Reactions of OH and SO4.- with Some Halobenzenes and Halotoluenes: A Radiation Chemical Study

Article abstract of DOI:10.1021/j100088a012

The optical absorption and kinetic characteristics of the transients formed in the reactions of OH and SO4^.- with bromobenzene, ortho and meta-isomers of chloro- and bromobenzenes, and monobromotoluenes have been studied by pulse radiolysis technique.The rates for OH reaction are generally higher (k = (1.7-4.4)X10⁹ M^-1s^-1) than those found for the SO4^.- reaction (k = (0.4-2.3)x10⁹ M^-1s^-1). ρ⁺ values of -0.4 for OH and -1.2 for SO4^.- reactions were obtained from the Hammett analysis.The formation of substituted hydroxycyclohexadienyl radicals (λ_max = 315-330 nm) is the major reaction channel, and the phenoxyl type radical (λ >/=400 nm) formation is an additional minor process in the SO4^.- reaction.Abstraction of H by SO4^.- from the -CH3 group is only significant with the para-isomers of bromo- and chlorotoluenes.This result is in accord with the observed yields (70 percent of SO4^.-) of the products resulting from the oxidation of the 4-chlorobenzyl radical in the presence of K3Fe(CN)6 under steady-state conditions.The total yields of the phenolic products accounting for >90 percent of OH and SO4^.- suggest that the attack at the ipso positions is considerably small.The rate constants for OH reactions relative to benzene at positions 3 and 6 of 2-chlorotoluene and positions 2 and 3 of 4-chlorotoluene are between 1.18 and 1.39, indicating that the directing effects of -CH3 and -Cl groups are comparable.This is also reflected in the additive effects of activation of the ortho and para-positions and deactivation of meta-positions by these substituents in 3-chlorotoluene.