperatures. Excitation of the SO2 (3B1) molecules was accomplished by two very different methods: (1) a 3829 A laser pulse generated the triplet directly through absorption within the “forbidden” SO2 (3B1) → SO2 (1A1) band; (2) a broadband Xe-flash system generated SO2(3B1) molecules and triplets were formed subsequently by intersystem crossing, SO2(1B1) + M → SO2(3B1) + M. The measured rate constants were independent of the method of triplet formation employed. For the atmospheric gases, the activation energies (kcal/mole) were identical within the experimental error: N2, 2.9 ± 0.4; 02, 3.2 ± 0.5; Ar, 2.8 ± 0.6; CO2, 2.8 ± 0.4; CO, 2.7 ± 0.4; CH4, 2.5 ± 0.6. This energy corresponds to the first region of the SO2(3B1) → SO2(1A1) absorption spectra in which Brand and coworkers observe strong perturbations. It is suggested that the quenching in these cases results largely from the physical process involving potential energy surface crossing to another electronic state. Activation energies for SO2(3B1) quenching by the paraffinic hydrocarbons show a regular decrease in the series ethane, neopentane, propane, n-butane, cyclohexane, and isobutane, which parallels closely the decrease in CH bond energies in these compounds. These and other data are most consistent with the dominance of chemical quenching in these cases. The rate constants for the olefinic and aromatic hydrocarbons and nitric oxide show only very small variations with temperature change, and they are near the kinetic collision number. These data support the hypothesis that quenching in these cases is associated with the formation of a charge-transfer complex and subsequent chemical interactions between the SO2(3B1) molecule and the π-system of these compounds.

The pyrolysis of neopentane, at small extents of reaction, was studied by gas chromatography, in Pyrex reaction vessels between 450° and 530°C and in the initial pressure range 25–200 mm Hg. At initial time, this thermal decomposition can be essentially represented by a homogeneous long-chain radical mechanism. The rate constant of the unimolecular initiation process is approximately given by the expression The initial rate constant of the global reaction (order 3/2) is nearly equal to This reaction is strongly inhibited by propene or isobutene and self-inhibited by the isobutene formed; an interpretation of all these inhibition phenomena of the neopentane pyrolysis is proposed. Our observations and conclusions, which have been summarized in communications during 1968 and 1969, are compared to those of other authors, particularly to the recent ones of Purnell and colleagues [13] and of Taylor and colleagues [14], [15].

An experimental study of the decomposition kinetics of chemically activated 2-methyl-l-butene and 3-methyl-l-butene produced from photolysis of diazomethane-isobutene-neopentane-oxygen mixtures is reported. The experimental rate constants for 3-methyl-l-butene decomposition were 1.74 ± 0.44 × 108 sec−1 and 1.01 ± 0.25 × 108 sec−1 at 3660 and 4358 A, respectively. 2-Methyl-l-butene experimental decomposition rate constants were found to be 5.94 ± 0.59 × 107 sec−1 at 3660 A and 3.42 ± 0.34 × 107 sec−1 at 4358 A. Activated complex structures giving Arrhenius A-factors calculated from absolute rate theory of 1016.6 ± 0.5 sec−1 for 3-methyl-l-butene and 1016.2 ± 0.4 sec−1 for 2-methyl-l-butene, both calculated at 1000°K, were required to fit RRKM theory calculated rate constants to the experimental rate constants at reasonable E0 and E* values. Corrected calculations (adjusted E0 values) on previous results for 2-pentene decomposition gave an Arrhenius A-factor of 1016.45 ± 0.35 sec−1 at 1000°K. The predicted A-factors for these three alkene decompositions giving resonance-stabilized methylully radicals are in good internal agreement. The fact that these A-factors are only slightly less than those for related alkane decompositions indicates that methylallylic resonance in the decomposition products leads to only a small amount of tightening in the corresponding activated complexes. This tightening is a significantly smaller factor than the large reduction in the critical energy due to resonance stabilization.

A check of the data from comparative rate single-pulse shock tube experiments have been carried out through the use of a new standard reaction, the decyclization reaction of ethylcyclobutane. The rate expressions for cyclohexene and 2,2,3-trimethylbutane have been found to be in excellent agreement with previously published results. Most of the small discrepancy that does exist is apparently due to the differences between the present and earlier (decomposition of isopropyl bromide) "standard" reaction. For the latter process, the present study yields These results confirm the correctness of previously published comparative rate single-pulse shock tube experiments. They demonstrate once again that for the decomposition of paraffin hydrocarbons, calculated preexponential factors are at least an order of magnitude higher than the directly measured number and that the accepted value of the heat of formation of t-butyl radicals ΔHf300(tC4H9·) = 29 kJ (6.8 kcals) is at least 10 kJ too low. Finally, attention is called to recent studies on neopentane decomposition in flow and static systems which are in complete agreement with the present conclusions.

The following Arrhenius parameters have been determined for the hydrogen-abstraction reactions: R + (CH3)4Si RH + (CH3)3SiCH3Text Table Text. R Temp. (°K) E (kcal/mole) Log A (mole−1 cc sec−1) Log k(400°K) (mole−1 cc sec−1) CF3 330–433 7.23 ± 0.09 11.90 ± 0.05 7.95 CH3 396–476 10.23 ± 0.36 11.55 ± 0.18 5.68 CD3 396–496 10.36 ± 0.12 11.84 ± 0.06 6.20 C2H5 423–522 11.40 ± 0.48 11.88 ± 0.22 5.68 The activation energies are in keeping with the strengths of the bonds formed during the reaction. By comparison with the activation energies for the analogous reactions of neopentane it is estimated that D((CH3)3SiCH2H) ≃ 97 kcal/mole. The A factors for the above series of reactions fall within the range predicted by transition-state theory for this type of process and the validity of previous results of Kerr, Slater, and Young is seriously in doubt.

A study of the structural isomerization rate of chemically activated 1,1-dimethylcyclopropane from singlet methylene addition to the double bond of isobutene is reported. Singlet methylenes were produced from the 4358- and 3660-A photolysis of diazomethane in the presence of added oxygen. Theoretical rates calculated via RRKM theory are in excellent agreement with experiment for calculations utilizing activated complex structures and critical energies consistent with known thermal Arrhenius parameters, and excitation energies consistent with previous determinations of ΔH(CH2) + E*(CH2) = 116.1 and 112.6 kcal/mole for diazomethane photolyses at 3660 and 4358 A, respectively.