Determination of Relative Rate Constants for Iodine Atom Abstraction By Phenyl Radicals

Winona, MN

Phenyl radicals (Ph·) are efficient hydrogen atom abstractors, and this reaction may have biological significance. However, the rate constants for H-abstraction from biomolecules (k_H) by Ph· are predominantly not known. Previous work from the Nalli lab used the reaction of phenyl radicals with iodoarenes (lit k_I = 1.0 x 10⁸ M^-1 s^-1) as a kinetic reference for measuring k_H­. However, this literature k_I value we have come to realize is unreliable. My research used the photolysis of p-fluorophenylazoisobutyronitrile (FPhAIN) to make p-fluorophenyl radicals in the presence of CCl₄ and an iodoarene (ArI). The products, p-chloro- and p-iodofluorobenzene (eq. 1) are measured using F-19 NMR, and the yield ratio allows the determination of the relative rate constant k_I/k_Cl (eq. 2), which in conjunction with a well-established k_Cl gives k_I. The results are compared to those reported by Tanner, Reed, and Setiloane in a 1982 paper that was later retracted.