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Rate constants for the reaction of halogenated alkylperoxyl radicals with iodide: influence of substituents, solvent polarity, and proton concentration
Author(s) -
Štefanić Igor,
Asmus KlausDieter,
Bonifačić Marija
Publication year - 2005
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.885
Subject(s) - chemistry , reaction rate constant , radical , electronegativity , iodide , radiolysis , solvent , adduct , substituent , photochemistry , solvent effects , medicinal chemistry , inorganic chemistry , kinetics , organic chemistry , physics , quantum mechanics
This 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 CF 3 OO · , CFCl 2 OO · , CCl 3 OO · , C 2 Cl 5 OO · , CHCl 2 OO · and CF 3 CHClOO · . 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 I 2 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 I 3 − /I 2 ·− , the spectrophotometrically accessible forms of I 2 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 × 10 5 to 7.0 × 10 8 M −1 s −1 . A good linear relationship is obtained between log k 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.