Electrochemical proton reduction and equilibrium acidity (pKa) in aprotic ionic liquids: phenols, carboxylic acids, and sulfonic acids

Many organic compounds contain acidic and/or basic functional groups that dictate their physical, chemical, and biological properties. For this reason, the acid dissociation constant, Ka, a quantitative measure of acid strength in solution, is a fundamentally important parameter. 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 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, using a range of oxyacids (phenols, carboxylic acids, or sulfonic acids) as the proton source. Triflic acid, H[OTf], a well-known superacid in aqueous media, has been shown to be a weak acid with a pKa of 2.0 in this ionic liquid. Hydrogen evolution from H[OTf] has been simulated using a CE mechanism, where acid dissociation is followed by proton reduction via the classical Volmer-Tafel route. Proton reduction from a range of other sulfonic or carboxylic acids has been shown to occur in two steps (electron stoichiometry = 1:1), which has been attributed to the formation of a stable intermediate species through homo hydrogen bonding (homoassociation) between the acid and its conjugate anion base. Simulations confirm that the experimental voltammetric response is consistent with an ECE mechanism, where C is the anionic homoassociation step. Finally, the pKa values of 10 weak oxyacids, covering 16 orders of magnitude in acid strength (2.0 ≥ pKa ≥ 17.8), have been calculated using a voltammetric method and compared with data from conventional solvents (acetonitrile and water) in order to gain insights into how the nature of the solvent (i.e., dielectric properties, Lewis acidity/basicity, hydrogen donating/accepting ability, etc.) influences equilibrium acidity.