Competing Hydrogen-Bonding, Decomposition, and Reversible Dimerization Mechanisms during the One- and Two-Electron Electrochemical Reduction of Retinal (Vitamin A)

Abstract

Retinal (R) can be sequentially voltammetrically reduced in CH3CN in two one-electron processes to form first the anion radical (R(•-)) at -1.75 (±0.04) V vs Fc/Fc(+) (Fc = ferrocene) then the dianion (R(2-)) at -2.15 (±0.04) V vs Fc/Fc(+). The anion radical undergoes a reversible dimerization reaction to form the dianion (R2(2-)) with a forward dimerization rate constant k(dim) = 8 × 10(2) L mol(-1) s(-1) and a reverse monomerization rate constant k(mon) = 2 × 10(-2) s(-1) at 295 K. All three anion species (anion radical, dianion, and dimer dianion) undergo hydrogen-bonding interactions with water that is present at millimolar levels in the solvent. As the water content of the solvent increases, the fate of the reduced compounds is determined by chemically irreversible hydrolysis reactions with H2O and decomposition reactions of the highly charged R(2-). Bulk-controlled potential electrolysis experiments combined with NMR analysis of the reaction solutions indicate that the reduction occurs at the aldehyde group of retinal. The electrochemical data obtained under a range of experimental conditions (varying voltammetric scan rates, temperatures, H2O content of solutions, and retinal concentrations) were modeled by digital simulation techniques to determine the kinetic and thermodynamic parameters associated with all of the homogeneous reactions.