Pulse radiolysis of aryl bromides in aqueous solutions: some properties of aryl and arylperoxyl radicals

Abstract

Aryl radicals were generated pulse-radiolytically in aqueous solutions by reacting the solvated electron with phenyl bromide and substituted phenyl bromides. The aryl radicals react rapidly (k 2.5 × 10⁹ dm³ mol^–1 s^–1) with O₂ giving rise to arylperoxyl radicals which display characteristic absorptions in the visible spectrum (490–600 nm, depending on the substituent). Aryl radicals react with alcohols by H-abstraction. Making use of the strong absorptions of the arylperoxyl radicals in the visible spectrum, rate constants of the reaction of the aryl radicals with various alcohols have been determined by competition kinetics [e.g. 4-methoxyphenyl: 1.1 × 10⁷(2-PrOH), 5.8 × 10⁶(EtOH), 1.1 × 10⁶(MeOH), 5.2 × 10⁵ dm³ mol^–1 s^–1(Bu^tOH)]. The temperature dependence of the reaction of the 4-CN-phenyl radical was studied and reaction parameters of its reaction with O₂(A= 4 × 10¹¹ s^–1; E_a= 12 kJ mol^–1), propan-2-ol (A= 1.5 × 10⁹ s^–1; E_a= 13 kJ mol^–1) and tert-butanol (A= 6 × 10⁹ s^–1; E_a= 25 kJ mol^–1) determined.

While most of the arylperoxyl radicals decay by second-order processes by reacting with one another and other peroxyl radicals present (k 3 × 10⁹ dm³ mol^–1 s^–1) HO- and H₂N-substituted arylperoxyl radicals also decay by first-order processes.

It has been noted that the reaction of 4-nitrobromobenzene with the solvated electron does not lead to the formation of an aryl radical by halide elimination.