Electrochemical Proton Reduction and Equilibrium Acidity (pKa) 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, K-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 (RxNH) acids as the proton source. The formal potential of the H-solvated(+)/H-2 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 pk(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(+)/H-2 process. Simulations reveal that weak acid dissociation is limiting on the voltammetric time scale when pK(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 pK(a) values for 10 weak nitrogen acids have been calculated (5.2 <= pk(a) <= 19.5) from voltammetrically derived reversible half-wave potentials (E-1/2) and diffusion coefficients (D), highlighting the utility of voltammetry as a convenient and relatively straightforward method for quantifying equilibrium acidity.