Oxidation of Secondary Hydroperoxides via αC-H Abstraction to Form Ketones and Hydroxyl Radicals: Fluorene Autoxidation as a Model System

Abstract

Organic and biological molecules with relatively weak C-H bonds can react with O2 via a free-radical reaction called autoxidation. The primary products of these peroxyl-radical-driven reactions are hydroperoxides, R2CHOOH. If autoxidation continues, the secondary oxidation of hydroperoxides is known to form ketones, R2C=O, but this mechanism is not well characterized. Importantly, we find that ketone formation produces a highly reactive hydroxyl radical, HO•. We can trap HO• using benzene as a solvent to form quantifiable amounts of phenol. Fluorene was chosen as a model system to study this secondary oxidation in great detail. Kinetic modeling allowed the measurement of rate constants for the primary and secondary autoxidation reactions as 11.3 M-1s-1 and 25 M-1s-1, respectively. DFT modeling likewise predicts a faster oxidation for the secondary autoxidation. This type of kinetic measurement and modeling approach could be useful to study the autoxidation of plastics, petrochemicals, and lipids.