Kinetic effects of fluorine substituents in free-radical chemistry. Hydrogen-atom abstraction reactions of perfluoro-n-alkyl radicals

Abstract

Using competitive methods, the rates of hydrogen-atom abstraction by a typical perfluoro-n-alkyl radical, n-C 7F 15·, have been determined for a series of silane and stannane reductants including nBu 3SnH (2.03×10 8 M −1 s −1), Et 3SiH (7.5×10 5), (TMS) 2SiMeH (1.63×10 7) and (TMS) 3SiH (5.1×10 7), all of which exhibited substantial rate enhancements relative to their analogous reductions of hydrocarbon radicals. The reduction by PhSH is, in contrast, ∼400-times slower than for hydrocarbon radicals. Transition state polar effects are invoked to rationalize the relative reactivity of perfluoro- versus hydrocarbon radicals in these hydrogen-transfer reactions. A Hammett study for H-atom transfer from arene thiols ( p + = −0.56) provided further substantiation for this conclusion. A discussion of the relative reactivity of t-butoxyl and perfluoro-n-alkyl radicals is presented.