Reactions of OH and SO₄·- with some halobenzenes and halotoluenes: a radiation chemical study

Abstract

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) × 10⁹ M^-1s^-1) than those found for the SO₄^.- reaction (k = (0.4-2.3) × 10⁹ M^-1 s^-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 (lambda greater than or equal to 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% 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% of OH and SO₄^.- suggest that the attack a 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.