Free radical addition to olefins. Part XVI. Photolysis of difluoroiodomethane in the presence of olefins

Abstract

Difluoroiodomethane has been photolysed in the presence of ethylene and fluoroethylenes. The products include di-iodopropanes formed by the addition of difluoroiodomethyl radicals, as well as mono-iodofluoropropropanes formed by the addition of difluoromethyl radicals. The proportions of the various products and the effect of changing concerntration and temperature can be rationalised in a reaction scheme in which the primary process leads to thermally excited difluoromethyl radicals [reacion (i)]. The excited difluoromethyl radicals can either add to the CHF₂I·[graphic omitted] CHF₂·*+ I(²P_½)(i) olefins or abstract hydrogen from the starting difluoroiodomethane [reaction (ii)]. The difluoroiodomethyl radicals CHF₂·*+ CHF₂I → CF₂I·+ CH₂F₂(ii) so formed are necessarily thermal and it has been possible to obtain approximate Arrhenius parameters for the relative rates of addition of these radicals to the olefins, although the very small amounts of the adducts limit the precision of the data. The excited difluoromethyl radicals, on the other hand, add rapidly and very unselectively to the olefins. At low temperatures there is no chain reaction with the less reactive olefins and only at the highest temperatures are the chains sufficiently long for addition by thermal as distinct from excited radicals to predominate.