The complex story of a simple Brønsted acid: Unusual speciation of HBr in an ionic liquid medium (original) (raw)
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Brønsted Acids in Ionic Liquids: Fundamentals, Organic Reactions, and Comparisons
Monatshefte für Chemie - Chemical Monthly, 2007
A background for studying acids in various solvents is developed, emphasizing the importance of knowing to what extent a solvent conducts electricity and is therefore ionized, the dissociation equilibria of common molecular solvents and the acidic and basic species generated by solvent leveling. Acidity measurements in the atypical solvent water are discussed and the common method of expressing acidity in other systems-by Hammett values-is introduced. Representative examples of reactions involving Brønsted acids in ionic liquids are presented and attention paid to the questions of speciation and acidity values. It is found that the gas phase proton affinity of a base is often a better guide to the acidity of its conjugate acid in an ionic liquid than is the dissociation constant of the said acid in water.
Aqueous Brønsted-Lowry Chemistry of Ionic Liquid Ions
Ionic liquids have become commonplace materials found in research laboratories the world over, and are increasingly utilised in studies featuring water as co-solvent. It is reported herein that proton activities, aH+, originating from auto-protolysis of H2O molecules, are significantly altered in mixtures with common ionic liquids comprised of Cl−, [HSO4]−, [CH3SO4]−, [CH3COO]−, [BF4]−, relative to pure water. paH+ values, recorded in partially aqueous media as −log(aH+), are observed over a wide range (∼0–13) as a result of hydrolysis (or acid dissociation) of liquid salt ions to their associated parent molecules (or conjugate bases). Brønsted–Lowry acid–base character of ionic liquid ions observed is rooted in equilibria known to govern the highly developed aqueous chemistry of classical organic and inorganic salts, as their well-known aqueous pKs dictate. Classical salt behaviour observed for both protic and aprotic ions in the presence of water suggests appropriate attention need be given to relevant chemical systems in order to exploit, or avoid, the nature of the medium formed.
Brønsted Acidity and the Medium: Fundamentals with a Focus on Ionic Liquids
ChemPhysChem, 2011
Fundamental aspects of Brønsted acidity in ionic liquid systems, in relation to those of simple protic molecules in the gas phase, pure protic molecules in the condensed phase and solutions of protic molecules in molecular systems, are presented. The variety of acidities possible, beyond those observed in aqueous systems, is emphasised and discussed in terms of differences of solvent levelling, ionisation, dissociation, homo-/ hetero-conjugate ion speciation and the stabilisation of proton-transfer products from solvent to solvent. It is argued that data regarding aqueous systems do not necessarily explain acid/base behaviour in other liquids satisfactorily. Methods of measuring acidity are reviewed, particularly by spectrophotometry and electrochemistry and recommendations proffered for estimating speciation and acidity of ionic liquids of various complexities.
1H, 13C NMR and DFT Study of Hydrogen Bonding in Imidazolium-based Ionic Liquids
Acta chimica Slovenica, 2011
The ionic liquid 1-decyl-3-methyl-imidazolium bromide [C10mim][Br], the neat material, and also dissolved (~0.01 mole fraction) in various dielectric media (acetonitrile, benzene, chloroform, dichloromethane, methanol, 2-butanol and H2O) was studied using 1H and 13C NMR spectroscopy. The most important interaction in this compound is considered to be the Br-...H-C2+ hydrogen bond, which is formed between the anions and cations. The obtained results show that dielectric medium influence mostly the behavior of the Br-...H-C2+ bridge proton. The changes observed in 1H and 13C NMR spectra of [C10mim][Br] with increasing solvents polarity and temperature can be explained applying the model of the lengthening of the H2...Br- bond with the accompanying thickening of the solvation shell of bromine anion and C2-H bond contraction. The short-range order effects related to the configuration of neighboring dipoles of solvent molecules are more important for the solvation ability of small anions...
Cation–anion–water interactions in aqueous mixtures of imidazolium based ionic liquids
Vibrational Spectroscopy, 2011
We have examined the cation-anion-water interactions in aqueous mixtures of imidazolium ionic liquids (ILs) over the whole composition range using FTIR spectroscopy. Changes in the peak positions or band areas of OH vibrational modes of water and CH vibrational modes of imidazolium cation as a function of IL concentration indicated a diminishing trend in hydrogen-bonding network of water and qualitative changes in solution structures. 1 H NMR chemical shifts of C(2)H, HC(4)C(5)H and alkyl chain protons of imidazolium cation provided useful information about the comparative strength of cation-anion-water interactions.
Brønsted Acid−Base Ionic Liquids as Proton-Conducting Nonaqueous Electrolytes
The Journal of Physical Chemistry B, 2003
A new series of Brønsted acid-base ionic liquids were derived from the controlled combination of a monoprotonic acid with an organic base under solvent-free conditions. Appropriate amounts of solid bis-(trifluoromethanesulfonyl)amide (HTFSI) and solid imidazole (Im) were mixed at various molar ratios to have compositions varying from an equimolar salt to HTFSI-or Im-rich conditions. The mixture at equivalent molar ratio formed a protic neutral salt with a melting point of 73°C, which was thermally stable at temperatures even above 300°C. The melting points of other compositions were lower than those of the equimolar salt and Im or HTFSI, giving eutectics between the equimolar salt and HTFSI or Im. Some of the compositions with certain molar ratios of Im and HTFSI were liquid at room temperature. For Im excess compositions, the conductivity was found to increase with increasing Im mole fraction, and the 1 H NMR chemical shift of the proton attached to the nitrogen atom of Im was shifted to a lower magnetic field. On the contrary, the conductivity decreased with increasing HTFSI mole fraction, and the 1 H NMR chemical shift of the proton attached to the TFSI imide anion also shifted to a higher magnetic field. Self-diffusion coefficients, measured by pulsed-gradient spin-echo NMR (PGSE-NMR) methods in Im-or HTFSI-rich compositions, indicated that fast proton exchange reactions between the protonated Im cation and Im take place in excess Im. The proton conduction follows a combination of Grotthuss-and vehicle-type mechanisms. Direct current polarization measurements were used for the confirmation of proton conduction in Im-rich compositions. Furthermore, reduction of molecular oxygen could be observed at the interface between a Pt electrode and these ionic liquids. This introduces the Brønsted acid-base ionic liquid system as a new candidate for proton conductor such as a fuel cell electrolyte to operate under anhydrous conditions and at elevated temperature.
Experimental measurements of density at different temperatures ranging from 293.15 to 313.15 K, the speed of sound and osmotic coefficients at 298.15 K for aqueous solution of 1-ethyl-3-methylimidazolium bromide ([Emim][Br]), and osmotic coefficients at 298.15 K for aqueous solutions of 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) in the dilute concentration region are taken. The data are used to obtain compressibilities, expansivity, apparent and limiting molar properties, internal pressure, activity, and activity coefficients for [Emim][Br] in aqueous solutions. Experimental activity coefficient data are compared with that obtained from Debye-Hückel and Pitzer models. The activity data are further used to obtain the hydration number and the osmotic second virial coefficients of ionic liquids. Partial molar entropies of [Bmim][Cl] are also obtained using the free-energy and enthalpy data. The distance of the closest approach of ions is estimated using the activity data for ILs in aqueous solutions and is compared with that of X-ray data analysis in the solid phase. The measured data show that the concentration dependence for aqueous solutions of [Emim] [Br] can be accounted for in terms of the hydrophobic hydration of ions and that this IL exhibits Coulombic interactions as well as hydrophobic hydration for both the cations and anions. The small hydration numbers for the studied ILs indicate that the low charge density of cations and their hydrophobic nature is responsible for the formation of the water-structure-enforced ion pairs. Ionic Interactions of [Emim][Br] and [Bmim][Cl] Ionic Interactions of [Emim][Br] and [Bmim][Cl]
The Interaction Nature of Brwith 1-(4-Butylamino)-3-Methyl Imidazolium
Journal of Applied Solution Chemistry and Modeling, 2012
An ionic liquid (IL) interaction system of Br with 1-(4-butylamino)-3-methyl imidazolium([Bamim] +) were investigated using B3LYP and MP2 methods at 6-311++G** level. Three possible stable geometry structures of this IL system were optimized. The energies of these three structures and ion-pair considering zero point energy (ZPE) and basis sets superposition energy (BSSE) correction were calculated. The results show that the interactions between ([Bamim] +) and Br are mainly hydrogen bonds(H-bonds); the interaction energies between ion-pairs are larger than-350KJ/mol, which are far beyond those of H-bonds; the interaction nature of Br with cation is studied based on natural bond orbital (NBO) calculation and the results show that the electrostatic attraction force plays a very important role in the interaction energy between ion-pair.
Ion Association of Imidazolium Ionic Liquids in Acetonitrile
The Journal of Physical Chemistry B, 2014
Molar conductivities, Λ, of dilute solutions of the ionic liquids (ILs) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF 4 ]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4 ]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6 ]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF 4 ]), and 1-hexyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)amide ([hmim][NTf 2 ]) in acetonitrile were determined as a function of temperature in the range 273.15−313.15 K. The data were analyzed with Barthel's lcCM model to obtain limiting molar conductivities, Λ ∞ (T), and association constants, K A°( T) of these electrolytes. The temperature dependence of these parameters, as well as the extracted limiting cation conductivities, λ i ∞ , were discussed. Additionally, dielectric spectra for [hmim][NTf 2 ] + AN were analyzed in terms of ion association and ion solvation and compared with the inference from conductivity. It appears that in dilute solutions the imidazolium ring of the cations is solvated by ∼6 AN molecules that are slowed by a factor of ∼8−10 compared to the bulk-solvent dynamics. Ion association of imidazolium ILs to contact ion pairs is only moderate, similar to common 1:1 electrolytes in this solvent.