KINETIC EFFECTS OF FLUORINE SUBSTITUENTS IN FREE-RADICAL CHEMISTRY - HYDROGEN-ATOM ABSTRACTION REACTIONS OF PERFLUORO-N-ALKYL RADICALS

Using competitive methods, the rates of hydrogen-atom abstraction by a typical perfluoro-n-alkyl radical, n-C7F25; have been determined for a series of silane and stannane reductants including (n)Bu(3)Sn-H (2.0 3 X 10(8) M(-1) s(-1)), Et(3)Si-H (7.5 X 10(5)), (TMS)(2)SiMe-H (1.63 X 10(7)) and (TMS)(3)Si-H (5.1 X 10(7)), all of which exhibited substa ntial rate enhancements relative to their analogous reductions of hydr ocarbon radicals. The reduction by PhSH is, in contrast, similar to 40 0-times slower than for hydrocarbon radicals. Transition state polar e ffects 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 (rho+ = -0.56) pro vided further substantiation for this conclusion. A discussion of the relative reactivity of t-butoxyl and perfluoro-n-alkyl radicals is pre sented.