Basicity comparison for di-substituted 4-nitropyridine derivatives in polar non-aqueous media

Acid dissociation, as well as cationic homoconjugation equilibria have been studied potentiometrically in systems involving four di-substituted 4-nitropyridines and conjugate cationic acids in the polar non-aqueous solvents - aprotic protophobic acetonitrile (AN) and propylene carbonate (PC), the amphiprotic methanol (MeOH), and in the aprotic protophilic dimethyl sulfoxide (DMSO). The influence of solvent effect on the obtained acidity constants has been discussed. The acidity constants (expressed as pK_a values) were compared with those previously determined in another polar protophobic aprotic solvent - acetone (AC), and obtained for the unsubstituted pyridine (Py). A comparison of the acid dissociation constants determined in all media studied has proved that the strength of the cationic acids increases on going from acetonitrile through propylene carbonate, acetone, and methanol to dimethyl sulfoxide. Furthermore, the values of acidity constants in the non-aqueous media have shown that in all the solvents studied they change according to the substituent effects. It has been also found that substituted 4-nitropyridine derivatives studied exhibit no tendency towards cationic homoconjugation in acetonitrile, propylene carbonate, and methanol and dimethyl sulfoxide. Moreover, it has been demonstrated that the acid dissociation constants determined by potentiometric titration method in all the solutions investigated correlate well with the calculated energy parameters of the protonation reactions in the gaseous phase.     

Acidity of Several Anilinium Derivatives in Pure Tetrahydrofuran

The acidity constant (pK _a) of eleven substituted anilinium ions and the dissociation constants of their perchlorate salts (pK _salt) were determined in pure tetrahydrofuran by potentiometry and conductometry. The pK _a values of the studied aniliniums extend downward the range of previously determined pK _a values. The resolution of acid strength for cationic acids in tetrahydrofuran was compared with those obtained in other amphiprotic and aprotic solvents. It is shown that the resolution in tetrahydrofuran is higher than the ones in water and methanol, similar to those in acetone, dimethyl sulfoxide and isobutylmethylketone, but lower than those in acetonitrile and nitromethane.     

The basicity of pyridine and its tendency towards cationic homoconjugation in non‐aqueous media

Acid dissociation constant, pK_a, of protonated pyridine, determined in the polar protophobic aprotic solvent, acetone, has been compared with pK_a values of the pyridinium ion in a variety of other polar solvents including aprotic protophobic ones, acetonitrile, benzonitrile, nitrobenzene, nitromethane, and propylene carbonate, in the protophilic aprotic dimethyl sulfoxide and N,N‐dimethylformamide, as well as in the amphiprotic methanol. On the basis of the set of these pK_a values, the effect of the medium on the basicity of pyridine is discussed. Further, based on the cationic homoconjugation constants of pyridine conjugated with the pyridinium cation in the non‐aqueous solvents, the tendency of pyridine towards homoconjugation reactions has also been discussed. Finally, both the basicity of pyridine and its tendency towards cationic homoconjugation has been compared with analogous properties of pyridine N‐oxide.     

Studies on acid–base equilibria of 4-R-2,6-dimethylpyridine N-oxide systems in non-aqueous solvents

Acid–base equilibrium constants, i.e. acid dissociation, cationic homoconjugation and cationic heteroconjugation constants in 4-R-2,6-dimethylpyridine N-oxide systems (where R denotes methoxy-, methyl-, chloro- or nitro-group) in polar non-aqueous solvents: protophobic aprotic nitromethane, acetonitrile and acetone, protophilic aprotic N,N-dimethylformamide and amphiprotic methanol have been determined. The acidity constant values of protonated N-oxides in solvents studied have been found to change according to the substituent effect and to the sequence of acidity changes in water. This finding allowed to correlate the pK_a values determined in water with those determined in the solvents studied. Further, it was found that the cationic homo- and heteroconjugation constant values increased with decreasing solvent basicity and with increasing basicity of the N-oxide, and in the case of heteroconjugating systems, with increasing basicity of the the proton acceptor.     

Acid-base equilibria of substituted pyridine N-oxides in methanol

Acid dissociation constants in methanol for eight substituted pyridine N-oxides having a wide range of acid-base properties, [quinoline N-oxide (bi-cyclic amine N-oxide) and pyridine (heterocyclic amine)] have been determined using the potentiometric titration method. A linear correlation between ourmethanol data and aqueous pK _a values from the literature has been found. As in polar aprotic solvents cationic homoconjugation phenomenon has been found to be present for sufficiently basic N-oxides. The tendency of substituted pyridine N-oxides towards cationic homoconjugation in methanol is weaker than in polar aprotic solvents and increases with increasing basicity of N-oxides. It has also been found that, in contrast to polar aprotic solvents, the cationic homoconjugation phenomenon in methanol is much more pronounced for heterocyclic amines than their N-oxides.     

Evaluation of the acidic properties of hydroxycoumarins and choice of solvents for performing potentiometric analysis

The relative acidity constants (pK_a) for 17 hydroxycoumarins in water, methanol, acetone (Ac), dimethylformide (DMFA), and dimethyl sulfoxide (DMSO) have been determined by Henderson's method. The existence of a linear relationship between pK_a in water and pK_a in acetone, dimethylformamide, and dimethyl sulfoxide has been established. From the pK_a values the sequence of neutralization of the hydroxy groups has been determined: their acidic properties decrease in the sequence 4-OH > 7-OH > 6-OH > 8-OH. A quantitative evaluation of the conditions of titration in five solvents on the basis of the titration constants (pK_t) and of the values of the potential jumps and the shape of the potentiometric titration curves has permitted acetone to be proposed as the optimum solvent for the performance of potentiometric analysis.     

Evaluation of the acidic properties of hydroxycoumarins and choice of solvents for performing potentiometric analysis

The relative acidity constants (pK_a) for 17 hydroxycoumarins in water, methanol, acetone (Ac), dimethylformide (DMFA), and dimethyl sulfoxide (DMSO) have been determined by Henderson's method. The existence of a linear relationship between pK_a in water and pK_a in acetone, dimethylformamide, and dimethyl sulfoxide has been established. From the pK_a values the sequence of neutralization of the hydroxy groups has been determined: their acidic properties decrease in the sequence 4-OH > 7-OH > 6-OH > 8-OH. A quantitative evaluation of the conditions of titration in five solvents on the basis of the titration constants (pK_t) and of the values of the potential jumps and the shape of the potentiometric titration curves has permitted acetone to be proposed as the optimum solvent for the performance of potentiometric analysis.All-Union Scientific-Research Institute of Drug Chemistry and Technology, Khar'kov. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 770–773, November–December, 1985.     

Quantitative estimation of the conditions of titrating hydroxyanthraquinones in nonaqueous solvents

In order to determine the optimum conditions of potentiometric titration, an investigation has been made of the relative acidities of 13-hydroxyanthraquinones in water, methanol, acetone (ac), dimethylformamide (DMFA), and dimethyl sulfoxide (DMSO) on the basis of a calculation of the indices of the relative acidity constants (pK_a) by Henderson's method. The existence of a relationship between pK_a in water and pK_a in acetone, dimethylformamide, and dimethyl sulfoxide has been established which is characterized by the linear equations $$pK_a^{DMSO} = 1.54pK_a^{H_2 O} + 11.88$$ , $$pK_a^{DMFA} = 1.38pK_a^{H_2 O} + 8.50$$ , $$pK_a^{ac} = 1.11pK_a^{H_2 O} + 10.26$$ . The sequence of neutralization of the hydroxyls in the titration of polyhydroxy-anthraquinones has been determined from the pK_a values in DMSO and the results of a calculation of electronic structures by the Pariser-Parr-Pople method. A quantitative evaluation of the conditions of titration in the five solvents on the basis of indices of the titration constants (pK_t) has shown that the optimum conditions for the quantitative determination by potentiometric titration are achieved in dimethyl sulfoxide.     

Structural and Physicochemical Characteristics of Tetrabutylammonium Tetrahalogenoferrates(III), [(C4H9)4N][FeBr4−nCln]

A series of tetrahalogenoferrates(III), [FeBr_4?nCl_n]^? (n=0‐4) stabilized with the tetrabutylammonium cation, of general formula [(C₄H₉)₄N][FeBr_4?nCl_n], has been synthesized. The crystal and molecular structure of [(C₄H₉)₄N][FeCl₄] was determined. The iron cation adopts slightly distorted tetrahedral coordination with two opposite angles smaller than tetrahedral one, two equal to tetrahedral and two larger than tetrahedral. The bond valences were computed. The total valence of iron atom is equal to 3.08. In the structure can be found only one hydrogen bond C(1)–H···Cl. Except mentioned there are no unusually intermolecular short contacts between ions existing in the structure. All [(C₄H₉)₄N][FeBr_4?nCl_n] (n=0‐4) compounds are isostructural in solid state. On the basis of conductometric measurements, relative stabilities of the anions have been estimated in methanol (MeOH), dimethyl sulfoxide (DMSO), acetone (AC), acetonitrile (AN) and dichloromethane (CH₂Cl₂) representing both polar (including amphiprotic and aprotic) as well as non‐polar solvents. Further, the dissociation constants of the compounds were calculated from the expanded Pitt's conductivity equation. The results of the conductometric measurements were supported by electronic spectra.     

Direct determination of pKa values of cationic acids conjugated to heterocyclic amine N-oxides in polar aprotic and amphiprotic solvents

The applicability of the direct method of pK_a determination in the case of protonated heterocyclic amine N-oxides in a series of polar non-aqueous (aprotic and amphiprotic) solvents has been tested. The method is based on the pH determination of the non-aqueous solution of complex salt (the semiperchlorate in this case) formed by the N-oxides studied. The direct method not only provides for quick (one data point per each pK_a determined), but also relatively accurate estimates of acidic dissociation constants. It has been experimentally shown on the example of substituted pyridine N-oxides that this method is precise enough in all studied non-aqueous solvents when applied to compounds of not too low basicity (the pK_a being of the order of 5 or higher). To prove this, the pK_a values of protonated monocyclic N-oxides obtained by the direct method have been compared with those resulting from the potentiometric titration curve. The agreement between the results found by using both methods is very good in most cases, the differences being within standard deviations. Based upon this observation it can be inferred that the pK_a values of protonated bicyclic N-oxides in solvents studied determined by using the direct method can be also considered reliable, especially in the case of polar aprotic solvents.     

Proton solvation in protic and aprotic solvents

Protonation pattern strongly affects the properties of molecular systems. To determine protonation equilibria, proton solvation free energy, which is a central quantity in solution chemistry, needs to be known. In this study, proton affinities (PAs), electrostatic energies of solvation, and pK_A values were computed in protic and aprotic solvents. The proton solvation energy in acetonitrile (MeCN), methanol (MeOH), water, and dimethyl sulfoxide (DMSO) was determined from computed and measured pK_A values for a specially selected set of organic compounds. pK_A values were computed with high accuracy using a combination of quantum chemical and electrostatic approaches. Quantum chemical density functional theory computations were performed evaluating PA in the gas‐phase. The electrostatic contributions of solvation were computed solving the Poisson equation. The computations yield proton solvation free energies with high accuracy, which are in MeCN, MeOH, water, and DMSO ?255.1, ?265.9, ?266.3, and ?266.4 kcal/mol, respectively, where the value for water is close to the consensus value of ?265.9 kcal/mol. The pK_A values of MeCN, MeOH, and DMSO in water correlates well with the corresponding proton solvation energies in these liquids, indicating that the solvated proton was attached to a single solvent molecule. © 2016 Wiley Periodicals, Inc.     

Spectrophotometric study of complex formations between 1-(2-pyridylazo)-2-naphthol (PAN) and some metal ions in organic solvents and the determination of thermodynamic parameters

The complexation reactions between Ni(2+), Co(2+) and Zn(2+) metal ions with PAN in methanol (MeOH), acetonitrile (AN) and dimethyl sulfoxide (DMSO) were studied using a spectrophotometric method. The stability constants of the resulting complexes were determined from computer fitting absorbance mole-ratio data. The results revealed that the stability constants of complexes are varying in order of Ni(2+)相似文献   

Spectrophotometric and conductometric study of complexation of salophen and some transition metal ions in nonaqueous polar solvents

The complexation reaction between Cu(2+), Co(2+) and Ni(2+) metal cations with N,N'-bis(salicylidene)-1,2-phenylenediamine (salophen), in three nonaqueous polar solvents such as: acetonitrile (AN), dimethyl sulfoxide (DMSO), methanol (MeOH) and two binary mixtures of AN:DMSO and AN:MeOH at 25 degrees C were studied by spectrophotometric and conductometric methods. All investigated metal ions form 1:1 ML complex which their stability constants were determined and increase as Irving-Williams stability order of Co(2+)相似文献   

Catalytic effects of monolayers of metals deposited at underpotentials: Oxidation of formic acid on platinum

An independent potentiometric method is developed for the determination of autoprotolysis constants of nonaqueous amphiprotic solvents. The data of potentiometric titrations of a weak protolyte (base or acid) with a strong one are used for calculation of both autoprotolysis constant of the solvent and acid—base constant of the protolyte. The calculations are based on the method of least-squares in combination with the “pit-mapping” procedure developed by Sillén. The validity of the method was proved by the determination of the autoprotolysis constant of water. The method has no restriction concerning the strength of the weak protolyte, the latter being its main advantage. The autoprotolysis constants of methanol and ethylene glycol were determined by means of the proposed method too. The obtained values of pK_MeOH=17.00 and pK_EG=15.91 are in a good agreement with some recent data.     

Isopoly Tungstate Anions in Water–Dimethyl Sulfoxide Solutions

The formation of isopoly tungstate anions (IPTAs) in water–dimethyl sulfoxide (DMSO) solutions was studied by potentiometric titration, followed by mathematical modeling, and electronic absorption spectroscopy. Logarithmic equilibrium constants and Gibbs free energies were calculated for IPTA formation from \(\rm{WO}_4^{2-}\) and Н⁺ in the \(\rm{WO}_4^{2-}\)–H⁺(DMSO = dimethyl sulfoxide) system for various solution acidities Z=v(H⁺)/v(\(\rm{WO}_4^{2-}\)). A scheme was suggested for IPTA interconversions at various DMSO contents in solution. Dimethyl sulfoxide was found to have a stabilizing effect on [W₇O₂₄]^6– and [W₁₀O₃₂]^4– anions. The distribution diagrams and the calculated thermodynamic parameters of IPTA formation were used to determine optimal conditions for preparing decatungstates from water–DMSO solutions, which consisted in using sodium tungstate solutions acidified to Z = 1.60 and containing ?(DMSO) = 40%.     

Excess enthalpies of alkane-1-amines {CnH2n+1NH2, n = 3-8} + methyl methylthiomethyl sulfoxide and +dimethyl sulfoxide at 298.15 K

Excess enthalpies of binary mixtures between each of alkane-1-amines {C_nH_2n+1NH₂, n=3-8} and methyl methylthiomethyl sulfoxide (MMTSO) or dimethyl sulfoxide (DMSO) have been determined at 298.15 K. All mixtures showed positive enthalpy changes over the whole range of mole fractions.The limiting excess partial molar enthalpies of the aliphatic amines, H₁^E,∞, of all the mixtures with MMTSO or DMSO studied were smaller than those of MMTSO or DMSO, H₂^E,∞, respectively. Linear relations are obtained between limiting excess partial molar enthalpies and number of methylene groups.     

Spectrophotometric study of acidity constants of Alizarine Red S in various water-organic solvent

The acidity constants of Alizarine Red S were determined spectrophotometrically at 25 degrees C and at constant ionic strength 0.1 M (KNO3) in pure water as well as in aqueous media containing variable mole percentages (5-70%) of organic solvents. The organic solvents used were methanol, ethanol, N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), acetonitrile and dioxan. The acidity constants of all related equilibria are estimated using the whole spectral fitting of the collected data to an established factor analysis model. DATAN program was applied for determining of acidity constants and pure spectra of different form of Alizarine Red S. The obtained results indicated that acidity constants decrease as the content of an organic solvent in the medium increases. There are linear relationship between acidity constants and the mole fraction of various organic solvents in the solvent mixtures. Effect of various solvents on acidity constants and pure spectrum of each component are also discussed.     

A simple and convenient synthesis of 2-(perfluoroalkyl)-4H-chromenes from salicyl N-tosylimines or salicylaldehydes and methyl 2-perfluoroalkynoates

Et₃N-catalyzed reactions of salicyl N-tosylimines or salicylaldehydes with methyl 2-perfluoroalkynoates proceed smoothly at room temperature in dichloromethane (DCM) or dimethyl sulfoxide (DMSO) to give the corresponding fluorinated chromenes in good to excellent yields with high regioselectivity.     

High-performance liquid chromatography of 5,7-dinitrobenzofurazan and nitrobenzodifurazan derivatives of aromatic amines

The retention times of 5,7-dinitrobenzofurazan and nitrobenzodifurazan derivatives of aromatic amines in reversed-phase high-performance liquid chromatography on a column filled with Hypersil ODS correlate with the acidity constants of these derivatives in dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) and with the σ constants of substituents in the aromatic nucleus. Procedures were developed for the determination of aniline, N-methylaniline, N,N-dimethylaniline, and N-ethylaniline as their 5,7-dinitrobenzofurazan and nitrobenzodifurazan derivatives with detection limits of 70 μg/mL. The procedures can be used for the analysis of reaction mixtures in the synthesis of these substituted amines.