Determination of Rate Constants for Hydrogen Abstraction by Phenyl Radicals from Fatty Acid Esters

Kryzsko Commons, Oak Rooms E&F

Radicals are molecules that contain an unpaired electron in place of a bond. As such they are generally very reactive and only exist as short-lived intermediates in chemical reactions. Phenyl radicals (C6H5•) are well known reactive intermediates that rapidly undergo typical radical reactions such as hydrogen abstraction and double bond addition. Their H-abstraction reactions with lipids and other biomolecules are of particular interest, yet rate constants (kH) for these have not been previously reported. We used the visible photolysis of p-fluorophenylazoisobutyronitrile (FPAIN) to generate p-fluorophenyl radicals, which were allowed to react with a fatty acid ester (FAE) in competition with abstraction of iodine from an iodoarene (m-iodobenzotrifluoride, ArI). The relative yields of the competitively formed products, fluorobenzene and p-fluoroiodobenzene were measured by integration of the fluorine-19 nuclear magnetic resonance (19F NMR) spectrum allowing the relative rate constants (kH/kI) to be determined. A literature derived value for kI (kI = 2.2 x 108M-1 s -1 ) then serves as a kinetic reference point for determining kH. We also report the results of high-level density functional theory (DFT) calculations that support the supposition that the reactivities of p-fluorophenyl and unsubstituted phenyl radicals are very similar.