Formation of radical anions on the reduction of carbonyl-containing perfluoroaromatic compounds in aqueous solution: a pulse radiolysis study

Abstract

Radical anions are formed on addition of hydrated electrons to pentafluoroacetophenone (PFA) and pentafluorobenzaldehyde (PFB) in aqueous solutions. On the other hand, addition of hydrated electrons to pentafluorobenzoic acid (PFBA) leads to rapid fluoride elimination. The spectrum of the radical anion of PFA has λ _max at 300 and 440 nm with absorption coefficient at 440 nm ε ₄₄₀ = 2100 L mol^-1 cm^-1. PFA^•- decays with a rate constant of (7 ± 3.0) × 10³ s^-1. It has a pK_a = 7.5 and the spectrum of the conjugate acid has λ _max at 270 and 460 nm with ε ₄₆₀ = 900 L mol^-1 cm^-1. The spectrum of the radical anion of PFB has λ _max at 285 and 430 nm with ε ₄₃₀ = 800 L mol^-1 cm^-1. PFB^•- decays with a rate of (4 ± 2) × 10³ s^-1. It has a pK_a = 7.2 and the spectrum of the conjugate acid has weak absorption at 330 nm. Evidence for the formation of the radical anion was obtained from intermolecular electron transfer from the radical anions of PFA and PFB to p-benzoquinone (Q), methyl viologen (MV²⁺), and 9,10-anthraquinone-2-sulfonate (AQS^-). Strong reductants derived from reduction of 2,2-bipyridine (BpyH^•) and 1,10-phenanthroline (PhenH^•) can reduce both PFA and PFB. From the kinetics of these electron transfer reactions the reduction potentials of PFA and PFB have been determined to be -0.86 ± 0.1 and -0.75 ± 0.1 V vs NHE at pH 9.4. Addition of OH^• radical to the aromatic ring of these fluorinated compounds led to rapid HF elimination and the formation of phenoxyl radicals, and addition of H^• atoms led to the formation of cyclohexadienyl radical.