Electrochemical Proton Reduction and Equilibrium Acidity (p<i>K</i>_a) in Aprotic Ionic Liquids: Protonated Amines and Sulfonamide Acids

Many organic compounds contain acidic and/or basic groups that dictate their physical, chemical, and biological properties. For this reason, the acid dissociation constant, <i>K</i>_a, a quantitative measure of acid strength in solution, is a fundamentally important parameter in organic (synthetic) chemistry and related fields. In this study, the thermodynamics, kinetics, and mechanisms of the proton reduction (hydrogen evolution) reaction at a platinum electrode have been investigated in the room temperature ionic liquid (IL) 1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­imide, using a range of nitrogen (R_<i>x</i>NH) acids as the proton source. The formal potential of the H⁺_solvated/H₂ process (simulated by combining the classical Volmer and Tafel reactions) has been shown to be strongly dependent on the identity of the IL anion, making direct comparison of p<i>K</i>_a data between ILs with different constituent anions impossible. Hydrogen evolution from weak nitrogen acids (protonated amines or sulfonamides) as the proton source is a diffusion controlled process which occurs in the potential region negative of the H⁺_solvated/H₂ process. Simulations reveal that weak acid dissociation is limiting on the voltammetric time scale when p<i>K</i>_a > 4, meaning proton reduction via a CE mechanism (where “C” is the acid dissociation step) cannot account for the experimentally observed mass-transport-limited currents. Under these conditions, proton reduction must proceed via an alternate pathway, where the weak acid undergoes direct reduction at the platinum electrode surface. Finally, the p<i>K</i>_a values for 10 weak nitrogen acids have been calculated (5.2 ≤ p<i>K</i>_a ≤ 19.5) from voltammetrically derived reversible half-wave potentials (<i>E</i>_1/2) and diffusion coefficients (<i>D</i>), highlighting the utility of voltammetry as a convenient and relatively straightforward method for quantifying equilibrium acidity