Effect of Temperature on the Solubility of Short-Chain Carboxylic Acids in Water

The aqueous solubilities of pentanoic and hexanoic acids were measured between 298.15 and 333.15 K at intervals of 5.00 K. Nonlinear dependences were found for the variation of solubility with temperature. The temperature dependences of the solubility of the acids in water are described by nonlinear van’t Hoff plots. Experimental data were fitted by the method of least-squares to a second-order polynomial equation, which was then used to determine the differential solution enthalpies at selected temperatures. The values for the apparent Gibbs energy and the apparent entropy of solution are calculated.     

The solubilities of benzene polycarboxylic acids in water

The solubilities in water of all benzene polycarboxylic acids are discussed, using data determined in this work (benzoic, terephthalic, trimellitic, trimesic, and pyromellitic acids) and available from the literature (benzoic, phthalic, isophthalic, terephthalic, hemimellitic, trimelitic, trimesic, mellophanic, prehnitic, pyromellitic, benzene-pentacarboxylic and mellitic acids). The apparent molar enthalpies of solution at the saturation point for these benzene polycarboxylic acids were determined from the temperature dependence of the solubilities.     

Photoacid generators (PAGs) based on N-acyl-N-phenylhydroxylamines for carboxylic and sulfonic acids

Simple and efficient photoacid generators (PAGs) for carboxylic and sulfonic acids based on N-acyl-N-phenylhydroxylamines have been demonstrated. Irradiation of o-carboxylates and thermally rearranged o-arenesulfonates of N-acyl-N-phenylhydroxylamines using UV light (≥254 nm) in aqueous methanolic solution resulted in efficient generation of carboxylic and sulfonic acids, respectively. The carboxylic acid generation ability of N-acyl-N-phenylhydroxylamines was found to be dependent on their N-acyl substituents. Further, polymer bearing o-arenesulfonates of N-acyl-N-phenylhydroxylamine was synthesized and demonstrated as PAG for sulfonic acids.     

Enthalpy of crystallization of lithium chloride from aqueous solution at 25°C

The enthalpies of crystallization of LiCl and LiCl·H₂O from aqueous solutions at 25°C are reported as measured by a calorimetric method and derived from the previously published concentration dependence of the enthalpy of solution data. The results are compared with those obtained from the concentration dependence of activity coefficients and from the temperature dependence of solubilities.     

Predicting the enthalpies of melting and vaporization for pure components

A mathematical model of the melting and vaporization enthalpies of organic components based on the theory of thermodynamic similarity is proposed. In this empirical model, the phase transition enthalpy for the homological series of n-alkanes, carboxylic acids, n-alcohols, glycols, and glycol ethers is presented as a function of the molecular mass, the number of carbon atoms in a molecule, and the normal transition temperature. The model also uses a critical or triple point temperature. It is shown that the results from predicting the melting and vaporization enthalpies enable the calculation of binary phase diagrams.     

The thermodynamic characteristics of solution of L-α-histidine and L-α-phenylalanine in water at 273–373 K

The solubility of L-phenylalanine and L-histidine in water at 298.15 and 318.15 K and the heat effects of solution of the amino acids at 328.15 K were determined. These results and the data obtained earlier were used to calculate all the standard thermodynamic functions of solution of the amino acids and the solubilities of L-phenylalanine and L-histidine over the temperature range 273–373 K. The selection of the form of the Δ_sol H ^o = f(T) dependence had a negligible effect on the free energies of solution and solubilities of the amino acids. This selection primarily influenced the entropy and heat capacity characteristics of the process.     

Calculations of pKa values of carboxylic acids in aqueous solution using density functional theory

With the specific aim of calculating the acidity equilibrium constant (K_a) of carboxylic acids in aqueous solution we investigated the solute-solvent interactions of these acids and their corresponding anions. The pK_a (−lg K_a) values have been calculated using density functional theory (DFT). The polarized continuum model (PCM) is used to describe the solvent. Using these methods, we successfully predicted the pK_as of 66 carboxylic acids in aqueous with the average error of 0.5 in pK_a units. Two different thermodynamic cycles have been studied. The theoretical values are in better agreement with the experimental results for those acids with moderate strength of acidity with the pK_a value higher than 3.     

Isotope effect on the solubility of amino acids in water

The solubilities of glycine, alanine, phenylalanine, and proline in H₂O and D₂O from 283 K to 335 K were determined. It was found that glycine and alanine are less soluble in heavy water than in light water but proline is more soluble in heavy water over the whole temperature range studied. Phenylalanine is more soluble in H₂O than D₂O below 310 K but above that temperature heavy water becomes a better solvent. An influence of H/D isotope substitution on the enthalpies of solution is also observed. In the case of glycine and alanine enthalpies of solution in heavy water increase by a small amount and in the same time the solution enthalpy for phenylalanine in D₂O increases markedly. No change in the solution enthalpy for proline was observed. The isotope effects on solubility and the solution enthalpy are qualitatively discussed.     

Synthesis of carboxylic acids based on the closo-decaborate anion

A series of new boron-containing carboxylic acids was prepared by the ring-opening reaction of cyclic oxonium derivatives of the closo-decaborate anion [B₁₀H₁₀]²⁻ with methyl esters of hydroxybenzoic acids or the cyanide anion followed by hydrolysis of the obtained nitrile and esters. Acid hydrolysis of the esters results in protonation of the oxygen atom connected to the boron cage, with the formation of the corresponding O-protonated acids, isolated in the solid state. The compounds synthesized can be used in radionuclide diagnostics and boron neutron capture therapy of cancer.     

High performance liquid chromatography of aromatic carboxylic acids on p-tert-butyl-calix[8]arene-bonded silica gel stationary phase

A new p-tert-butyl-calix[8]arene-bonded silica gel stationary phase (CABS) was used for the separation of some aromatic carboxylic acids by HPLC. The chromatographic behaviour of the solutes on CABS was studied in comparison with conventional ODS. The results show that the calix[8]arene-bonded phase exhibits a stronger retention and better selectivity than ODS for the aromatic carboxylic acids. The different elution order of the analytes was also observed on both packings, which show the existence of distinct retention mechanisms. According to chromatographic data, it can be concluded that the high selectivity of CABS for aromatic carboxylic acids ascribes to various interactions between CABS and the analytes, such as hydrophobic interaction, hydrogen bonding interaction, π-π interaction and inclusion interaction.     

Binary and ternary phenylboronic acid complexes with saccharides and Lewis bases

Cumulative formation constants for the association of three boronic acids (phenylboronic acid and its ortho-anilinomethyl and ortho-benzylaminomethyl derivatives) with four saccharides (fructose, glucose, mannitol, and sorbitol) were determined by potentiometric titration. Similarly, the constants for the formation binary complexes of the three boronic acids with (hydrogen) phosphate, (hydrogen) citrate, or imidazole were determined. Finally, the formation of ternary complexes of the boronic acids, phosphate, citrate or imidazole, and the saccharides were determined based on the determined values of the binary complexes. The previously unrecognized ternary complexes are significant in all systems investigated, and under some solution compositions, they can be the dominant species in solution over a wide pH range. A value of 15-25 kJ mol⁻¹ was determined for the energy of the B-N interaction in the benzylmethyl derivative based on the relative stabilities of the ternary phosphate complexes of the three boronic acids. The data are used to rationalize the medium dependence of stepwise formation constants and the apparent acidity constants of previous literature reports.     

Efficient enantioselective synthesis of orthogonally protected (R)-α-alkylserines compatible with the solid phase peptide synthesis

The Schöllkopf methodology for the asymmetric synthesis of α-amino acids, which was previously not applicable to the construction of quaternary α-amino acids, has been rendered not only suitable but also practical for this purpose and applied to a highly efficient enantioselective synthesis of orthogonally protected (R)-α-alkylserines suitable for the solid phase synthesis.     

The diffusion and solubility of Irganox 1098 in polyamide 6

Diffusion coefficients (D) and solubilities (S) of a phenolic antioxidant (Irganox^® 1098) in polyamide 6 were determined in the temperature range of 139-180 °C. D and S show Arrhenius and van’t Hoff dependences on temperature respectively, with discontinuities at the melting temperature of the antioxidant. The activation energies for diffusion and enthalpies of solution are lower above than below the melting temperature. Based on the determined parameters for diffusion and solubility, the blooming of Irganox^® 1098 as a function of temperature and time was calculated.     

Enthalpies of solution and of crystallization of lithium nitrate and of lithium nitrate trihydrate in water at 25°C

The enthalpies of crystallization of LiNO₃ and LiNO₃–3H₂O from aqueous solutions at 25°C, measured by a calorimetric method and determined from the previously published data on the concentration dependence of the enthalpy of solution, are reported. The results are compared with the values obtained from the concentration dependences of the activity coefficients and from the temperature dependences of the solubilities. The enthalpy of solution at infinite dilution and the enthalpy of hydration are given.     

Evaluation of different electrolyte systems and on-line preconcentrations for the analysis of haloacetic acids by capillary zone electrophoresis

The separation of various carboxylic acids was performed on a polymethacrylate-based weakly acidic cation-exchange resin (TSKgel OApak-A) using ion-exclusion chromatography under the acidic elution conditions. When a diluted sulfuric acid solution was used as the eluent, highly sensitive conductimetric detection of carboxylic acids was achieved without increasing the background conductance of the eluent. This method was more sensitive than using benzoic acid eluent and enabled a good resolution of dicarboxylic as well as monocarboxylic acids. The addition of 5–20% methanol to the eluent considerably reduced the retention times of carboxylic acids with hydrophobic nature.     

Thermodynamic properties of caffeine: Reconciliation of available experimental data

Molar enthalpies of sublimation of two crystal forms of caffeine were obtained from the temperature dependence of the vapour pressure measured by the transpiration method. A large number of primary experimental results on the temperature dependences of vapour pressure and phase transitions have been collected from the literature and have been treated in a uniform manner in order to derive sublimation enthalpies of caffeine at T = 298.15 K. This collection together with the new experimental results reported here has helped to resolve contradictions in the available sublimation enthalpies data and to recommend a consistent and reliable set of sublimation and formation enthalpies for both crystal forms under study. Ab initio calculations of the gaseous molar enthalpy of formation of caffeine have been performed using the G3MP2 method and the results are in excellent agreement with the selected experimental data.     

Ion-pair chromatography on a porous graphitic carbon stationary phase for the analysis of twenty underivatized protein amino acids

The analysis of twenty underivatized protein amino acids has been achieved on porous graphitic carbon packing material (Hypercarb). Five perfluoroalkyl carboxylic acids (trifluoroacetic, heptafluorobutyric, nonafluoropentanoic, tridecafluoroheptanoic and pentadecafluorooctanoic acid) have been studied as ion-pairing reagent. Several parameters (equilibration time, quantities adsorbed onto the chromatographic support, concentration and nature of the ion-pairing reagent, as well as temperature effect) have been studied leading to the complete separation of these compounds in gradient elution mode. Evaporative light scattering detector has allowed the detection of these non UV–visible absorbent molecules. The chromatographic methodology developed can also be easily coupled with pneumatically assisted electrospray mass spectrometry.     

Capillary electrophoresis-electrospray ionization mass spectrometry using uncoated fused-silica capillaries and alkaline buffer solution for the analysis of small carboxylic acids

A simple and cost-effective capillary electrophoresis/mass spectrometric (CE/MS) method for the analysis of small carboxylic acids including succinate, malate, tartarate, maleinate and citrate, is described. All CE/MS experiments were performed with uncoated fused-silica capillaries and with alkaline volatile buffer solution (ammonium formate buffer, pH 10). Since sheath liquids have significant effects on the sensitivity in typical CE/MS applications, the effects of type and flow rate of the sheath liquids on the sensitivity of carboxylic acids were investigated. As the result, the best sensitivity was obtained with the alkaline sheath liquid (5 mM ammonium hydroxide in water/methanol (50/50, v/v) solution) at 6 μl min⁻¹. With the alkaline volatile buffer solution, sufficient electroosmotic flow (EOF) to carry all small carboxylic acids toward the cathode (MS side) was obtained, although all analytes had different electrophoretic mobilities toward the anode (the CE inlet). Taking advantage of the relatively higher EOF velocity, several carboxylic acids could be detected by MS in ESI-negative mode with a short analysis time. The R.S.D. values (n=5) for the migration time and the peak area of the carboxylic acids tested were less than 0.6 and 4.2%, respectively. The method was applied to the CE/MS analysis of carboxylic acids in apple juice to demonstrate the applicability to real samples.     

Solid phase extraction purification of carboxylic acid products from 96-well format solution phase synthesis with DOWEX 1x8-400 formate anion exchange resin

The anion exchange resin DOWEX 1x8-400 formate has been developed for the isolation or resin capture of carboxylic acids from solution phase reactions in a 96-well format using a batchwise solid phase extraction technique. Eleven different anion exchange resins (formate forms) were evaluated for their efficiency at scavenging aryl and aliphatic carboxylic acids from solution. The model carboxylic acids had pK(a)s ranging from 3.40 to 4.89. Exchange efficiency onto the resin was pK(a) dependent with the carboxylic acids but not with their diisopropylethylammonium salts. Exchange off of the resin also showed pK(a) dependence with the stronger acids requiring more concentrated solvent acid for exchange. DOWEX 1x8-400 formate was determined to have superior capacity and the fastest exchange rate. Solvents suitable for exchanging the acids onto the resin were CH2Cl2, methanol, and various solvent/water mixtures. Solvents suitable for exchanging the carboxylic acids off of the resin were TFA/solvent or HCO2H/solvent mixtures. The resin was found to swell best in CH2Cl2 and in polar protic solvents such as water, alcohols, and acids. Application of this technique to the crude product mixtures from an arrayed reductive amination and an arrayed Stille reaction provided product carboxylic acids in yields averaging 57% and purities averaging 89%.