Influences of ClH and BrH on the pyrolyses of neopentane and ethane at small extents of reaction

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 Cl? H(?103 kcal/mol) is higher than that of C₂H₅—H (?98 kcal/mol), whereas that of Br—H (?88 kcal/mol) is lower.     

Site-site potentials in neopentane and tetramethylsilane

Neopentane and TMS are used as model M(CH(3))(4) systems to investigate intramolecular interactions. The nonbonded site-site potential between two proximal hydrogen atoms on different methyl groups, V(nb)(d(HH)), is not Lennard-Jones- or Morse-like but is found to be pseudolinear in hydrogen-hydrogen internuclear separation, d(HH), for both neopentane and TMS. The Morse potential is found to be a poor basis in which to expand V(nb)(d(HH)). The nonbonded site-site potential is conformation-dependent and not transferable between molecules. The individual contributions to V(nb)(d(HH)) are presented. The local mode parameters for neopentane and TMS are calculated ab initio for a variety of molecular conformations. The ab initio values of the local mode frequency and local mode anharmonicity are increasingly blue-shifted with increasing steric hindrance. Electron correlation is found to be increasingly important with decreasing internuclear separations, d(HH).     

Muonium atoms observed in neopentane and tetramethylsilane

Muonium has been observed in neopentane and tetramethylsilane by a pulsed MuSR method. The probability of muonium formation was ≈ 20%, and its relaxation times were 8 and 2.6 μs for liquid neopentane and tetramethylsilane, respectively. The rate of muonium relxation has been shown to be correlated well with the dissociation energy of the CH bonding.     

The pyrolyses of tri- and tetramethylethylene in the presence of nitric oxide

The pyrolyses of trimethylethylene and tetramethylethylene have been investigated in the presence and absence of nitric oxide. It appears that apart from a unimolecular split, e.g., a disproportionation reaction such as may play an important role in initiation. Nitric oxide had no effect on H₂ production, which is probably a molecular process. There was similar behavior of both compounds in the presence of NO, indicating that the olefinic hydrogen atom does not play a decisive role. Other aspects of the mechanisms are discussed.     

Bandwidths in the gas phase overtone spectra of neopentane

The overtone spectrum of neopentane vapor is measured from 63 to 670 Torr. The bandwidth for δ_vCH = 5 is substantially smaller for the gas than the liquid. The local mode overtone bandwidths are much smaller than for other molecules. The results are discussed in terms of local mode coupling and vibrational state dynamics.     

Volumetric and acoustic behaviour of systems containing n-hexane,or n-heptane and isomeric chlorobutanes

Densities and speeds of sound at atmospheric pressure and temperatures of (283.15, 298.15, and 313.15) K for the binary mixtures formed by n-hexane or n-heptane with isomeric chlorobutanes were determined. Afterwards excess volumes, excess isentropic compressibilities and excess speeds of sound were calculated using the experimental data and correlated using a Redlich–Kister type equation. Finally, the results were studied using the Prigogine–Flory–Patterson theory showing excellent predictions for speed of sound and isentropic compressibility values of mixtures as well as a strong influence of the interactional contribution term for excess volume values.     

Generation of alkenyl-type carbocations from neopentane in HNaY zeolite

In the transformation of neopentane in HNaY zeolite, the presence of alkenyl-type carbocations could be detected by UV-VIS spectroscopy. Generation of these ions was not accompanied by the formation of methane which could be the primary cracking product of neopentane.

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HNaY, - . , .

     

Direct kinetic parameters for the reaction of Br atoms with neopentane

Laser flash photolysis coupled with resonance fluorescence detection of Br atoms was employed to investigate the temperature dependence of the reaction Br + neo‐C₅H₁₂ (1) between 688 and 775 K. The following Arrhenius preexponential factor and activation energy were determined (±1 σ): A₁ = (6.89 ± 2.27) 10¹⁴ cm³ mol⁻¹ s⁻¹ and E_A,1 = 57.61 ± 2.05 kJ mol⁻−¹ The only other kinetic parameters reported for the reaction of Br atoms with neo‐C₅H₁₂ were obtained from competitive kinetic experiments relative to Br + C₂H₆. Comparison with our direct results is hampered by uncertainties in the kinetic data for the reference reaction that may need reinvestigation. The standard enthalpy of formation for the neo‐C₅H₁₁ radical was estimated to be 34.7 and 41.6 kJ mol⁻¹, depending on the value of the activation energy assumed for the reverse reaction neo‐C₅H₁₁ + HBr (−1). © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 33: 49–55, 2001     

Fine structure in the VUV absorption spectrum of neopentane at 16 eV

The photoabsorption of neopentane (C(CH₃)₄) has been measured for photon energies from 10 to 30 eV with synchrotron radiation. Sharp absorption bands are observed around 16 eV on top of a strong continuum. These can be grouped into a p-like Rydberg series with accompanying vibrational sub-bands. The nature of the Rydberg states is discussed.     

Molecular structures of neopentane and di-tert-butylmethane by vapor-phase electron diffraction

Although the present molecules are much less strained than the tri-t-butyl member of the series CH_n(t-Bu)_4–n studied previously, di-t-butylmethane nevertheless exhibits striking steric deformations due to its pair of inescapable GG' conformations. The two adjacent t-butyl groups respond to the steric stress by undergoing torsional displacements of 15 ± 6° (3σ), by tilting away from each other by 3–5°, and by opening up the central CCC bond angle to 125–128° (parameter value sensitive to assumptions in analysis). Carbon-carbon bonds, with mean lengths of 1.545 ± 0.005Å, are stretched on the average by about 0.008Åfrom the neopentane reference value. Derived molecular parameters are in substantial agreement with values calculated by molecular mechanics using model fields MUB-1 and MUB-2. The methylene¹³C-H nmr coupling constant was found to be 125 Hz, a value indistinguishable from those reported for unstrained alkanes but not in accord with predictions from the formulas of Foote or Mislow for severely distorted methylene groups.Molecular parameters for neopentane included r_g(C-C) = 1.534 ± 0.003Å, r_g(C-H) = 1.114 ± 0.008Åand ∠CCH = 112 ± 3°. The new value for the C-C bond satisfactorily resolves a discrepancy between previously reported bond lengths. These had disagreed significantly with each other but, to within their uncertainties, they are consistent with the new, intermediate value. Amplitudes of vibration were determined for both neopentane and di-t-butylmethane.     

Relative-rate kinetic study of the reaction of bromine atom with neopentane

The rate coefficient ratio ofk ₁/k ₂=0.83±0.21 has been determined for the reactions Br+neo-C₅H₁₂ (1) and Br+C₂H₆ (2) by applying the relative-rate kinetic method atT=298 K.     

Catalytic cracking and dehydrogenation of butanes and neopentane on reduced copper-exchanged Y-zeolites

The activity and selectivity of reduced Cu–Y zeolites containing acidic sites either only of Brönsted or only of Lewis type suggest that the Lewis acid sites are active in the cracking of butanes and neopentane, while both Lewis acid and Lewis base sites are involved in the mechanism of the dehydrogenation of these reactants.

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Cu–Y , , , , . , .

     

Shock tube explorations of roaming radical mechanisms: the decompositions of isobutane and neopentane

The thermal decompositions of isobutane and neopentane have been studied using both shock tube experiments and ab initio transition state theory based master equation calculations. Dissociation rate constants for these molecules have been measured at high temperatures (1260-1566 K) behind reflected shock waves using high-sensitivity H-ARAS detection. The two major dissociation channels at high temperature are iso-C(4)H(10) → CH(3) + i-C(3)H(7) (1a) and neo-C(5)H(12) → CH(3) + t-C(4)H(9) (2a). Ultrahigh-sensitivity ARAS detection of H-atoms produced from the rapid decomposition of the product radicals, i-C(3)H(7) in (1a) and t-C(4)H(9) in (2a), through i-C(3)H(7) + M → H + C(3)H(6) + M (3a) and t-C(4)H(9) + M → H + i-C(4)H(8) + M (4a) allowed measurements of both the total decomposition rate constants, k(total), and the branching to radical products, which were observed to be equivalent in both systems, k(1a)/k(total) and k(2a)/k(total) = 0.79 ± 0.05. Theoretical analyses indicate that in isobutane, the non-H-atom fraction has two contributions, the dominant fraction being due to the roaming radical mechanism leading to molecular products through iso-C(4)H(10) → CH(4) + C(3)H(6) (1b) with k(1b)/k(total) = 0.16, and a minor fraction that involves the isomerization of i-C(3)H(7) to n-C(3)H(7) that then subsequently forms methyl radicals, i-C(3)H(7) + M → n-C(3)H(7) + M → CH(3) + C(2)H(4) + M (3b). In contrast to isobutane, in neopentane, the contribution to the non-H-atom fraction is exclusively through the roaming radical mechanism that leads to neo-C(5)H(12) → CH(4) + i-C(4)H(8) (2b) with k(2b)/k(total) = 0.21. These quantitative measurements of larger contributions from the roaming mechanism for larger molecules are in agreement with the qualitative theoretical arguments that suggest long-range dispersion interactions (which become increasingly important for larger molecules) may enhance roaming.     

Steric influence in the direction of elimination of gas-phase pyrolyses of several highly branched secondary acetates

The rate coefficients for the gas-phase pyrolyses of a series of structurally related secondary acetates have been measured in a static system over the temperature range of 289.1–359.5°C and the pressure range 50.0–203.0 torr. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 3-hexyl acetate, log k₁ (s^?) = (12.12 ± 0.33) ? (176.1 ± 3.9)kJ/mol/2.203RT; for 5-methyl-3-hexyl acetate, log k₁ (s^?) = (13.17 ± 0.20) ? (186.2 ± 2.3)kJ/mol/2.303RT; and for 5,5-dimethyl-3-hexyl acetate, log k₁ (s^?) = (12.70 ± 0.19) ? (177.4 ± 2.2)kJ/mol/2.303RT. The direction of elimination of these esters has shown from the invariability of olefin distributions at different temperatures and percentages of decomposition that steric hindrance is a determining factor in the eclipsed cis conformation. Moreover, a more detailed analysis indicates that the greater the alkyl–alkyl interaction, the less favored the elimination tends to be. Otherwise, an increase of alkyl–hydrogen interaction caused steric acceleration to be the determining factor.     

Modelling of the homogeneously catalyzed and uncatalyzed pyrolysis of neopentane: Thermochemistry of the neopentyl radical

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 kinetics and mechanisms of methyl 2-bromopropionate and 2-bromopropionic acid pyrolyses under maximum inhibition

The kinetics of the gas phase pyrolyses of methyl 2-bromopropionate and 2-bromopropionic acid were studied in a seasoned, static reaction vessel and under maximum inhibition of the free radical suppressor toluene. The working temperature and pressure range was 310–430°C and 26.5–201.5 torr, respectively. The reactions proved to be homogeneous, unimolecular, and obeys a first-order rate law. The rate coefficients are expressible by the following equations: for methyl 2-bromopropionate, log k₁(s^?1) = (13.10 ± 0.34) ? (211.4 ± 4.4)kJ mol^?1(2.303RT)^?1; for 2-bromopropionic acid, log k₁(s^?1) = (12.41 ± 0.29) ? (180.3 ± 3.4)kJ mol^?1(2.303RT)^?1. The bromoacid yields acetaldehyde, CO and HBr. Because of this result, the mechanism is believed to proceed via a polar five-membered cyclic transition state.     

Predicting neopentane isosteric enthalpy of adsorption at zero coverage in MCM-41

The isosteric enthalpy of adsorption for neopentane at relative pressures down to 3 × 10(-8) in MCM-41 was predicted for the temperature range from -15 to 0 °C. At such low pressures and temperatures, experimental measurements become problematic for this system. We used an atomistic model for MCM-41 obtained by means of a kinetic Monte Carlo method mimicking the synthesis of the material. The model was parametrized to represent experimental nitrogen adsorption isotherms at 77 K using grand canonical Monte Carlo simulations. The simulated isosteric enthalpy of adsorption shows very good agreement with available experimental data, demonstrating that GCMC simulations can predict heats of adsorption for conditions that are challenging for experimental measurements. Additional insights into the adsorption mechanisms, derived from energetic analysis at the molecular level, are also presented.