Kinetics of chain reaction driven by proton-coupled electron transfer: α-hydroxyethyl radical and bromoacetate in buffered aqueous solutions

Abstract

We measured and computed the rate constants of the reaction between the α-hydroxyethyl radical (˙CH(CH₃)OH) and bromoacetate (BrCH₂CO₂⁻) in the non-buffered (NB), as well as in the bicarbonate (HCO₃⁻) and hydrogen phosphate (HPO₄²⁻) buffered aqueous solutions in the presence of ethanol. These complex multistep reactions are initiated by the proton-coupled electron transfer (PCET) which reduces BrCH₂CO₂⁻ and incites its debromination. The PCET is followed by the step in which the resulting carboxymethyl radical propagates a radical chain reaction thus recovering ˙CH(CH₃)OH and enhancing the debromination yields. It is found that the rate constants for the initial PCET step (k₁) are raised by ca. an order of magnitude in the presence of the buffers (k₁(NB) = 1.4 × 10⁵ dm³ mol⁻¹ s⁻¹; k₁(HCO₃⁻) = 1.4 × 10⁶ dm³ mol⁻¹ s⁻¹; k₁(HPO₄²⁻) = 1.1 × 10⁶ dm³ mol⁻¹ s⁻¹). To rationalize this, we used density functional theory at the M06-2X-D3/6-311+G(2d,p) level in conjunction with the polarizable continuum model (PCM) for an implicit description of the aqueous environment. To acceptably reproduce the measured rate constants, the minimal solute, consisting of ˙CH(CH₃)OH, BrCH₂CO₂⁻ and the buffer anion, has to be expanded by at least 2–3 explicit molecules of the water solvent. The used kinetic model consisting of a set of coupled differential equations indicates the sigmoid dependence of yields vs. k₁ thereby confirming the autocatalytic trait of these reactions. The computations unravel the profound influence of the presence of buffers on these reaction systems. On the one hand, the buffer anions promote the PCET by accelerating the proton transfer; on the other hand, they slow down the propagation step by forming the strong hydrogen bonds with the carboxymethyl radical. The two opposing effects cancel out and cause the Br⁻ yields to remain approximately comparable in the non-buffered and buffered media.