The mercury (6³P₁) photosensitized hydrogenation of ethylene. Kinetics of the reaction C₂H₅ + H₂ = C₂H₆ + H

Abstract

A quantitative study has been made of the reaction of ethyl radicals with molecular hydrogen in the gas phase in the temperature range 240 to 320 "C. The mercury (63P1) photosensitized decomposition of hydrogen in the presence of ethylene was used to generate ethyl radicals. Extinction coefficients for the absorption of 2537 &, by mercury vapor were measured and Beer's law was shown to be obeyed under the experimental conditions used. T h e corrections required to allow for the nonuniformity of radical concentrations in the cell were small. After delineating the experimental conditions necessary to minimize secondary reactions, the rate constant (cn13 mole-' s-') for the reaction CzHS + Hz = CzH6 + H was found to be given by log,, k = 12.57 -13.718. Experiments in the presence of added carbon dioxide showed the absence of hot radical effects at the working pressure of 92 Torr of hydrogen.Canadian Jorlrnal of Chemistry, 46, 3275 (1968) Relatively few atom transfer reactions of the I type R + X, = R X + X, where X = H or D, have been studied (1-3), and in the even fewer cases where the reaction of a given radical with both H, and D, has been investigated there is some doubt about the magnitude of the isotope effect on the activation energy. For n-C,F, radicals, values of 14.0 (4) and 13.8 (5) kcal mole-' have been found for the reaction with D,, 1.9 (4) and 1.5 (5) kcal mole-' greater than for the reaction with Hz. For the reaction of C2F5 with D, and Hz (4) the activation energies were almost the same: 12.6 and 12.4 kcal mole-', respectively. An intermediate isotope effect was found in the case of CF,; values of 9.5 (6) and 10.2 (7) kcal mole-' were found for the reaction with D,, 0.9 (6) and 0.7 (7) kcal mole-' greater than for the reaction with Hz, although the probable errors quoted by the authors in these cases were such that a negligible isotope effect would also be compatible with their results. In the case of C2H5 radicals the value found for the reaction with D, was 13.3 kcal mole-' (8) while the value for the reaction with H, (9), as subsequently corrected (lo), was only 11.3 kcal mole-'.The apparent isotope effect for C2H5 radicals (8, lo), 2.0 kcal mole-', is very close to the difference between the zero point energies of Hz and D,. However, there is no theoretical justification for equivalence with zero point energy differences, and recent measurements in this laboratory (1 1) show no such correlation in the 'Present address: Research Department, Imperial Oil Enterprises Ltd., Sarnia, Ontario.case of the three-center exchange reactions of atomic and molecular hydrogen and deuterium.If we accept a value of 6.8 kcal mole-' (12) or greater (13) for the activation energy of the reaction H + CzH6 = CzHs + Hz and a value of about 6.2 kcal mole-' (14, 15) for -AH, then the activation energy for the reaction CzHs + Hz = CzH6 + H should be > 13.0 kcal mole-'. In view of the uncertainty in this value we felt it necessary to reinvestigate the reaction.Aside from possible errors in the interpretation of the ea...

The formation of alkanes in the pyrolysis of n-hexadecane: effect of an inert gas on the decomposition of alkyl radicals

“…Let n' = 116' be the number of collisions of R, * with the inert molecules A necessary for R,'% to obtain energy E,. Because of reaction [9] the number iz = 116 of collisions with C,,H,, molecules necessary to obtain the same energy will be greater than n'. After the R, * radicals have experienced on the average il' collisions, only a fraction (1 -a)"' of these radicals remain unreacted; the necessary number of collisions to transfer an energy AE to the radicals R," becomes n = nf(l -a)-"'.…”

Reaction of hydrogen atoms with propane: relative Arrhenius parameters for the attack on the 1 and 2 positions

“…largely that by the ratio R(l,,/R(,,+i,. The other terms having only minor significance can then be evaluated and that k-, is equal to that of the analogous reaction of ethyl radical, 10'2.57 exp (-13 7301 RT) cc mole-' s-' (6), and lastly that k -, is equal to exp (-17 730/RT) cc mole-' s-'. Using the recently determined values of k3 and k-, as lo9.'…”

Temperature Dependence of Disproportionation–Combination Ratios for Alkyl Radicals in Liquid Methane

“…A recent study by Reid and LeRoy (12) concluded tentatively that Ec -Ed = -900 cal mol-' for ethyl radicals in the gas phase, and a paper by James and Suart (13) reported that Ec -Ed = 250 cal mol-' for the mutual interaction of tert-butyl radicals. Konar (14) treated literature data by the thermodynamic formulation of reaction rates, and concluded that Ec is probably not equal to Ed for all alkyl radicals.…”