Electrochemical Proton Reduction and Equilibrium Acidity (pKa) in Aprotic Ionic Liquids: Protonated Amines and Sulfonamide Acids

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• 作者：Cameron L. Bentley ; Alan M. Bond ; Anthony F. Hollenkamp ; Peter J. Mahon ; Jie Zhang
• 刊名：Journal of Physical Chemistry C
• 出版年：2015
• 出版时间：September 24, 2015
• 年：2015
• 卷：119
• 期：38
• 页码：21828-21839
• 全文大小：581K
• ISSN：1932-7455

文摘

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 (R_xNH) 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 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/H₂ 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 鈥淐鈥?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 鈮?pK_a 鈮?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.