Direct evidence for H⁺-catalysed dehydration of fluorohydroxycyclohexadienyl radical: a pulse radiolysis study

Abstract

The OH-adduct of fluorobenzene (λ _max= 310 nm) formed on reaction of ^•OH radicals in a neutral aqueous solution reacts with H⁺ to form solute radical cations (λ _max= 385, 395, < 270 nm) when [HClO₄] > 1.5 mol dm^-3. When the concentration of HClO₄ is between 1.5 and 4.0 mol dm^-3, the H⁺-catalysed dehydration of the OH-adduct is observed with a bimolecular rate constant of 1.4 × 10⁵ dm³ mol^-1 s^-1. The formation of solute radical cations of halogenated benzene depends on the electron affinity of the halogen. The λ _max values of solute radical cations of halogenated benzene vary linearly with the size of the halogen. On the other hand, SO₄^•¯ is able to undergo a direct electron-transfer reaction with fluorobenzene (k= 1.4 × 10⁹ dm³ mol^-1 s^-1); the OH-adduct is then formed on hydrolysis of solute radical cations. The radical cation of fluorobenzene is a strong one-electron oxidant with a reduction potential value of >2.0 V. In acidic aqueous solution ([HClO₄] >6.0 mol dm^-3) of 3-chlorofluorobenzene, ^•OH radicals react to form radical cations; and their formation is influenced by chlorine but not by fluorine.