Studies on structure–reactivity in the reaction of OH radicals with substituted halobenzenes in aqueous solutions

Abstract

The reaction of OH radicals with a number of substituted halobenzenes (C₆H_{5 –n}X_nY, where X = F, Cl or Br and Y = CH₃, CH₂Cl, CHCl₂, CF₃ or OCH₃) produces a transient absorption band whose λ_max is in the range 310–340 nm. This band is assigned to the OH adduct and decayed with second-order kinetics, the bimolecular rate constant being in the region of 0.6 × 10⁹– 5.0 × 10⁹ dm³ mol^–1 s^–1. The OH radical reacts mainly by addition to the benzene ring. The rate constants for the reaction of OH radicals, as determined from formation kinetics studies, are between 1.7 and 9.3 × 10⁹ dm³ mol^–1 s^–1, and are found to depend on the nature of the substituents. A linear correlation is observed with the Hammett parameter. The distribution pattern for the OH adduct is discussed. The OH adduct can be oxidized by K₃[Fe(CN)₆]. The rate constant for the oxidation depends on the position of OH addition and the extent of oxidation on the nature of the substituents. The reaction of O^– with 2-chlorotoluene is by H-atom abstraction (14%), producing an absorption band with λ_max= 258 nm, and by addition to the benzene ring (86%), producing absorption bands at 290, 320 and 410 nm. The bimolecular rate constant for the reaction of O^– with 2-chlorotoluene is 2.0 × 10⁹ dm³ mol^–1 s^–1 and ε_320nm= 2.6 × 10³ dm³ mol^–1 cm^–1. For the reaction of e_aq^– with 2-chlorobenzyl chloride, 44% of the e_aq^– react with the Cl of the CH₂Cl group producing an absorption band at 258 nm and 56% react with the Cl of the benzene ring producing an absorption band at 320 nm. The bimolecular rate constant as determined from the decay of e_aq^– is 2.2 × 10¹⁰ dm³ mol^–1 s^–1.