MECHANISMS OF PEROXIDE STABILIZATION - AN INVESTIGATION OF SOME REACTIONS OF HYDROGEN-PEROXIDE IN THE PRESENCE OF AMINOPHOSPHONIC ACIDS

It has been established by continuous-flow studies in conjunction with EPR spectroscopy that the aminophosphonic acids 1-4 accelerate significantly the Fenton reaction between Fe(II) and H2O2 in aqueous solution via complexation of the metal ion (with values of the rate constant k for the generation of the hydroxyl radical up to 2 x 10(5) dm3 mol-1 s-1 at room temperature). To a certain extent this behaviour parallels that of EDTA and some structurally-related aminocarboxylic acids. It is also shown that the N-oxides of the aminophosphonic acids 1-3 react readily with the hydroxyl radical to give long-lived nitroxides via beta-scission of first-formed carbon-centred radicals. Neither of these findings is believed to correspond to the major chemistry which underlies the efficacy of these ligands as peroxide stabilizers. It is suggested instead that the crucial role of these compounds depends upon their ability to stabilize the higher valence state of iron, and hence not only to encourage oxidation of Fe(II) by O2.- and H2O2 but also to prevent effective reduction of Fe(III) by O2.-, HO2. and H2O2. However, radical scavenging by N-oxides may be a secondary, contributory factor in this stabilizing function, especially in peroxide systems when the sequestrant is added before storage, when slow N-oxidation is to be expected.