Rate constants for the reaction of halogenated alkylperoxyl radicals with iodide: influence of substituents, solvent polarity, and proton concentration

Abstract

AbstractThis pulse radiolysis study deals with the reaction of iodide ions (I−) with a series of halogenated alkylperoxyl radicals in 2‐propanol–water mixtures. Under investigation were CF3OO·, CFCl2OO·, CCl3OO·, C2Cl5OO·, CHCl2OO· and CF3CHClOO·. Each of these peroxyl radicals oxidizes a total of three I− ions in a fast and practically simultaneous reaction sequence. An overall two‐electron step, involving a transient peroxyl–iodide adduct radical, results in the oxidation of two iodide ions to yield molecular I2 and the corresponding halogenated alkoxyl radical, R(Hal)O·. The latter is responsible for the oxidation of the third iodide to yield iodine atoms, I·. The overall second‐order rate constants for these multi‐electron oxidation processes were determined by following the formation kinetics of I3−/I2·−, the spectrophotometrically accessible forms of I2 and I· in I−‐containing solutions. These rate constants were determined as a function of pH, polarity of solvent mixtures and electronegativity of the substituents at the α‐C atom of the peroxyl radicals. They strongly depend on all these variables, with the actual numbers covering a range from 3 × 105 to 7.0 × 108 M−1 s−1. A good linear relationship is obtained between logk and the dielectric constant of the solvent mixture. Within a particular solvent mixture the rate constants depend strongly on the substituents at the alkylperoxyl moiety and correlate with Taft's inductive substituent constants σ*. The overall multi‐electron oxidation mechanism of alkylperoxyl radicals is discussed in the light of the new results. Copyright © 2004 John Wiley & Sons, Ltd.