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

Autori
Džeba, Iva ; Bonifačić, Marija ; Nikšić-Franjić, Ivana ; Ljubić, Ivan

Izvornik
PCCP. Physical chemistry chemical physics (1463-9076) 20 (2018); 19829-19840

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
Free radicals, haloacetates, aqueous reactions, buffers, reaction mechanism, proton-coupled electron transfer, steady state gamma-radiolysis, density functional theory

Sažetak
The reaction systems of a-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 gamma-radiolysis) and theoretical (density functional theory) studies of reactions of the alpha-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.

Izvorni jezik
Engleski

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