Acid–base equilibria in systems involving substituted pyridines in polar aprotic protophobic media and in the amphiprotic methanol (original) (raw)
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Journal of Molecular Liquids, 2021
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Journal of Molecular Structure, 1999
Temperature dependence of acid–base equilibrium constants in systems consisting of substituted pyridine N-oxides was studied in acetonitrile, a strongly ionizing polar protophobic aprotic solvent. N-oxides, which have an enhanced tendency towards formation of cationic homo- and heteroconjugates, (OHO+) as a result of their relatively strong basicity, were investigated. For the sake of comparison, also aliphatic trimethylamine N-oxide and pyridine, the parent compound of its N-oxide, were included. Of particular interest were acid dissociation constants of cations of the protonated bases, homoconjugation equilibria of the cationic acids with conjugate bases, as well as cationic heteroconjugation equilibria of the protonated bases with non-conjugated bases. The constants were determined at three temperatures, 292.1, 298.1 and 304.1 K. On the basis of the temperature dependence of the constants, enthalpy changes were calculated for the acid dissociation of the cationic acids, as well as for cationic homo- and heteroconjugation. These values were derived from the known thermodynamic relationships, ΔG = −RTln K and ΔG = ΔH − TΔS. It has been found that enthalpy changes for the dissociation reactions of the cationic acids and for cationic homoconjugation reactions are consistent with the variations of the protonation and homoconjugation energies, respectively calculated by both the semi-empirical and the ab initio methods on the 4-31G level.
Analytica Chimica Acta, 1999
Acid-base and hydrogen-bonding equilibria in acetic acid -acetate, n-butylammonium cation -n-butylamine and acetic acid (acetate) -n-butylamine (n-butylammonium cation) systems were studied in five polar non-aqueous solvents with varying prototropic and dielectric properties: acetonitrile, acetone, dimethyl sulfoxide, methanol, and propylene carbonate using the potentiometric-titration technique. These systems were designed to model the acid-base and hydrogen-bonding phenomena that involve acid and basic amino-acid side chains in proteins. The obtained order of pK a values of acetic acid and n-butylamine is consistent with the variation of prototropic, basic, dielectric, and hydrogen-bonding properties of the solvents used. The homoconjugation and heteroconjugation constant values decrease with increasing basicity of the solvents; they turn out to be indeterminable in dimethyl sulfoxide (the most basic solvent) for cationic homoconjugation and in methanol (the second most basic solvent) for anionic homoconjugation and heteroconjugation. : S 0 0 0 3 -2 6 7 0 ( 9 9 ) 0 0 5 0 9 -7
A study of cationic heteroconjugation equilibria of substituted pyridine N-oxides in acetonitrile
Analytica Chimica Acta, 1997
Cationic heteroconjugation equilibria (resulting in the formation of the [O-H. .O]+ hydrogen-bonded bridges) of proton-N-oxide systems in acetonitrile (a polar aprotic protophobic solvent) were studied using the potentiometric titration method. For comparison, the heteroconjugation equilibria of the pyridinium cation with N-oxides (the [N-H. .O]+ bridges) were also studied. For the [O-H. .O]+ bridges, the heteroconjugation constants were found to increase with increasing basicity of the proton acceptor and decrease with increasing basicity of the proton donor. Consequently, the highest values of the heteroconjugation constants were obtained for the [O-H.. .O]+ bridges of N-oxides with almost the same basicity. Another factor influencing the heteroconjugation is the hydrogen-bonding propensity of the components defined as the arithmetic average of their homoconjugation constants. A correlation was found that expresses the logarithm of heteroconjugation constant as a linear function of the two foregoing quantities. For the [N-H.. .O]+ bridges, the heteroconjugation constants were found independent of the proton-acceptor basicity.
International Journal of Thermophysics, 2007
The acidity constants of molecular acid, K a (HA), cationic acid, K a (BH+), as well as the equilibrium constants of anionic homoconjugation, K_{\rm AHA^{-}},cationichomoconjugation,, cationic homoconjugation,,cationichomoconjugation,K_{\rm BHB^+}$$ , and molecular heteroconjugation, KAHB, have been determined in (n-butylamine + acetic acid) systems without proton transfer in binary [dimethyl sulfoxide (DMSO) + 1,4-dioxane (D)] solvent mixtures. The constants were determined by using the potentiometric titration method at a fixed ionic strength. It is concluded that the molecular heteroconjugation constants in the mixed solvent systems studied are linearly related to the 1,4-dioxane content. Furthermore, in the (acid + base) systems without proton transfer, the direction of titration (direct B + HA or reverse HA + B) has been found to affect the precision of determination of reliable values of molecular heteroconjugation constants. Moreover, it has been found that the relative dielectric constants of the solvent mixtures st...
Amino acid chemistry in dipolar aprotic solvents: dissociation constants and ambident reactivity
The Journal of Organic Chemistry, 1986
The pK,'s of 10 amino acids and their esters were measured spectrophotometrically in dilute MezSO solution. The pK,'s of the a-amino acids ranged from 6.3 to 7.5 and the esters from 6.4 to 8.7. The ratio of zwitterion to uncharged form of the amino acids in MezSO ranged from 2 to 40, compared to 104-105 in aqueous solution. The primary reason for this difference is the greater solvation of the carboxylate anion in water compared to MGO, with solvation of the ammonium ion being similar in the two solvents. Although the zwitterion predominates in dipolar aprotic solvents, N-alkylation was competitive with 0-alkylation. Reaction of the unprotected amino acids with alkyl halides gave 50-70% yield of esters. Using alkylating agents with "harder" leaving groups increased the ester yield to 75-85%. 0-Alkylation yields were improved from 50-70% to 85-90% by addition of 2 equiv of LiBr to the reaction mixture. The improvement in 0 VI. N selectivity was attributed to a salting in of the zwitterionic form of the amino acid.
The Journal of Chemical Thermodynamics, 2005
By using the potentiometric titration method, standard equilibrium constants have been determined of acid dissociation of molecular acid, K a (HA), cationic acid, K a (BH + ), of anionic and cationic homoconjugation, K AHA À and K BHB þ , respectively, and of molecular heteroconjugation, K AHB (K BHA ), in (acid + base) systems without proton transfer consisting of n-butylamine and acetic acid in binary (acetone + cyclohexane) solvent mixtures. The results have shown that both the pK a (HA) and pK a (BH + ), as well as lg K AHA À values change non-linearly as a function of composition of the solvent mixture. On the other hand, standard molecular heteroconjugation constants without proton transfer do not depend on the cyclohexane content in the mixture, i.e. on solvent polarity.
Journal of The Serbian Chemical Society, 2005
The rate constants for the reaction of three isomeric pyridine mono-carbocylic acid N-oxides with diazodiphenylmethane were determined at 30 ºC in thirty two protic and aprotic solvents by the well known UV spectrophotometric method. The rate constants are generally higher than for pyridine mono-carboxylic acids in a similar range of solvents, except for picolinic acid N-oxide, and also higher in protic than in aprotic solvents. The determined rate constants were correlated with solvent parameters using the Kamlet-Taft solvatochromic equation by means of multiple regression analysis. The sign of the equation coefficients were in agreement with the postulated reaction mechanism. The mode of the influences of the solvent is discussed on the basis of the correlation coefficients, taking into account the specific structures of the pyridine mono-carboxylic acid N-oxides.
Journal of the Serbian Chemical Society, 2006
The acid-base properties of 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-one (L 1) and 1-amino-5-benzoyl-4-phenyl-1H-pyrimidine-2-thione (L 2) were investigated potentiometrically at an ionic strength of 0.10 M (LiCl) in 19.8, 33.6 and 55.9 % (v/v) methanol-water mixtures at 25.0 ± 0.1 ºC. The apparent dissociation constants (p s K a) were calculated for the di-protonated form (L 1 H 2 +2 and L 2 H 2 +2) of pyrimidine bases, using a software package TITFIT, which were then extrapolated to pure water to derive the dissociation constants in aqueous solution (pK a). The aqueous pK a constants were found to be: L 1 , pK a1 = 3.76 and pK a2 = 6.95; L 2 , pK a1 = 3.57 and pK a2 = 6.90. At pH £ 2.00, the dominant species in solution were the protonated form of the amino group substituted at the 1-position, while at a pH around 5.00, they were the protonated form of the pyrimidine ring nitrogen at the 3-position. An effect of intramolecular hydrogen bonding on the p s K a values was observed with L 1 but not L 2. The effects of molecular structure and solvent medium on the p s K a values are also discussed.
Investigations of (acid+base) equilibria in systems modelling interactions occurring in biomolecules
The Journal of Chemical Thermodynamics, 2006
By using the potentiometric microtitration method, acidity constants, K a , anionic, K AHA À , and cationic, K BHB þ , homoconjugation constants, as well as molecular heteroconjugation, K BHA , constants have been determined in (acid + base) systems formed by the following compounds: acetic acid, phenol, n-butylamine, imidazole, and 4(5)-methylimidazole. These compounds constitute fragments of the side chains of amino acids capable of proton exchange in active sites of enzymes. The (acid + base) equilibria were studied in five polar solvents of different properties, namely in aprotic protophobic acetonitrile, acetone and propylene carbonate, in aprotic protophilic dimethyl sulfoxide and in amphiprotic methanol. The lowest values of the acidity constants of the molecular and cationic acids have been found in aprotic protophobic polar solvents-acetonitrile, propylene carbonate and acetone. Their acid strength have been found to depend on solvent basicity expressed as donor numbers, DN. These media, in particular acetonitrile and acetone, are also favourable for establishing molecular homo-and heteroconjugation equilibria. The most stable homocomplexes are formed in the case of acetic acid (K AHA À values range from 2.26 to 3.56 in these media, being more than an order of magnitude higher than those for the remaining compounds). The magnitudes of lg K BHA reveal that the most stable heterocomplexes are formed by n-butylamine and acetic acid that are characterized by the smallest differences in pK a values.