Effect of lone-pair repulsion on the reactivity and selectivity of fluoroalkyl radicals

The relative rates of addition of trifluoromethyl, pentafluoroethyl, heptafluoroisopropyl, and nonafluoro-t-butyl radicals to ethylene and vinyl fluoride show that these radicals become increasingly selective as they become branched and there is a direct correlation between the logarithm of orientation of addition to vinyl fluoride of each radical and the pK_a's of the corresponding perfluoroalkyl hydrides [CF₃H, CF₃CF ₂H, (CF₃)₂CFH, and (CF₃) ₃CH]; these observations suggest that repulsion between the lone pair electrons on an α-fluorine atom and the unpaired electron on the trivalent carbon atom plays an important part in determining the reactivity of these radicals.