Kinetics of chemically activated ethane

Abstract

Chemically activated ethane, with an excitation energy of 114.9 f 2 kcal/mole, was forme; by reaction with methane of excited singlet methylene radicals produced by the 4358 A photolysis of diazomethane. A decomposition rate constant of (4.6 =k 1.2) X l o 9 sec-' was measured for the chemically activated ethane. This result agrees, via RRKM theory, with most other chemically activated ethane data, and the result predicts, via RRKM and absolute rate theory for Eo = 85.8 kcal/mole, E* = 114.9 kcal/niole, and k E = 4.6X 10' sec-I, a thermal A-factor at 600°K of 1016.6io * sec-I, in approximate agreement with the more recent experimental values. Combining 2 kcal/mole uncertainties in Eo and E * with the uncertainty in our rate constant yields an A-factor range of lox6 6i0.7 sec-'. It is emphasized that this large uncertainty in the A-factor results from an improbable combination of uncertainty limits for the various parameters. These decomposition results predict, via absolute rate theory (with &(recombination) = 0) and statistical thermodynamic equilibrium constants, methyl radical recombination rates at 25°C of between 4.4X l o 8 to 3.1 X 109 l.-rnole-'-sec-', which are 60 to 8 times lower, respectively, than the apparently quite reliable experimental value. A value of &(recombination) greater than zero offers no improvement, and a value less than zero would be quite unusual. Activated complexes consistent with the experimental recombination rate and Eo(recombinati0n) = 0 greatly overestimate the experimental chemical activation and high pressure thermal decomposition rate data. Absolute rate theory as it is applied here in a straightforward way has failed in this case, or a significant amount of internally consistent data are in serious error. Some corrections to our previous calculations for higher alkanes are discussed in Appendix 11.

Comparison between experimental and calculated results on the decomposition of chemically activated alcohols

“…excellent agreement between the life times of excited hydrocarbon molecules (from the insertion reaction of singlet methylene into alkanes) as measured by Simons and coworkers [6] and that which has been calculated from comparative-rate single-pulse shock-tube experiments 171. For the purposes of the present study we have confirmed this agreement.…”

“…last is based on the earlier analysis of Cvetanovic's results on O(l0) addition to propane and is the same procedure used by Simons and coworkers [6] in determining the excess energy carried by lCH2 into reaction with alkanes. Results are summarized in Table IV.…”

“…I t is only possible to make comparatively accurate estimates of the first two processes. For the former the transition state structure is similar to that used by Benson and O'Neal [14] for the dehydrochlorination of ethylchloride, while for the latter it is similar to that for the hydrocarbon decomposition processes [ 15,6]. With regard to the C-0 bond cleavage reaction, the observation of Lin and DeMore (2d] that the specific rate of this reaction is M that of the C-C bond cleavage process taken in conjunction with the calculated value of the latter yields the number in the table.…”

Chemically activated methylcyclobutane exothermicity of singlet methylene reactions and the heat of formation of singlet methylene

“…The small discrepancies (the c-c rupture of alkanes is an exception) [25] that have been observed are frequently explained as due to neglect of anharmonicities in the density of states calculations, approximate treatments of hindered internal rotations, uncertainties in absolute values of collision cross sections, and the unit collisional energy transfer efficiency assumption. Considerable attention has been given to the correct estimates of the magnitudes of these factors.…”

Pressure Effect on the Interaction of Hydrogen Atoms with Ethyl Radicals

“…However, Simons and coworkers reinvestigated this system in 1973. 6 ) In that study, also usiug the RRKM theory but with a revised model, k«= 3 X lOT sec-1 was suggested. This value reduces PSID=-l of the excited ethane molecules to 2 torr.…”