engleski

Reductive Halogen Elimination from Phenols by Organic Radicals in Aqueous Solutions ; Chain Reaction Induced by Proton-Coupled Electron Transfer

 -Radiolysis and measurements of halide ions by means of ion chromatography have been employed to investigate reductive dehalogenation of chloro-, bromo-, and iodophenols by carbon-centered radicals, • CH(CH3)OH, • CH2OH, and • CO2 , in oxygen free aqueous solutions in presence of ethanol, methanol or sodium formate. While the reactions of 4-IC6H4OH with • CH(CH3)OH and • CH2OH radicals are endothermic in water/alcohol solutions, the addition of bicarbonate leads to iodide production in high yields indicative for a chain reaction. The maximum effect has been observed at about 10 mM of sodium bicarbonate present. The complex formed of an  -hydroxyalkyl radical and a bicarbonate anion is considered to cause the enhancement of the reduction power of the former to the extent at which the reduction of iodophenol molecule becomes exothermic. No such effect has been observed with phosphate, which is a buffer with higher proton affinity, if added in the concentration of up to 20 mM at pH 7. This indicates one-electron reduction reactions by  -hydroxyalkyl radicals to occur by the concerted proton-coupled electron transfer, PCET, and not by a two-step ET/PT or PT/ET mechanisms. The reason for the negative results with phosphate buffer could be thus ascribed to a less stable complex or to the formation of a complex with a less suitable structure for an adequate support to reduce iodophenol. The reduction power of the carbonate radical anion is shown to be high enough to reduce iodophenols by one-electron transfer mechanism. In the presence of formate ions as H-atom donors, the dehalogenation also occurs by a chain reaction. In all systems, the chain lengths have depended on the rate of reductive radicals reproduction in the propagation step, i.e. on the rate of H-atom abstraction from methanol, ethanol or formate by 4-• C6H4OH radicals liberated after iodophenol dehalogenation. The rate constants of those reactions were determined from the iodide yield measurements at constant irradiation dose rate. They were estimated to be 6 M-1s-1 for methanol, 140 M-1s-1 for ethanol and 2100 M-1s-1 for formate. Neither of the tested reductive C-centered radicals was able to dehalogenate bromo- or chloro-derivative of phenol.

radiolysis; dehalogenation; phenol; radicals

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