engleski

Reductive iodide elimination from iodophenols by alcohol derived radicals; chain reaction induced by proton-coupled electron transfer

Gamma-Radiolysis and measurements of iodide by means of ion chromatography have been employed to investigate reductive dehalogenation of iodophenols by carbon-centered radicals, .CH(CH3)OH, .CH2OH, and .CO2-, in oxygen free neutral aqueous solutions in the presence of ethanol, methanol or sodium formate. While the reaction of 4-IC6H4OH with .CH(CH3)OH of .CH2OH radicals is endothermic in water/alcohol solutions, addition of bicarbonate leads to iodide production in high yields indicative for a chain reaction. No such effect was observed with phosphate, a buffer with higher proton affinity. Formation of a complex between alpha- hydroxy alcohol radicals and the bicarbonate anion is proposed enhancing the reduction power of the former to the extentthat the electron transfer to iodophenol meolecule becomes exothermic. This indicates one-electron reduction reactions by alpha-hydroxy alkyl radicals to occur by the concerted proton- coupled electron transfer and not by a two step ET/PT or PT/ET mechanism. Reduction power of the carbonate radical anion is shown to be high enough to reduce iodophenols by the one- electron transfer mechanism. In the presence of formate ions as an H-atom donor, dehalogenation also occurs by a chain reaction. In all systems the chain lenghts were dependent on the rate of reductive radicals reproduction in the propagation step, i.e. the H-atom abstraction from methanol, ethanol or formate by 4-.C6H4OH radicals. The rate constant of this reaction was determined from the iodide yield measurements at constant irradiation dose rate and was estimated to be 6 M-1 s-1 for methanol, 140 M-1 s-1 for ethanol and 2100 M-1 s-1 for formate. Results will be discussed in view of their significance for the environmental remediation.

PCET mechanism ; Chain reactions ; Rate constant ; Dehalogation

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