COMBINATION AND DISPROPORTIONATION OF ETHYL RADICALS: INFLUENCE OF THE REACTION H+C 2 H 5 = C 2 H 6

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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...

Section: Reactions Likely To Afj'ect the Validity O F The Basicsupporting

confidence: 75%

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

Reid¹,

Roy²

1968

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“…Values of kE/k4 for both ethyl and propyl radicals appear to be rather higher than those observed for thermally equilibrated radicals (5,6,7,8). Some of this discrepancy may be attributed to inaccuracy in the extrapolation to zero time.…”

Section: Discussionmentioning

confidence: 74%

The Mercury-Photosensitized Decompositions of Propane and Ethane

Back¹

1959

The mercury-photosensitized decompositions of propane and ethane have been studied a t pressures of 300 and 600 mm, a t 2 4 O C, a t very low conversions. Hydrogen was the only product identified and measured. The hydrogen yield fell rapidly a t first, with increasing time of irradiation, and finally became constant. This behavior is attributed t o the accumulation in the system of propylene or ethylene formed by the disproportionation of propyl or ethyl radicals, and the constant rate of hydrogen production finally achieved is thought t o correspond to a steady-state concentration of the unsaturate. Previous values of the quantum yields of these decompositions were probably measured in this steady-state region, and must be corrected upwards.Relative values have been estimated for the various rate constants involved in this mechanism. There is some evidence that energy-rich radicals, formed by the addition of hydrogen atoms to the ~~nsaturates, tend to disproportionate rather than dimerize. INTRODUCTIONIn a previous study of the mercury-photosensitized decomposition of n-pentane a t very low conversion, the hydrogen production decreased rapidly a t first, then more slowly, and finally became constant after about 0.57, reaction (1). This was attributed to the accumulation in the system of pentene, formed by disproportionation of pentyl radicals, and the final constant rate of hydrogen production was thought to correspond to a low, steadystate concentration of pentene. The hydrogen yield was reduced, and pentene consumed, by the addition of hydrogen atoms to pentene. Similar behavior has been observed with the cyclo-alkanes (2), and more recently with n-butane (3). Previous studies of the mercury-photosensitized decomposition of propane and ethane did not show this effect, as hydrogen production was independent of time (4). However, rough calculations from the relative rates of the reactions concerned suggest that even the smallest conversions achieved in these experiments were probably much too large for the initial behavior to be observed. I t was decided to study these reactions a t very low conversions to see whether they behaved in the same way.The apparatus, a conventional static system, and the method were essentially similar to those described previously (1). Two reaction vessels, of 38-and 180-cc volume, were used in the course of the work, the larger volume permitting measurements to be made a t lower conversion. After irradiation, gases non-condensable a t -210' C were removed and analyzed, and the hydrocarbon, together with any condensable products, was returned to the reaction vessel for further irradiation. In this way, the hydrogen yield could be studied as a function of total irradiation time, using a single sample of hydrocarbon. Concentrations of hydrogen and unsaturates produced in the system were always low enough so that all quenching of the excited mercury atoms was by the hydrocarbon alone. Measurements were made with propane a t 300-and 600-mm pressure, and with ethane a t 305-mm pr...

“…T h e primary process in the presence of mercury has long been considered to be a C-I bond rupture, followed by the reactioil of the iodine atom with mercury, R I = R + I [lo1 , as well as [9], is considered to be a reversible process then in the absence of reactions other than those indicated it is immaterial whether [9] or [ l l ] is considered t o be the primary process, since K11 = K9.IClo. The present experimellts confirm previous evidence that larger 1-alkyl radicals are built up by the successive addition of ethylene to ethyl radicals, etc.…”

Section: Discussionmentioning

confidence: 99%

“…According to their mechanism all the products should be saturated, for a t low radical concentrations they assumed that chain termination would take place by the reactions while a t high radical concentrations they assumed t h a t it would occur by radical combination. Radical disproportionation would also be expected to occur, in the light of more recent work (9).…”

mentioning

confidence: 98%

The Polymerization of Ethylene Sensitized by Ethyl Iodide

Sefton¹,

Roy²

1956

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The polymerization of ethylene sensitized by the thermal decomposition of ethyl iodide in the presence of mercury vapor has been studied a t 250°, 275", and 300°C. C14-labelled ethyl iodide was used in a number of experiments. The increase in the rate of decomposition of ethyl iodide in the presence of ethylene and the formation of butyl iodide are accounted for by equilibrium processes of the type R I f H g = R f H g I . 'The important features of the reaction were established from the identity, quantity, and activity of the various products. The predominant formation of olefins is attributed to the isomerization and decomposition of large radicals. Very little of the butane is formed by the combination of ethyl radicals. Radical disproportionation is the most important chain termination step.The decompositioil of the lower allcyl iodides has been studied exteilsively (10) and it is generally agreed that in both the thermal reaction and in the photolysis in the region of the contiiluum (ca. 2600 A) the primary process consists in the formation of alkyl radicals and iodine atoms. The quantum yields of the photochemical process are low because of the baclc reaction but can be greatly increased by trapping the iodine atoms with mercury or silver. The thermal or photochemical decompositioil of alkyl iodides in the presence of ethylene and either mercury or silver should therefore afford a conveilieilt method for studying the free radical sensitized polymerizatioil of ethylene.Juilgers and Yeddailapalli (5) have studied the polymerizatioil of ethylene sensitized by the photolysis of methyl, ethyl, ?z-propyl, and isopropyl iodides in the presence of mercury a t temperatures in the range 130' to 275OC. Under conditions lilcely to yield low radical concentrations, viz. low iodide concentrations, high ethylene concentrations, and low light intensities, they found that the "yield", defined as the ultimate pressure decrease on prolonged irradiation divided by the initial pressure of iodide, reached maximum values which were in the approximate ratio 3.5 : 3.0 : 2.5 for methyl, ethyl, and the propyl iodides, respectively. They concluded from this that the final polymer had approximately the same molecular weight in each case and that for steric reasons the growing radicals formed by the successive addition of ethylene molecules uilderwerlt chain termination when they reached a more or less common critical size. According to their mechanism all the products should be saturated, for a t low radical concentrations they assumed that chain termination would take place by the reactions while a t high radical concentrations they assumed t h a t it would occur by '~Manuscripl received September 82, 1955.