Abstract
| Original language | English |
|---|---|
| Pages (from-to) | 21840-21851 |
| Number of pages | 12 |
| Journal | Journal of Physical Chemistry C |
| Volume | 119 |
| Issue number | 38 |
| DOIs | |
| Publication status | Published - 2015 |
Cite this
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Electrochemical proton reduction and equilibrium acidity (pKa) in aprotic ionic liquids: phenols, carboxylic acids, and sulfonic acids. / Bentley, Cameron L; Bond, Alan Maxwell; Hollenkamp, Anthony Frank; Mahon, Peter J; Zhang, Jie.
In: Journal of Physical Chemistry C, Vol. 119, No. 38, 2015, p. 21840-21851.Research output: Contribution to journal › Article › Research › peer-review
TY - JOUR
T1 - Electrochemical proton reduction and equilibrium acidity (pKa) in aprotic ionic liquids: phenols, carboxylic acids, and sulfonic acids
AU - Bentley, Cameron L
AU - Bond, Alan Maxwell
AU - Hollenkamp, Anthony Frank
AU - Mahon, Peter J
AU - Zhang, Jie
PY - 2015
Y1 - 2015
N2 - 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.
AB - 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.
UR - http://pubs.acs.org.ezproxy.lib.monash.edu.au/doi/pdf/10.1021/acs.jpcc.5b05724
U2 - 10.1021/acs.jpcc.5b05724
DO - 10.1021/acs.jpcc.5b05724
M3 - Article
VL - 119
SP - 21840
EP - 21851
JO - Journal of Physical Chemistry C
JF - Journal of Physical Chemistry C
SN - 1932-7447
IS - 38
ER -