Rates and Mechanism for Oxidation of Paraquat and Diquat Radical Cations by Several Peroxides

Article abstract of DOI:10.1021/jo00320a006

The rates of oxidation of the free radicals MV(+)-radicals and DQ(+)-radicals derived from herbicides Paraquat and Diquat by hydrogen peroxide, peroxodiphosphate species, and peroxodisulfate have been investigated.Where consistent data were obtainable, the reactions are first order each in peroxide and radical.Results for H2O2 and MV(+)-radical are k = 2.0 (M*s)^-1 at 25 deg C, ΔH(excit.) = 92 kJ*mol^-1, and ΔS(excit.) = 73 J*(mol*K)^-1; similar results were found with H2O2 and DQ(+)-radical.Although hydroxyl radicals are likely intermediates, the predicted inhibition when methanol is present did not materialize; a mechanistic rationalization is presented.The rates with peroxodiphosphate were pH dependent and could be interpreted as different contributions from H2P2O8(2-), HP2O8(3-), and P2O8(4-) in the same order as found for peroxodiphosphate and unstable radicals.The rate with S2O8(2-) is fastest, but complications prevented the evaluation of a rate constant.