engleski

The Mechanism of Reduction by Aliphatic α-Hydroxyalkyl Radicals

Carbon-centred radicals, •CH2OH, •CH(CH3)OH, •CH(CH3)O-, and •CO2- have been used to dehalogenate iodophenol, some halogenated aliphatic compounds, and 5-bromouracil in oxygen free aqueous solutions in the presence of formate, methanol or ethanol. For radicals production 60Co γ-radiolysis was employed and the yield of halide ions was measured by means of ion chromatography. In systems where reduction is operative, dehalogenation occurs in a chain process. While reduction potentials of •CH(CH3)OH and •CH2OH radicals is not negative enough to transfer an electron to e.g. iodophenols or 4-bromouracil in just water/alcohol solutions, the addition of sodium bicarbonate or acetate (mM concentrations, pH 7) leads to halide ions production in high yields indicative for a chain reaction. Phosphate buffer failed to show such an effect in spite of its higher bulk proton affinity. This indicates one-electron reduction reactions by α-hydroxyalkyl radical to occur by the concerted proton-coupled electron transfer, PCET, nechanism. It is considered that bicarbonate or acetate anions form a suitable complex with an α-hydroxyalkyl radical coursing the enhancement of the radical reduction power in comparison with only water molecules as proton acceptors. Pulse radiolysis experiments with halophenols in similar systems were useless in the sense of the reduction mechanism investigations. The results enabled incidentally to determine absolute rate constants for the phenol/phenolate proton transfer reactions in water.

PCET mechanism; reductive dehalogenation; radiolysis

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