Proton-coupled electron transfer is the dominant mechanism of reduction of haloacetates by the α-hydroxyethyl radical in aqueous media

Abstract

The reaction systems of α-hydroxyalkyl radicals with halogenated organics in aqueous solutions are uniquely suited for studying the fundamentally important proton-coupled electron transfer (PCET) mechanism in competition with alternatives such as substitution, hydrogen abstraction, halogen atom abstraction etc. We report experimental (steady state γ-radiolysis) and theoretical (density functional theory) studies of reactions of the α-hydroxyethyl radical (˙EtOH) with the four monohaloacetate anions (XAc⁻): fluoroacetate (FAc⁻), chloroacetate (ClAc⁻), bromoacetate (BrAc⁻) and iodoacetate (IAc⁻). The reactions are conducted in non-buffered and buffered (bicarbonate or phosphate) aqueous solutions of ethanol. In these conditions, only IAc⁻ and BrAc⁻ are reduced by ˙EtOH, and the PCET is predicted to be the most feasible reaction mechanism. In contrast to analogous reaction systems with alkyl halides, halophenols and 5-bromouracil, the radical-mediated one-electron reduction and subsequent dehalogenation of IAc⁻ and BrAc⁻ proceed regardless of the presence of buffers as the external proton acceptors. This implies that the proton can be efficiently transferred to the carboxyl group. The proton transfer is predicted to take place directly as interposition of one water molecule raises the barriers to the PCET. The addition of HCO₃⁻ or HPO₄²⁻ accelerates the PCET owing to their larger proton affinities compared to that of the carboxyl group. The reduction of IAc⁻ and BrAc⁻ generates daughter carboxymethyl radicals thus initiating a radical chain reaction which considerably enhances the Br⁻ and I⁻ yields. In contrast, ClAc⁻ and FAc⁻ are not degraded by ˙EtOH even at elevated temperatures. These comparatively simple reaction systems enable general insights into PCET processes in which the carboxyl group may assume the role of proton acceptor.