Oxidizing intermediates generated in the Fenton reagent: kinetic arguments against the intermediacy of the hydroxyl radical.

It has long been recognized that the aqueous mixture of hydrogen peroxide and ferrous ion, known as the Fenton reagent, generates powerful oxidants. Furthermore, the chemical intermediates and reaction pathways of the type generated by this reagent have been implicated in oxidative damage in biological systems. Although the subject continues to be debated, the hydroxyl radical (.OH) is generally invoked as the predominant oxidizing intermediate formed by the Fenton reagent. However, recent results from this laboratory have demonstrated that the principal pathway for the Fenton-mediated oxidation of N-nitrosodimethylamine does not involve .OH, but instead must involve the intermediacy of another strongly oxidizing species. This conclusion was based on stopped-flow spectrophotometric observation of a transient, A, believed to be an iron(II) nitrosyl adduct, which was formed at a rate five-fold faster than that predicted for formation of .OH. Subsequent experiments have shown that methanol and other organic compounds can quench the formation of A. This quenching procedure provides a unique spectrophotometric probe with which to examine the relative reactivities of putative Fenton-type oxidizing intermediates toward organic substrates. Presented here are the results of several such quenching studies, plus an overview of our mechanistic investigations of the Fenton reaction.