Zenon Pawlak

Solute-solvent interactions in acid-base dissociation: nine protonated nitrogen bases in water-DMSO solvents

A titration method utilizing glass electrodes and silver-silver chloride electrodes in a cell without liquid junction has been used to determine the acidic dissociation constants at 15, 25, and 35°C of nine protonated nitrogen bases in mixtures of water and dimethyl sulfoxide (DMSO). The mole fraction of DMSO in the mixed solvents was 0.2, 0.4, 0.6, and 0.8. The cell was calibrated with HCl (molality=0.01 mole-kg−1) in the mixed solvents, and the ionic strength and chloride molality remained substantially unchanged during the titration with added base. This method minimizes the errors resulting from the formation of AgCl2− in the media rich in DMSO. The pKa of all the protonated bases passes through a minimum at a solvent composition close to that at which H2O-DMSO mixtures display a maximum solvent structure. The results are discussed in terms of the preferential solvation of ions by the two types of solvent molecules. They are consistent with the hypothesis that increased solvent structure is accompanied by increased desolvation of the cation acids.

Calorimetric studies of hydrogen-bond formation in propylene carbonate II. Some cationic complexes at 298.15 K

Abstract The enthalpies of formation of some (BHB) + and (BHB′) + complexes in propylene carbonate at 298.15 K have been determined by calorimetry. A solution of the perchlorate or picrate of a protonated base in propylene carbonate was titrated enthalpimetrically with a solution of the same or another uncharged base. The bases studied were pyridine, pyridine-1-oxide, n -butylamine, dibenzylamine, and piperidine. In one experiment, the protonated form of tris(hydroxymethyl)aminomethane was titrated with n -butylamine. An attempt was made to rationalize the observed enthalpies of reaction on the basis of hydrogen bonding between the reactants in mole ratio 1. The acidities of the donors and the protonated forms of the acceptors. steric hindrance, and other structural factors have been taken into account in assessing the nature and strength of the hydrogen bonds in the individual cases.

Solvent effects on acid-base behavior: Five uncharged acids in water-sulfolane solvents

An electrometric titration method utilizing glass electrodes and silver-silver bromide electrodes in a cell without liquid junction has been applied to a determination of the dissociation constants of five uncharged weak acids (HA) in four mixtures of water and sulfolane (tetramethylene sulfone) at 25°C. The acids studied were monochloroacetic, formic, benzoic, and acetic acids andp-nitrophenol, and the mole fractions (x2) of sulfolane in the mixed solvents were 0.2, 0.4, 0.6, and 0.8. All measurements were made at a constant ionic strength of 0.02 mole-kg−1 in a solution containing NaBr at a molality of 0.01 mole-kg−1. The cell was standardized by measurements of HClO4 at molalities from 0.002 to 0.01 mole-kg−1 and, in subsequent measurements, a solution of NaA was titrated with HClO4. To obtain the thermodynamic pK, an activity correction derived from the Debye-Hückel theory was applied. The pK of all five acids was found to vary linearly withx2 in the range 0 to 0.8. By comparison of data for acetic acid in water-methanol, water-dioxane, and water-sulfolane solvents, it was shown that the results are consistent with the known properties of sulfolane (low acidity, basicity, and solvating power), but the linear variation of pK with solvent composition remains unexplained.

Calorimetric studies of hydrogen-bond formation in propylene carbonate I. Some hydride-bis-carboxylates and hydride-bis-phenolates at 298.15 K☆

Abstract The changes of enthalpy for the formation of hydrogen-bonded complexes of the types AHA − or AHR − by the interaction of acids HR or HA with bases A − have been determined at 298.15 K in propylene carbonate as solvent by a calorimetric procedure. Aliphatic carboxylates, benzoates, and phenolates were titrated enthalpimetrically with a series of acids to form homocomplexes AHA − and heterocomplexes AHR − . It is useful to regard the formation process as an acid-base interaction. The hydrogen-bond energy of homocomplexes decreases nearly linearly with increase in the p K of the proton donor. Two types of heterocomplex were identified. When A − is a weaker base than R − , complexation occurs without proton transfer, as confirmed by the low values of − ΔH for the reaction. On the other hand, the enthalpies of reaction are large if A − is a stronger base than R − , as both the energy of proton transfer and the energy of formation of a hydrogen bond are included in the measurement.

Solute-solvent interactions in acid-base dissociation: Seven protonated nitrogen bases in water-N-methyl-2-pyrrolidinone solvents

Acidic dissociation constants of seven protonated nitrogen bases, namely “Bis-tris”, 2-aminopyridine, “Tris”, morpholine, 2-amino-2-methyl-1,3-propanediol,n-butylamine, andt-butylamine, have been determined at 15, 25, and 35° C in mixtures of water andN-methyl-2-pyrrolidinone (NM2Py). The mole fractions of cosolvent were 0.2, 0.4, 0.6, and 0.8. Glass and silver-silver chloride electrodes were used in a cell without liquid junction, and both the ionic strength and molality of chloride ion remained substantially constant in all of the measurements. The pKaof all the protonated bases decreases initially and passes through a minimum as the aqueous solvent is enriched with NM2Py. The order of acidic strengths is the same as in the H2O-dimethyl sulfoxide (DMSO) solvent system, but the pKavalues are all lower by up to 2 units. NM2Py appears to be a more basic solvent than DMSO or H2O.

Conductance of tetraalkylammonium salts in propylene carbonate at 25C

Conductance measurements of 12 quaternary ammonium salts in propylene carbonate (PC) have been made at 25°C. The cations were either tetramethylammonium, tetraethylammonium, or tetra-n-butylammonium, and the concentrations of salt varied from about 2×10−4 to 5×10−3M. The data were analyzed by the equation of Pitts. The results showed that the benzoate, nitrobenzoate, and pentachlorophenolate salts are completely dissociated. The nitrophenolate, chlorophenolate, methylsulfonate, and nitrate salts are only slightly associated (KA from 2.5 to 6.5), while the acetate, phenylacetate, and nitrophenolate salts display somewhat more extensive association, with ionpair association constants from 17 to 45. Limiting molar conductances for the anions were derived. The factors affecting ionic mobilities in this dipolar aprotic solvent are discussed.

Thermometric titration of some amines in water—acetone mixtures

Abstract The enthalpies of protonation of some N-bases in H 2 O—acetone, 0, 0.25, 0.5, 0.75 and 0.95 mole fraction at 298 K have been determined by calorimetry. Solutions of amines in each solvent system were titrated enthalpimetrically with HClO 4 solution. The bases studied were piperidine, n -butylamine, 2-amino-2-methyl-1,3-propanediol, (AMP), tris(hydroxymethyl) aminomethane, tris, 2,2-bis-(hydroxymethyl-2,2′,2″-nitrilotriethanol, (bis-tris) and pyridine. The results are discussed in terms of the preferential solvation of ions by two types of solvent molecules. The enthalpy of protonation of all the bases passes through a minimum at a solvent composition of 0.75 mole fraction of acetone.

Calorimetric studies of hydrogen-bond formation in propylene carbonate III. Conjugation involving nitrate and halide ions at 298.15 K

Abstract An enthalpimetric study of the energetics of hydrogen-bond formation between the ions NO 3 − , Cl − , Br − , and I − and the proton donors HCl, CHCl 2 COOH, CCl 3 COOH, 3,5-dinitrobenzoic acid, pentachlorophenol, and picric acid in propylene carbonate ( pc ) is reported. The results for the conjugation of HCl, the chloroacetic acids, 3,5-dinitrobenzoic acid, and pentachlorophenol with the halide ions are in general agreement with the relative acidities of the proton donors and the sizes of the anions. The unusually large enthalpy effect found for the reaction of NO 3 − with HCl, however, might indicate a special role for the three oxygens of this anion. No energy is evolved when picric acid reacts with any of the three halide ions in pc . Furthermore, addition of small amounts of water reduces the amount of energy evolved when Cl − reacts with CCl 3 COOH in pc . The interpretation of these observations in terms of solvation enthalpies and selective solvation of reactants and products is discussed.

Conductance of HCl in water-sulfolane solvents at 25, 30, and 40C; a comparison of conductance equations

The molar conductance of solutions of HCl (concentrations from 3×10−4 to 0.033 mol-l−1) in water-sulfolane (tetramethylenesulfone) mixtures of mole fractions (X2) of sulfolane of 0.25, 0.50, 0.75, and 0.85 at 25, 30, and 40°C have been determined. The dielectric constants of the solvents varied from 45 to 60 at 25°C. The data were analyzed by the full Pitts equation, the expanded Pitts equation, and the expanded Fuoss-Hsia equations, all of which give comparable results for the limiting molar conductanceAo and for the ion-pair association constant KA for HCl. These equations were unsuccessful for the analysis of supplemental data in pure sulfolane. At 25°C, the pK for dissociation of HCl varies from 0.4 (X2=0.25) to 2.9 (X2=0.85). The extent of ion pairing is apparently strongly influenced by selective ion-solvent interactions.

The conductance of 1:1 electrolytes in sulfolane in the presence of proton donors at 30°C

Abstract The molar conductances of LiCl, LiNO 3 , KCHCl 2 COO and Na p -(NO 2 )C 6 H 4 O in sulfolane, in the presence of HCl, CH 3 COOH, CHCl 2 COOH and p -(NO) 2 C 6 H 4 OH have been determined. The data were analysed by the full Pitts equation and interpreted in terms of the ionic association constant ( K A ) of the salts K + A − and K + RHA − , and the heteroconjugation constant ( K f ) of RHA − . Values of K f have been calculated from the ratio of association constant K A (K + A − )/ K A (K + RHA − ).