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Brønsted Basicity of Solute Butylamine in an Aprotic Ionic Liquid Investigated by Potentiometric Titration
Abstract
The quantitative Bronsted basicity of butylamine (BuNH2) in the typical aprotic ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [C(2)mIm(+)][TFSA(-)], is reported. Potentiometric titration was performed for BuNH2 in the aprotic IL to experimentally determine the acid dissociation constant (pK(a)) for butylammonium (BuNH3+). The pK(a) value was estimated to be 16.6(1), and it was found that the value is significantly larger than that in aqueous solution (pK(a) = 10.6).
... We recently reported the gelation mechanism of TetraPEG prepolymers, i.e., the A−B-type cross-end coupling reaction of TetraPEG-NH 2 and TetraPEG-NHS, in a typical aprotic imidazolium-based IL, in terms of the gelation time and acid−base reaction of the TetraPEG. 32,40,41 We noted that the gelation reaction in this system strongly depends on the concentration of H + ([H + ] or pH). That is, the acid−base reaction of the TetraPEG-NH 2 prepolymer, TetraPEG-NH 2 + H + ⇆ TetraPEG-NH 3 + , in the IL solution is important for understanding the gelation reaction because the protonated TetraPEG-NH 3 + cannot react with TetraPEG-NHS. ...
We report a high-toughness ion gel with a nearly ideal polymer network prepared in an imidazolium-based aprotic ionic liquid (aIL) with a controlled solution pH. We formed the ion gel from tetra-armed poly(ethylene glycol) (TetraPEG), i.e., an A–B-type cross-end coupling reaction of two different TetraPEG prepolymers. To complete this A–B-type reaction, we needed to optimize the reaction rate such that the two TetraPEGs were mixed homogeneously, which strongly depends on the pH or [H+] in the aIL solution. To control solution pH, we established a “pH-buffering IL” by adding an imidazolium-based protic IL (as a proton source) and its conjugated base to the solvent aIL. We demonstrated that the pH-buffering IL exhibits a successful pH-buffering effect to maintain a constant pH (≈16.2, apparent value) during the gelation reaction. From a kinetic study, we found that the gelation reaction undergoes a simple second-order reaction of the two TetraPEGs in the pH-buffering IL. The gelation rate constant, kgel, in the present ion gel system was 2 orders of magnitude smaller than that in the corresponding hydrogel system, which is ascribed to the difference in the activation entropy, ΔS‡, of the cross-end coupling reactions. The reaction efficiency at the cross-linking point was experimentally estimated to be 92% by spectroscopic measurements. We thus conclude that a nearly ideal polymer network was formed in the pH-buffering IL system. This is reflected in the excellent mechanical property of the ion gel, even at a low polymer content (=6 wt %).
We selected and validated the pH values of three standard materials that function in the protic ionic liquid, ethylammonium nitrate (EAN). The pH values of 0.05 mol kg‒1 phthalic acid, oxalic acid, and phosphoric acid were 4.93 (0.04), 2.12 (0.04), and 7.13 (0.06), respectively (the values in the parentheses denote the standard deviation). Since the pH of EAN ranges from 0 to 10, with a neutral pH of 5, these materials are usable as acidic, basic, or neutral standards. The standard electrode potential of silver-silver chloride in EAN was 127.2 (1.7) mV. The activity coefficients of hydrogen and chloride ions remain equal to unity in EAN of wide concentration range, which indicates that the effective ionic strength is independent of the solute ion concentration. In addition, the estimated value of the transfer activity coefficient of chloride ion suggests a weaker solvation in EAN compared with water in spite of a ubiquitous cation (C2H5NH3+). These behaviors of ions in EAN can be explained by the unique solvation in the ionic liquid through direct ion-ion electrostatic interactions.
Recently, chemical fixation of CO2 catalyzed by protic ionic liquids (PILs) has been successfully achieved, suggesting that CO2 absorption behavior plays an important role on CO2 activation and transformation. For this reason, the influence of the alkalinity and physicochemical properties on CO2 absorption behavior of four azole-based PILs, namely [DBNH][Pyr], [DBNH][Im], [DBUH][Pyr], and [DBUH][Im] was discussed in this work. The alkalinity of PILs was determined by potentiometric titration, and free space of PILs was evaluated by thermal expansion coefficient and refractive index. Solubility parameter of PILs was obtained from the activation energy for viscous flow. The results show that PILs with [Pyr]⁻ exhibits stronger alkalinity and larger free space, which can help to improve CO2 capture capacity. Whereas PILs with larger cation size [DBUH]⁺ exerts higher viscosity, resulting in slower absorption rate of CO2. Furthermore, the CO2 absorption capacity is inversely proportional to the solubility parameter of PILs. The absorption mechanism proved by FT-IR and ¹³C NMR spectra indicates that [Pyr]⁻ or [Im]⁻ can react with CO2 to form carbamate. Accordingly, considering the CO2 capture capacity and absorption rate, [DBNH][Pyr] may be a good candidate for efficient CO2 capture and activation with low enthalpy of absorption of −38.2 kJ mol⁻¹.
We report on the acid-base reaction of an amine solute in an aprotic ionic liquid from a structural point of view. Thus, the solvation structures of n-butylamine and n-butylammonium (BuNH2 and BuNH3(+), respectively, acid-base reaction: BuNH2 + H(+) ⇄ BuNH3(+)) in 1-methyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mIm][TFSA]) were investigated by high-energy X-ray total scattering combined with molecular dynamics simulations. We found that the solvation structure drastically changed as a result of the protonation reaction in [C2mIm][TFSA]. The NH3(+) group was preferentially solvated by TFSA(-) anions in the protonated BuNH3(+) system, whereas the neutral NH2 group was surrounded by both TFSA(-) and C2mIm(+) ions in the BuNH2 system. With regard to the nearest neighbor solute-TFSA interaction, the solvation number for TFSA(-) anions (nTFSA) increased upon protonation (i.e. BuNH2:nTFSA = 2 and BuNH3(+):nTFSA = 3). Both BuNH2 and BuNH3(+) interacted with O atoms within TFSA(-) through hydrogen bonding (i.e. N-HO) interactions, and the N-HO distance was appreciably shorter for positively charged BuNH3(+) as compared to neutral BuNH2. These results revealed that the solvation is more stable in energy for the protonated BuNH3(+) group because of its stronger hydrogen bonding as compared to neutral BuNH2. This is the origin of the large Gibbs energy for the protonation reaction (ΔG = -94.7 kJ mol(-1)) and the high acid dissociation constant (pKa = 16.6) of BuNH2 in [C2mIm][TFSA] solution.
Absolute pKas of 25 sulfonamides in four ionic liquids (ILs) were measured spectroscopically with high precision and subsequently compared with these in commonly seen molecular solvents. It is found that the acidity order of these sulfonamides follows: in water > in DMSO > in ILs > in acetonitrile (ACN). The well-known solvent polarity index Epsilon fails to explain the observed stronger bond-weakening effect of ILs than that of ACN whose Epsilon is much greater. In addition, the regression analyses show that the pKas of sulfonamides determined in ILs linearly correlate with these in molecular solvents of distinct properties, but with various slopes. A rationale and related discussion on the effect of solvation in ILs are presented.
To quantify the properties of protic ionic liquids (PILs) as acid-base reaction media, potentiometric titrations were carried out in a neat PIL, ethylammonium nitrate (EAN). A linear relationship was found between the 14 pKa values of 12 compounds in EAN and in water. In other words, the pKa value in EAN was found to be roughly one unit greater than that in water regardless of the charge and hydrophobicity of the compounds. It is possible that this could be explained by the stronger acidity of HNO3 in EAN than that of H3 O(+) in water and not by the difference in the solvation state of the ions. The pH value in EAN ranges from -1 to 9 on the pH scale based on the pH value in water.
In contrast to the great success of computational methodologies in molecular solvents, effective and accurate calculation of the fundamental bond energetic properties in ionic liquids (ILs) is essentially absent. This is largely due to the unusual complexity of handling solvation quantities of ILs and the lack of precisely determined bond parameters to serve as the authentic benchmark to calibrate the modeling methodology. Herein, we report the first accurate calculations of absolute pKa values in a commonly used IL, [Bmim][NTf2], with the carefully developed “ion-biased” cluster-continuum model. Experimental pKa values of benzoic acids and benzenethiols in [Bmim][NTf2] were reproduced with mean unsigned errors of only 0.3 and 0.5 pKa units, respectively, which enables theoretical approaches with a suitable strategy as a powerful tool to handle complicated problems in ILs and to eventually realize the rational design of the IL chemistry.
We report the gelation mechanism of tetra-armed prepolymer chains in typical aprotic ionic liquid (aIL), i.e., A-B type cross-end coupling reaction of Tetra-armed poly(ethylene glycol)s with amine- and activated ester-terminals (TetraPEG-NH2 and TetraPEG-NHS, respectively) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mIm][TFSA]). In the ion gel system, we focused on the pH (or H+ concentration) dependence of the gelation reaction. We thus applied the protic ionic liquid (pIL), 1-ethylimidazolium TFSA ([C2ImH][TFSA]) as a nonvolatile H+ source, and added it into the solvent aIL. It was found that the gelation time of TetraPEG ion gel can be successfully controlled from 1 min to 3 hours depending on the concentration of pIL (cpIL = 0 ~ 3 mM). This suggests that the acid-base properties of TetraPEG-NH2 showing acid-base equilibrium (-NH2 + H+ ⇆ -NH3+) in the solutions play a key role on the gelation process. The acid dissociation constants, pKas of TetraPEG-NH3+ and C2ImH+ (cation of pIL) in aIL were directly determined by potentiometric titration to be 16.4 and 13.7, respectively. This indicates that most of H+ ions bind to TetraPEG-NH2 and then C2ImH+ exists as neutral C2Im. The reaction efficiency of amide bond (cross-linked point) systematically decreased with increasing cpIL, which was reflected to the mechanical strength of the ion gels. From these results, we discuss the gelation mechanism of TetraPEG in aIL to point out the relationship between polymer network structure and [H+] in the solutions.
A high-performance CO2 separation membrane consisting of ion gel has been prepared. The ion gel, tetra- armed poly(ethylene glycol) (Tetra-PEG) ion gel contains a large fraction of ionic liquid (94 wt%) and shows excellent CO2 permselectivity over the high temperature range up to 100 °C. We also demonstrate that the ion gel can absorb CO2 without solvent-seeping at the high pressure of 3 MPa.
An SMD implicit-explicit approach was developed that allows reproduction of experimental pKa values of various carbon acids in ionic liquids with a mean unsigned error of 1.28 pK units, which is comparable in magnitude to those authentic pKa calculations in conventional molecular solvents. This model may be used for predicting more pKa values in RTILs and may also offer clues for studying other thermodynamic quantities.
We present for the first time Gutmann donor and acceptor numbers for a series of 36 different ionic liquids that include 26 distinct anions. The donor numbers were obtained by (23)Na NMR spectroscopy and show a strong dependence on the anionic component of the ionic liquid. The donor numbers measured vary from -12.3 kcal mol(-1) for the ionic liquid containing the weakest coordinative anion [emim][FAP] (1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate), which is a weaker donor than 1,2-dichloroethane, to 76.7 kcal mol(-1) found for the ionic liquid [emim][Br], which exhibits a coordinative strength in the range of tertiary amines. The acceptor numbers were measured by using (31)P NMR spectroscopy and also vary as a function of the anionic and cationic component of the ionic liquid. The data are presented and correlated with other solvent parameters like the Kamlet-Taft set of parameters, and compared to the donor numbers reported by other groups.
A comprehensive basicity study of alpha,omega-alkanediamines and related bases has been carried out. Basicities in acetonitrile (AN, pK(a) values), tetrahydrofuran (THF, pK(alpha) values), and gas phase (GP, GB values), were measured for 16, 14, and 9 diamine bases and for several related monoamines. In addition the gas-phase basicities and equilibrium geometries were computed for 19 diamino bases and several related monoamines at the DFT B3LYP 6-311+G** level. The effects of the different factors (intrinsic basicity of the amino groups, formation of intramolecular hydrogen bonds, and molecular strain) determining the diamine basicities were estimated by using the method of isodesmic reactions. The results are discussed in terms of molecular structure and solvation effects. The GP basicity is determined by the molecular size and polarizability, the extent of alkylation, and the energy effect of intramolecular hydrogen bond formation in the protonated base. The basicity trends in the solvents differ very much from those in GP: 1) The solvents severely compress the basicity range of the bases studied (3.5 times for the 1,3-propanediamine family in AN, and 7 times in THF), and 2) while stepwise alkylation of the basicity center leads to a steady basicity increase in the gas phase, the picture is complex in the solvents. Significant differences are also evident between THF and AN. The high hydrogen bond acceptor strength of THF leads to this solvent favoring the bases with "naked" protonation centers. In particular, the basicity order of N-methylated 1,3-propanediamines is practically inverse to that in the gas phase. The picture in AN is intermediate between that of GP and THF.
When the potential-volume curve obtained during potentiometric titrations shows only a small potential change at the end-point, it has been customary to plot a ΔE/ΔV-volume curve and to take the peak of this curve as the equivalence point. In 1950 the author proposed a method of transforming these curves by a numerical manipulation into straight lines intersecting at the equivalence point. In this paper another way of transforming titration curves into straight lines has now been developed. A simple theoretical treatment shows that the method can be applied to titrations involving acids and bases, ionic precipitations formation of complexes, and oxidation - reduction reactions. To facilitate the use of the method a table has been compiled giving quantities to be calculated and plotted against volume of titrant added. These quantities can be evaluated by simple slide rule calculations and, since straight line relationships hold, end-points can be obtained by simple extrapolation. The practice of the method is applicable to potentiometers calibrated either in millivolts or in pH units, even when titrations other than acid-alkali reactions are in use.
Ethylammonium nitrate (EAN) is a protic room-temperature ionic liquid and classified into an amphoteric solvent. The equilibrium constant KS = [EtNH2][HNO3] has directly been obtained for the first time by using an ion-selective field effect transistor (ISFET) electrode. The obtained KS value is of 10-10.0 mol2 dm -6, indicating that the neutral EAN involves HNO3 and EtNH2 molecules as much as 1.0 × 10-5 mol dm -3, or the pH (= -log[HNO3]) is 5 at 298 K.
We describe the behavior of the conductivity, viscosity, and vapor pressure of various binary liquid systems in which proton transfer occurs between neat Brönsted acids and bases to form salts with melting points below ambient. Such liquids form an important subgroup of the ionic liquid (IL) class of reaction media and electrolytes on which so much attention is currently being focused. Such "protic ionic liquids" exhibit a wide range of thermal stabilities. We find a simple relation between the limit set by boiling, when the total vapor pressure reaches one atm, and the difference in pK(a) value for the acid and base determined in dilute aqueous solutions. For DeltapK(a) values above 10, the boiling point elevation becomes so high (>300 degrees C) that preemptive decomposition prevents its measurement. The completeness of proton transfer in such cases is suggested by the molten salt-like values of the Walden product, which is used to distinguish good from poor ionic liquids. For the good ionic liquids, the hydrogen bonding of acid molecules to the proton-transfer anion is strong enough that boiling points, but not melting points, may maximize at the hydrogen-bonded dianion composition. High boiling liquids of this type constitute an interesting class of high-temperature protonic acid that may have high-temperature fuel cell applications.
The Brönsted acidities of several neutral NH-acids (substituted diphenylamine, substituted anilines and imides) were measured in the gas phase (pulsed FT ICR spectrometry), dimethyl sulfoxide and aqueous solution. Comparison of the Brönsted acidities of neutral NH-acids in the gas phase, dimethyl sulfoxide and water was also carried out. It was shown that substituent effects on the acidity of the studied compounds are significantly attenuated by the transfer of the reaction series of acidic dissociation of neutral acids from the gas phase into dimethyl sulfoxide and water. The strongest solvent-induced attenuation of the substituent effects is characteristic of the meta-substituted anilines whose sensitivity towards substituent effects decreases with transfer from the gas phase into DMSO by 2.83 times and with transfer into water by 4.13 times. At the same time, the reaction series of para and/or ortho-π-acceptor substituted anilines, amides, imides and substituted diphenylamines are less sensitive to a change in gas phase for DMSO or water.In the special case of para-acceptor substituted anilines it was demonstrated that the specific solvation induced an increase in the acidity of the para- and/or ortho-acceptor substituted anilines as compared with the behavior of the corresponding meta-substituted anilines by amounts up to 10 pKa units.
The Brønsted acidities of a number of neutral CH-acids (substituted toluenes, aryl- and diarylacetonitriles, fluorenes, ethyl esters of phenylcyanoacetic acids, substituted methanes, etc.) were measured in the gas phase (pulsed FT-ICR spectrometry) and in dimethyl sulfoxide (potentiometric titration). Comparison of the Brønsted acidities of the neutral CH-acids in the gas phase and in dimethyl sulfoxide (DMSO) was also carried out. It was shown that, as a rule, substituent effects on the acidity of the studied compounds are significantly attenuated by the transfer of the reaction series of acidic dissociation of neutral acids from the gas phase into DMSO. The weakest attenuation was monitored in the case of aromatic hydrocarbons, which are the conjugate acids of carbanions with very extensive charge delocalization (fluoradene, substituted fluorenes, aryl-substituted cyclopentadienes and indenes, toluene, diphenyl and triphenylmethanes, etc.). The strongest solvent-induced attenuation of the substituent effects is characteristic of meta-substituted phenylacetonitriles and phenylmalononitriles, whose sensitivity towards substituent effects decreases with transfer from the gas phase into DMSO by up to 2.8–3.3 times. At the same time, the reaction series of para and/or ortho-π-acceptor substituted phenylacetonitriles are less sensitive to a change from the gas phase to DMSO.
Proton-donating and ionization properties of several protic ionic liquids (PILs) made from N-methylimidazole (Mim) and a series of acids (HA) have been assessed by means of potentiometric and calorimetric titrations. With regard to strong acids, bis(trifluoromethanesulfonyl) amide (Tf2NH) and trifluoromethanesulfonic acid (TfOH), it was elucidated that the two equimolar mixtures with Mim almost consist of ionic species, HMim+ and A-, and the proton transfer equilibrium corresponding to autoprotolysis in ordinary molecular liquids was established. The respective autoprotolysis constants were successfully evaluated, which indicate the proton-donating abilities of TfOH and Tf2NH in the respective PILs are similar. In the case of trifluoroacetic acid, the proton-donating ability of CF3COOH is much weaker than those of TfOH and Tf2NH, while ions are predominant species. On the other hand, with regard to formic acid and acetic acid, protons of these acids are suggested not to transferred to Mim sufficiently. From calorimetric titrations, about a half of Mim is estimated to be proton-attached at most in the CH3COOH-Mim equimolar mixture. In such mixture, hydrogen-bonding adducts formation has been suggested. The autoprotolysis constants of the present PILs show a good linear correlation with dissociation constants of the constituent acids in an aqueous phase.
The difficulties in estimating uncertainty of pKa values determined in nonaqueous media are reviewed and two different uncertainty estimation approaches are presented and applied to the pKa values of the compounds on a previously established self-consistent spectrophotometric basicity scale in acetonitrile. One approach is based on the ISO GUM methodology (the “ISO GUM” approach) and involves careful analysis of the uncertainty sources and quantifying the respective uncertainty components. The second approach is based on the standard-deviation-like statistical parameter that has been used for characterization of the consistency of the scale (the “statistical” approach). It is demonstrated that the ISO GUM approach somewhat overestimates the uncertainty. The statistical approach is based on long-term within-laboratory statistical data and it is demonstrated that it underestimates the uncertainty. In particular it neglects the laboratory bias effects that are taken into account at least to some extent by the ISO GUM approach. Thus, together these two approaches allow to “bracket” the uncertainties of the pKa values on the scale. The uncertainties of the pKa values are defined in two different ways. Definition (a) includes the uncertainty of the pKa of the reference base (anchor base of the scale) pyridine. Definition (b) excludes it. It is demonstrated that both definitions have their virtues. Definition (a) leads to the uncertainty ranges of 0.12–0.22 and 0.12–0.14 pKa units at standard uncertainty level for different bases using the ISO GUM and statistical approach, respectively. Definition (b) leads to the uncertainty ranges of 0.04–0.19 and 0.02–0.08 pKa units, respectively. The uncertainty of the pKa of a given base is dependent on the quality of the measurements involved and on the distance from the reference base on the scale. The importance of the correlation between the pKa values of bases belonging to the same scale is stressed.
The dissociation constants of the conjugate acids of 31 amines of various structural types were measured in anhydrous acetonitrile as solvent by means of a glass electrode shown to respond reversibly to hydrogen ion activity in this solvent. The electrode was calibrated in a series of buffers containing picric acid and tetraethylammonium picrate, using the dissociation constant found for picric acid in a careful study by Kolthoff and Chantooni. All measurements were carried out at a constant and sufficiently low ionic strength to allow extrapolation of equilibrium constants to their limiting values. No simple, quantitative correlation was found between base strengths in acetonitrile and water. Although the pKa values of the majority of the ammonium ions studied are from 7.2 to 7.9 units greater in acetonitrile than in water, several ions differ significantly from this "norm." Aromatic ions are considerably stronger acids in acetonitrile than would be predicted from the strengths of aliphatic ions. In the series of monoprotonated diamines, H2N(CH2)nNH3+, the acids with n = 3 and 4 derive considerable stability from intramolecular hydrogen bonding. Although acetonitrile is much more inert than water, its salvation of ammonium ions still exerts an important influence. Steric inhibition of salvation appears to be more important in acetonitrile than in water. A correlation exists between the base.
Two electrochemical techniques have been used to measure the pK(a) of N-bases in several ionic liquids (ILs). The first method corresponds to a potentiometric titration of a strong acid with the N-base using a platinized Pt indicator electrode immersed in the IL solution and maintained under dihydrogen atmosphere via gas bubbling. The second approach involves performing cyclic voltammetry at a platinized Pt electrode in a solution containing both strong acid and the conjugate weak acid of the N-base. Values of pK(a) obtained by one or the other approach are in good agreement with each other. The experimental data clearly demonstrated that acid/base chemistry in ILs is similar to that observed in molecular nonaqueous solvents; i.e., the relative strengths of the bases were in the right order and spaced (ΔpK(a)). It was also observed that the strength of N-bases is highly dependent on the anion of the ionic liquid; this observation indicates that pH-dependent reactions could be controlled by the appropriate choice of anion for bulk ILs or as an added co-ion to bulk IL.
Proton transfer in protic ionic liquids is poorly understood. Some acid/base proton transfer reactions do not proceed to the extent that is expected from Delta pK(a)(aq) data from aqueous solutions, yet some do. In this work we have investigated protic ionic liquids obtained by proton transfer from a common acid, acetic acid, to a range of amine bases of similar pK(a)(aq) values. Probe indicator observations, transport property data allowing the construction of Walden plots and computational studies all suggest that there is a clear distinction between the behaviour of simple primary vs. tertiary amines, the proton transfer being more complete in the former case than the latter. The origins of this seem to be related to the hydrogen bonding ability of the ammonium ions in providing a good solvating environment for the ions produced by the proton transfer.
pH-ISFETs were used in the study of acid-base equilibria in gamma-butyrolactone (GBL). After the spectrophotometric determination of the pK(a) value of 3,5-dichloropicric acid, the pK(a) values and homo-conjugation constants of various acids (including the conjugate acids of bases) were determined potentiometrically using a Ta(2)O(5)-type pH-ISFET. The values of pK(a) in GBL were in a linear relation with those in propylene carbonate (PC) and 1.0 units smaller on average. The difference in pK(a) between GBL and PC was mainly attributable to the difference in proton solvation. The autoprotolysis constant of GBL, roughly estimated by a rapid titration with a Si(3)N(4)-ISFET, was about 30 on the pK(SH) scale. A comparative study was made of the response speeds of the Ta(2)O(5)- and Si(3)N(4)-type pH-ISFETs and a conventional pH-glass electrode. The result was Si(3)N(4)-ISFET > Ta(2)O(5)-ISFET > glass electrode. Because GBL is not stable against acids and bases, the use of pH-ISFETs was much more convenient than the use of the conventional glass electrode.
Solvation structures of manganese(II), cobalt(II), nickel(II) and zinc(II) ions in 1-ethyl-3-methylimidazolium bis(trifluoro-methanesulfonyl) amide (EMI(+)TFSA(-)) have been studied by UV-Vis, FT-IR and FT-Raman spectra. The ionic liquid involves TFSA(-) conformers with C(1) (cis) and C(2) (trans) symmetries, and both conformers coexist in equilibrium in the liquid state. The results showed that these metal(II) ions are all six-coordinated with three TFSA(-) ions, i.e., TFSA(-) ligates as a bidentate O-donor in the ionic liquid. Although the metal ion strongly prefers the C(1) conformer in crystals, the metal ion coordinates both the C(1) and C(2) conformers in the liquid state, and the conformational equilibrium in the bulk only slightly shifts to the C(1) conformer in the coordination sphere. We concluded that the conformational equilibrium in the coordination sphere is strongly temperature-sensitive.
Ethylammonium nitrate (EAN) is composed of C(2)H(5)NH(3)(+) and NO(3)(-) ions, which behave as an acid and a base, respectively. The ionic liquid thus involves small amounts of C(2)H(5)NH(2) and HNO(3) molecules owing to proton transfer from C(2)H(5)NH(3)(+) to NO(3)(-). The equilibrium constant K(s) (= [C(2)H(5)NH(2)][HNO(3)]), which corresponds to the autoprotolysis constant of water, was obtained to be ca. 10(-10) mol(2) dm(-6) by potentiometry using an ion-selective field-effect transistor and hydrogen electrodes at 298 K. The value indicates that C(2)H(5)NH(2) and HNO(3) molecules of ca. 10(-5) mol dm(-3) are involved in neat EAN. On the other hand, in an EAN-water mixture, a water molecule behaves as a base. The apparent pK(s) value was determined in EAN-water mixtures of various solvent compositions. Interestingly, the pK(s) value is remained at 10.5 in mixtures over the range of an EAN mole fraction of 0.05-0.9. The value is close to the pK(a) of C(2)H(5)NH(2), or the acid-dissociation constant of C(2)H(5)NH(3)(+), in aqueous solution. This implies that the reaction C(2)H(5)NH(3)(+) + H(2)O --> C(2)H(5)NH(2) + H(3)O(+) is responsible for the pK(s) over a wide range of solvent composition. The pK(s) value in neat EAN is thus slightly smaller than that in the mixtures, implying that H(3)O(+) is a stronger acid than HNO(3) in an EAN solution, unlike water.
The acidity scale of different Brønsted acids in ionic liquids such as [BMIM][NTf2], [BMIM][BF4], and [BMMIM][BF4] has been investigated by determination of Hammett functions, using a spectrophotometric indicator method. This scale should permit one to correlate the acidity strength of ionic liquid systems with their ability to achieve acid-catalyzed reactions.
Ionic liquids which are (weak) Lewis bases have a number of interesting and useful properties different to those of traditional ionic liquids, including volatility and the possibility of being distillable in some cases, a base catalysis effect in others and enhancement of the acidity of dissolved acids.
- W Yoshizawa
- C A Xu
- Angell
Yoshizawa, W. Xu, C. A. Angell, J. Am. Chem. Soc. 2003,
125, 15411.
- D R Macfarlane
- J M Pringle
- K M Johansson
- S A Forsyth
- M Forsyth
D. R. MacFarlane, J. M. Pringle, K. M. Johansson, S. A. Forsyth,
M. Forsyth, Chem. Commun. 2006, 1905.
- J Koppel
- P.-C Koppel
- J.-F Maria
- R Gal
- V M Notario
- R W Vlasov
- Taft
Koppel, J. Koppel, P.-C. Maria, J.-F. Gal, R. Notario, V. M.
Vlasov, R. W. Taft, Int. J. Mass Spectrom. Ion Processes 1998,
175, 61.
- I Sooväli
- A Kaljurand
- I Kütt
- Leito
Sooväli, I. Kaljurand, A. Kütt, I. Leito, Anal. Chim. Acta
2006, 566, 290.
- M Schmeisser
- P Illner
- R Puchta
- A Zahl
- R Van Eldik
M. Schmeisser, P. Illner, R. Puchta, A. Zahl, R. van Eldik,
Chem.®Eur. J. 2012, 18, 10969.
- H Kanzaki
- X Doi
- S Song
- S Hara
- Y Ishiguro
- Umebayashi
Kanzaki, H. Doi, X. Song, S. Hara, S.-i. Ishiguro, Y.
Umebayashi, J. Phys. Chem. B 2012, 116, 14146.
Tables of Rate and Equilibrium Constants of Heterolytic Organic Reactions
- T Fujii
- Y Nonaka
- Y Akimoto
- S Umebayashi
- Ishiguro
Fujii, T. Nonaka, Y. Akimoto, Y. Umebayashi, S.-i. Ishiguro,
Anal. Sci. 2008, 24, 1377.
13 K. Izutsu, M. Ohmaki, Talanta 1996, 43, 643.
14 G. Gran, Analyst 1952, 77, 661.
15 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html.
16 Tables of Rate and Equilibrium Constants of Heterolytic Organic
Reactions, ed. by V. Palm, VINITI, Moscow-Tartu, 19751985.
- R Barhdadi
- M Troupel
- C Comminges
- M Laurent
- A Doherty
R. Barhdadi, M. Troupel, C. Comminges, M. Laurent, A.
Doherty, J. Phys. Chem. B 2012, 116, 277.
- R Kanzaki
- K Uchida
- S Hara
- Y Umebayashi
- S Ishiguro
- S Nomura
R. Kanzaki, K. Uchida, S. Hara, Y. Umebayashi, S.-i. Ishiguro,
S. Nomura, Chem. Lett. 2007, 36, 684.
- E I Stoimenovski
- D R Izgorodina
- Macfarlane
Stoimenovski, E. I. Izgorodina, D. R. MacFarlane, Phys.
Chem. Chem. Phys. 2010, 12, 10341.
- H Thomazeau
- L Olivier-Bourbigou
- S Magna
- B Luts
- Gilbert
Thomazeau, H. Olivier-Bourbigou, L. Magna, S. Luts, B.
Gilbert, J. Am. Chem. Soc. 2003, 125, 5264.
- E.-I Rõõm
- A Kütt
- I Kaljurand
- I Koppel
- I Leito
- I A Koppel
- M Mishima
- K Goto
- Y Miyahara
E.-I. Rõõm, A. Kütt, I. Kaljurand, I. Koppel, I. Leito, I. A.
Koppel, M. Mishima, K. Goto, Y. Miyahara, Chem.®Eur. J.
2007, 13, 7631.
- I A Koppel
- J Koppel
- V Pihl
- I Leito
- M Mishima
- V M Vlasov
- L M Yagupolskii
- R W Taft
I. A. Koppel, J. Koppel, V. Pihl, I. Leito, M. Mishima, V. M.
Vlasov, L. M. Yagupolskii, R. W. Taft, J. Chem. Soc., Perkin
Trans. 2 2000, 1125.
- J Stoimenovski
- E I Izgorodina
- D R Macfarlane
J. Stoimenovski, E. I. Izgorodina, D. R. MacFarlane, Phys.
Chem. Chem. Phys. 2010, 12, 10341.
- R Kanzaki
- K Uchida
- X Song
- Y Umebayashi
- S -I
- Ishiguro
R. Kanzaki, K. Uchida, X. Song, Y. Umebayashi, S.-i. Ishiguro,
Anal. Sci. 2008, 24, 1347.
- I Koppel
- J Koppel
- P.-C Maria
- J.-F Gal
- R Notario
- V M Vlasov
- R W Taft
I. Koppel, J. Koppel, P.-C. Maria, J.-F. Gal, R. Notario, V. M.
Vlasov, R. W. Taft, Int. J. Mass Spectrom. Ion Processes 1998,
175, 61.
- K Fujii
- T Nonaka
- Y Akimoto
- Y Umebayashi
- S -I
- Ishiguro
K. Fujii, T. Nonaka, Y. Akimoto, Y. Umebayashi, S.-i. Ishiguro,
Anal. Sci. 2008, 24, 1377.