Addition of Radicals to Ethylene

Radical addition reactions are very difficult to study and despite considerable effort there is still a lack of accurate kinetic results for these reactions. The experiments which have been carried out using the rotating cryostat have nevertheless enabled limiting values of the activation energy to be obtained. From the results the radicals can be divided clearly into two categories, those which react under these conditions and those which do not. 

When t-butyl or trichloromethyl radicals were bombarded with ethylene no detectable reaction occurred. The reaction of normal alkyl radicals, apart from ethyl, with ethylene cannot be studied directly because the e.s.r. spectra, of the initial and final radicals are virtually identical, but this difficulty could be overcome by using deuteriated ethylene. However, when the reaction of n-heptyl radicals with deuteriated ethylene was studied, no reaction occurred. As the deuteriated ethylenes are known to undergo addition reactions more readily than the protium analogue (Feld et al., 1962), the results indicate that alkyl radicals would not add to ethylene under the conditions of these experiments. The absence of reaction allows a lower limit of about 5 kcal rnole (see earlier discussion VIIIA) to be placed on the addition of n-alkyl (apart from ethyl), t-butyl and trichloromethyl radicals to ethylene. 

Very few data exist for these reactions but for t-butyl and n-butyl radicals the activation energies determined in the gas phase are 7-1 kcal mole and 7-3 kcal mole respectively (Kerr and Trotman-Dickenson, 1960a, b), and are consistent with a lower limit of 5 kcal mole . However, the activation energy for the addition of trichloromethyl radicals 

In contrast to the other alkyl radicals both methyl and ethyl radicals add efficiently to ethylene, showing that the activation energies for these reactions are not greater than 5 kcal mole . This value for the methyl radical is rather lower than that obtained in the gas phase (7-9 kcal mole ) (Cvetanovic and Irwin, 1967). Values ranging from 5-5 to 9 kcal mole have been obtained for the addition of ethyl radicals to ethylene in the gas phase (Finder and Le Roy, 1957 James and Steacie, 1958 Lampe and Field, 1959 Kerr and Trotman-Dickenson, 1960c). Our results suggest that the lower value (5-5 kcal mole ) for the activation energy is the more reliable one. 

Tlie phenyl and trifluoromethyl radicals also add readily to ethylene at 7 7°K indicating that the activation energies are less than 5 kcal mole . There are no other values available. 

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