Radiolytic Studies of the Mechanism of Autoxidation of Triphenylphosphine and Related Compounds

Abstract

Triphenylphosphine (Ph3P) undergoes one-electron oxidation in irradiated CH2Cl2 solutions to yield the radical cation Ph3P•+. This short-lived species exhibits intense absorption with a maximum at 325 nm and an extended shoulder at 400−500 nm and decays by a second-order process in the absence of O2. In the presence of O2, however, the radical cation reacts with O2 with a rate constant of 7 × 106 L mol-1 s-1 to yield a peroxyl radical, Ph3P+OO•, which exhibits no significant absorbance at λ > 300 nm. Similar results, but with slightly different rate constants, were obtained also in CCl4 solutions and in CH3CN and cyclohexane solutions containing 1% CCl4. Tris(2,4,6-trimethoxyphenyl)phosphine in CH2Cl2 exhibits a behavior similar to that of Ph3P, but the reaction of its radical cation with O2 is an order of magnitude more rapid. On the other hand, the perfluorinated Ph3P•+ in CH2Cl2 reacts with O2 much more slowly, if at all. Diphenyl-2-pyridylphosphine yields a radical cation which exhibits a slightly narrower absorption band but reacts with O2 with the same rate constant as Ph3P•+. The peroxyl radicals propagate a chain reaction by further oxidation of phosphine molecules to Ph3P•+ either directly or indirectly. The final radiolysis product is the phosphine oxide, Ph3PO. The radiolytic yields for oxidation of the phosphines were much higher than the radiolytic yield of the solvent derived radicals, except for the case of the perfluoro derivative, and were dependent on the concentrations of phosphine and O2 and on the dose rate. At low dose rates and high solute concentrations the chain lengths exceeded 1000.