Mercury-photosensitized decomposition of dimethyl ether. Part III. The combination of radicals

1967 ◽  
Vol 45 (22) ◽  
pp. 2775-2783 ◽  
Author(s):  
Leon F. Loucks
Keyword(s):  
Dimethyl Ether ◽  
Rate Constant ◽  
Pressure Dependence ◽  
Rate Coefficient ◽  
Methyl Radicals ◽  
Methyl Ethyl ◽  
Methyl Radical ◽  
Unimolecular Decomposition ◽  
Third Body ◽  
Methyl Ethyl Ether

As part of the study of the mercury-photosensitized decomposition of dimethyl ether, the combination of methyl radicals has been investigated in the temperature range 200 to 300 °C and at pressures between 3 and 300 mm Hg. For pressures of less than 100 mm the second-order rate coefficient for the combination of methyl radicals shows a pressure dependence. The pressure dependence agrees qualitatively with that observed by others, but occurs at somewhat higher pressures. Calculations for the Kassel equation using the Arrhenius parameters for ethane decomposition and fitted to the pressure dependence of the methyl radical combination show that the number of effective modes for ethane decomposition is 8 or 9. Carbon dioxide was found to be a quite ineffective third body for energy transfer. The results for the mercury-photosensitized decomposition of dimethyl ether have also been analyzed to obtain information about the combination of methyl radicals with methoxymethyl radicals. The combination of these radicals becomes pressure dependent at pressures less than about 15 mm. Kassel integrations based on the rate constant [Formula: see text]for the unimolecular decomposition of methyl ethyl ether at the C—C bond, and fitted to the observed pressure dependence of the combination reaction, lead to s = 10 for these reactions.The rate constant for the abstraction of a hydrogen atom by a methyl radical from dimethyl ether was found to be [Formula: see text]

The reaction of hydrogen atoms with ethylene

1970 ◽  
Vol 316 (1527) ◽  
pp. 575-591 ◽  
Keyword(s):  
Hydrogen Atom ◽  
Rate Constant ◽  
Recombination Reaction ◽  
Methyl Radicals ◽  
Methyl Radical ◽  
Hydrogen Atoms ◽  
Ethyl Radical ◽  
Cracking Reaction ◽  
Computer Calculations ◽  
First Time

A detailed study has been made of the products from the reaction between hydrogen atoms and ethylene in a discharge-flow system at 290 ± 3 K. Total pressures in the range 8 to 16 Torr (1100 to 2200 Nm -2 ) of argon were used and the hydrogen atom and ethylene flow rates were in the ranges 5 to 10 and 0 to 20 μ mol s -1 , respectively. In agreement with previous work, the main products are methane and ethane ( ~ 95%) together with small amounts of propane and n -butane, measurements of which are reported for the first time. A detailed mechanism leading to formation of all the products is proposed. It is shown that the predominant source of ethane is the recombination of two methyl radicals, the rate of recombination of a hydrogen atom with an ethyl radical being negligible in comparison with the alternative, cracking reaction which produces two methyl radicals. A set of rate constants for the elementary steps in this mechanism has been derived with the aid of computer calculations, which gives an excellent fit with the experimental results. In this set, the values of the rate constant for the addition of a hydrogen atom to ethylene are at the low end of the range of previously measured values but are shown to lead to a more reasonable value for the rate constant of the cracking reaction of a hydrogen atom with an ethyl radical. It is shown that the recombination reaction of a hydrogen atom with a methyl radical, the source of methane, is close to its third-order region.

Mercury-photosensitized decomposition of dimethyl ether. Part II. The thermal decomposition of the methoxymethyl radical

1967 ◽  
Vol 45 (22) ◽  
pp. 2767-2773 ◽  
Author(s):  
Leon F. Loucks ◽  
Keith J. Laidler
Keyword(s):  
Carbon Dioxide ◽  
Dimethyl Ether ◽  
Pressure Range ◽  
High Pressures ◽  
Rate Coefficient ◽  
Rate Coefficients ◽  
Methyl Radical ◽  
Lower Efficiency ◽  
First Order ◽  
Order Rate

The decomposition of the methoxymethyl radical, generated in the mercury-photosensitized decomposition of dimethyl ether, has been investigated over the temperature range 200 to 300 °C and the pressure range 3 to 600 mm Hg. The radical decomposes to give a formaldehyde molecule and a methyl radical. The effects of pressure and temperature on the first-order rate coefficient for the decomposition of the methoxymethyl radical have been examined in detail. The rate coefficient shows a pressure dependence over the full pressure range studied. The order of the decomposition is about 1.4 at the middle of the pressure range studied, with a lower order at higher pressures and a higher order at lower pressures. At 100 mm Hg the observed activation energy for the decomposition of the methoxymethyl radical is 24.8 kcal/mole.The first-order and second-order rate coefficients, k∞ and k0, corresponding to the limiting conditions of high pressures and low pressures respectively, have been evaluated as [Formula: see text]Kassel integrations have been carried out for the methoxymethyl radical and have been fitted to the experimental data. It is concluded that 8 or 9 normal modes contribute to the energization of the radical. The rate coefficient is increased by the presence of carbon dioxide, but carbon dioxide has a lower efficiency than dimethyl ether for the transfer of energy in the energization process.

THE THERMAL DECOMPOSITION OF ETHANE: PART II. THE UNIMOLECULAR DECOMPOSITION OF THE ETHANE MOLECULE AND THE ETHYL RADICAL

1966 ◽  
Vol 44 (20) ◽  
pp. 2357-2367 ◽  
Author(s):  
M. C. Lin ◽  
M. H. Back
Keyword(s):  
High Pressure ◽  
Rate Constant ◽  
Order Rate Constant ◽  
Methyl Radicals ◽  
Unimolecular Decomposition ◽  
First Order ◽  
Order Rate ◽  
Ethyl Radical ◽  
Lower Temperature ◽  
Ethyl Radicals

The rate of the elementary dissociation of ethane into two methyl radicals has been measured in its pressure-dependent region at temperatures from 913–999 °K and at pressures from 1–200 mm. The high-pressure first-order rate constant obtained by extrapolation was in agreement with that obtained at lower temperatures,[Formula: see text]Comparison with calculated Kassel curves showed that the best fit of the data was obtained with the Kassel parameter s = 12 ± 1. The high-pressure first-order rate constant for the decomposition of the ethyl radical was obtained by extrapolation of the data reported in Part I, assuming the rate constant for combination of ethyl radicals is independent of temperature.[Formula: see text]From the measured constant for the dissociation of ethane, the rate constant for the combination of methyl radicals was calculated and compared with the values measured in a lower temperature region. Differences in the values of the rate constants and in the shapes of the unimolecular falloff curves are discussed.

Hydrogen abstraction from toluene by methyl radicals and the pressure dependence of the recombination of methyl radicals

1970 ◽  
Vol 48 (8) ◽  
pp. 1269-1272 ◽  
Author(s):  
A. N. Dunlop ◽  
R. J. Kominar ◽  
S. J. W. Price
Keyword(s):  
Pressure Dependence ◽  
Reasonable Agreement ◽  
Pressure Range ◽  
Hydrogen Abstraction ◽  
Methyl Radicals ◽  
Third Body ◽  
Kcal Mole

Using dimethylzinc, dimethylmercury, and trimethylbismuth as sources of methyl radicals, values of k1/k21/2[Formula: see text]have been calculated from 338 to 610 °C over the pressure range 4.5–204 mm. M is predominantly toluene. The observed pressure dependence of reaction [2] is in agreement with that found when M = benzene, but is somewhat greater, and the fall-off occurs at higher pressures, than for ethane dissociation. However, reasonable agreement is obtained if it is assumed that the efficiency of toluene as a third body in reaction [2] is about 1/10th that of ethane.Extrapolation to infinite pressure, where it is assumed that E2 = 0and A2 = 1013.34 cm3 mole−1 s−1, gives E1 = 8.0 ± 0.3 kcal mole−1 and A1 = 1011.07 cm3 mole−1 s−1.

The reaction of methyl radicals with formaldehyde in dimethyl ether pyrolysis

1978 ◽  
Vol 56 (10) ◽  
pp. 1307-1310 ◽  
Author(s):  
Kim C. Manthorne ◽  
Philip D. Pacey
Keyword(s):  
High Pressure ◽  
Dimethyl Ether ◽  
Rate Constant ◽  
Arrhenius Plot ◽  
Flow System ◽  
Methyl Radicals ◽  
Limiting Values

Dimethyl ether was pyrolyzed in a flow System at 788, 856, and 935 K and 38–401 Torr. Measurement of the yields of CH4 and C2H6 and of either H2 or CO enabled calculation of high pressure limiting values of the rate constant quotient k9k6−1/2, where reaction 9 is[Formula: see text]and reaction 6 is the recombination of two methyl radicals. Including literature data from 357–1005 K, the Arrhenius plot for this quotient is a curve.

Pressure dependence of the rate constant of the reaction atomic hydrogen + methyl radicals .fwdarw. methane

1977 ◽  
Vol 81 (21) ◽  
pp. 1982-1984 ◽  
Author(s):  
Jung-Tsang Cheng ◽  
Chuin-Tih Yeh
Keyword(s):  
Rate Constant ◽  
Pressure Dependence ◽  
Atomic Hydrogen ◽  
Methyl Radicals

Kinetics and mechanisms of the thermal decomposition of propane I. The Uninhibited of reaction

1962 ◽  
Vol 270 (1341) ◽  
pp. 242-253 ◽  
Keyword(s):  
Radical Mechanism ◽  
Methyl Radicals ◽  
Methyl Radical ◽  
Third Body ◽  
Third Order ◽  
First Order ◽  
The Third ◽  
Free Radical Mechanism ◽  
Pressure Region ◽  
Initiation Reaction

The uninhibited pyrolysis of propane was investigated from 530 to 670 °C and at pressures up to 600 mm. In an unpacked vessel the reaction was of the first order at lower temperatures and higher pressures. A transition to 3/2 order at higher temperatures and lower pressures was observed. The rates were somewhat reduced in a packed vessel, and an apparent order of 1.25 was obtained. The activation energy of the reaction in its first-order region was 67.1 kcal and that of the f-order reaction was 54.5 kcal. Added carbon dioxide had no effect on the rates either in the first-order or 3/2-order region. On the basis of this evidence, and of theoretical arguments, it is concluded that the reaction is largely homogeneous and occurs by a free-radical mechanism. The initiation reaction is considered to be the dissociation of propane into a methyl radical and an ethyl radical, this reaction being in its second-order low-pressure region under the conditions of the experiments. The termination reaction when the overall order is unity is concluded to be the recombination of a methyl and a propyl radical in the presence of a third body. In the 3/2-order region the termination reaction is believed to be the recombination of two methyl radicals, also in the third-order region. These mechanisms are shown to give a satisfactory interpretation of the overall behaviour.

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