Electrochemical Proton Reduction and Equilibrium Acidity (pKa) in Aprotic Ionic Liquids: Phenols, Carboxylic Acids, and Sulfonic 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：21840-21851
- 全文大小：606K
- ISSN：1932-7455
文摘
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鈥揟afel 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 鈮?pKa 鈮?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.