Direct evidence for H+-catalysed dehydration of fluorohydroxycyclohexadienyl radical: a pulse radiolysis study
Abstract
The OH-adduct of fluorobenzene (λmax= 310 nm) formed on reaction of ˙OH radicals in a neutral aqueous solution reacts with H+ to form solute radical cations (λmax= 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 λmax 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.