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

Cameron L Bentley, Alan Maxwell Bond, Anthony Frank Hollenkamp, Peter J Mahon, Jie Zhang

Research output: Contribution to journalArticleResearchpeer-review

12 Citations (Scopus)

Abstract

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.
Original languageEnglish
Pages (from-to)21840-21851
Number of pages12
JournalJournal of Physical Chemistry C
Volume119
Issue number38
DOIs
Publication statusPublished - 2015

Cite this

@article{67cb23a9a4744f59ab0acdd8b6f796af,
title = "Electrochemical proton reduction and equilibrium acidity (pKa) in aprotic ionic liquids: phenols, carboxylic acids, and sulfonic acids",
abstract = "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.",
author = "Bentley, {Cameron L} and Bond, {Alan Maxwell} and Hollenkamp, {Anthony Frank} and Mahon, {Peter J} and Jie Zhang",
year = "2015",
doi = "10.1021/acs.jpcc.5b05724",
language = "English",
volume = "119",
pages = "21840--21851",
journal = "Journal of Physical Chemistry C",
issn = "1932-7447",
publisher = "American Chemical Society",
number = "38",

}

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 journalArticleResearchpeer-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

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