Determination of the Rate Constant for H Atom Abstraction by Phenyl Radicals for THF

Kryzsko Commons Ballroom, Winona, Minnesota

Phenyl radicals are known to rapidly abstract hydrogens and add to double bonds and H-abstraction, particularly from the deoxyribose rings of DNA, is known to have biological significance. However, rate constants for H-abstraction (kH) by phenyl radicals from biomolecules are largely unknown. This research used p-fluorophenyl radicals generated by the photolysis of 4-fluorophenylazoisobutyronitrile (FPAIN), to allow for product yields to be measured using F-19 NMR. Specifically, the rate constant for phenyl radical iodine abstraction from 3-iodobenzotrifluoride (kI= 2.2 x 108 M-1 s-1) measured previously in our lab was used as a kinetic reference point for determining kH for hydrogen abstraction from tetrahydrofuran as a model for the deoxyribose rings of DNA. Thus, when FPAIN was photolyzed in the presence of THF and 3-iodobenzotrifluoride (ArI) at 23 °C the product yield ratio of p-hydro- to p-iodofluorobenzene ([FPhH]/[FPhI]) gave an excellent linear correlation (R2= 0.9985) with the reactant concentration ratio ([THF]/[ArI]). The slope of the trendline (slope = 0.048) gives the relative rate constant kH/kI, which in conjunction with the determined kI = 2.2 x 108 M-1 s-1 gives kH. The result kH = 1.1 x 107 M-1 s-1 at 23 °C compares favorably to the literature value kH = 4.8 x 106 M-1 s-1.