Protolytic equilibria in aqueous sodium deoxycholate solutions

Protolytic equilibria taking place in aqueous solutions of sodium deoxycholate (DCNa) have been studied at 25°C using 0.5M NaCl as ionic medium. Electromotive force measurements of a galvanic cell were carried out by means of a glass electrode.The reagent necessary to change the acidity of the solutions was produced in situ by supplying a constant small current.Solubility and acid constant of deoxycholic acid (HDC) have been determined for the chosen experimental conditions. Experimental data obtained in less acid solutions have been explained by assuming the presence of the species H(DC)₂. The relative stability constant has been determined. At higher deoxycholate concentration the presence of a polymeric micellar species has been assumed.

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Protolytische Gleichgewichte in wäßrigen Natriumdesoxycholat-Lösungen

Zusammenfassung Die Protonierung von Natrium-Desoxycholat (DCNa) in wäßrigen Lösungen mit 0.5M NaCl wurde bei 25°C mit Hilfe von E.M.K. Messungen mit einer Glaselektrode untersucht.Das notwendige Reagens für die Umwandlung der Säure in den untersuchten Lösungen wurde in situ durch einen konstanten schwachen Strom erzeugt.Löslichkeit und Dissoziationkonstante von Desoxycholsäure (HDC) wurden unter den gewählten experimentellen Bedingungen bestimmt. Die experimentellen Daten in schwach sauren Lösungen konnten mit der Annahme der Existenz von H(DC)₂ erklärt werden. Die entsprechende Konstante wurde bestimmt. Zur Erklärung der Daten in stärker konzentrierten Lösungen von Desoxycholat ist die Annahme einer polynuklearen Spezies nötig.

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Symbols H analytical excess of hydrogenions, if negative it corresponds to OH^–; - h free concentration of hydrogen ions; - A total concentration of deoxycholate; - a free concentration of deoxycholate; - K _a acid constant of deoxycholic acid (HDC) defined as follows: [HDC]K _a =ha; - _q,p stability constant of a speciesH _p (DC) _q defined as follows: [H _p (DC) _q ]= _q,p h ^p a ^q ; - C ⁰ solubility of HDC; - formation function representing the average number of H⁺ bonded to deoxycholate.     

Protolytic equilibria on the surface of carboxyl-containing silica

Potentiometric titration has been used to study the protolytic equilibria on the surface of carboxyl-containing silica (CS) prepared by the reaction of silica (Silokhrom S-80, S_sp = 80 m²/g) with Cl₃SiCH₂CH₂COOCH₃, followed by hydrolysis with 30% sulfuric acid. The titration curve of the vacuum-dried sample is irreversible. The titration curve of its Na⁺ form with hydrochloric acid proceeds lower than the titration curve of its H⁺ form and coincides with the titration curve of the air-dried sample (the last curve is reversible). The titration curve of CS coincides with the titration curve of butyric acid at pH < 6. At pH > 6 the titration curve of CS passes below the titration curve of butyric acid; this is due to the participation of silanol groups on the silica surface in the protolytic equilibria. The pK_a of the grafted CS groups is equal to 4.80 which is close to the pK_a value of butyric acid (4.78). A method has been proposed for the determination of the amount of weak acid groups grafted to the silica. It has been shown that in the titration of CS the equilibrium is established much faster than in the case of the unmodified silica.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 243–247, March–April, 1986.     

Protolytic equilibria of fluorescein halo derivatives in aqueous-organic systems

The dissociation constants of fluorescein halo derivatives containing substituents in the xanthene ring and/or in the phthalic acid moiety were determined in 91.4 wt % aqueous ethanol. The tautomerism of the dyes was inferred from the electronic absorption spectra of ions and molecules in this solvent and of the dichloromethane and chloroform extracts of associates of the anions with tetra-n-butylammonium cations. The equilibria in two-phase systems were considered.     

Protolytic equilibria of 2-hydroxytropolone (4,5-d) thiophene in homogeneous and micellar solutions

The present study comprises an investigation of the optical absorption and fluorescence spectra of the title compound (HT) in homogeneous solutions of ethanol, cyclohexane, and sulphuric acid, and in aqueous micellar systems of anionic (NaLS), cationic (CTABr) and non-ionic (Triton X-100) surfactants.This compound behaves as monoprotic acid in buffer solutions of pH = 1–13 containing 3% v/v ethanol. It has pK = 8.2, but in the first excited singlet state the p_K* drops to 3.9. However, another protolytic equilibrium is established with pK₂ = 0.45 and pK*₂ = 2.15 in concentrated H₂SO₄ solutions.Contrary to all other media studied, the dissociated form of HT was observed in CTABr micellar solution with an apparent red shift indicating that the HT molecules are incorporated into the detergent layer of the micelles and at the interface of the aggregates.The influence of micellar solutions on the acid—base equilibrium of HT reveals that the effect of the charge distribution of the counter ions in the double layer is much larger than the effective dielectric constant at the site of solubilization.     

Protolytic equilibria of picric acid in a mixture of protophobic and protophilic polar solvents non-donors of H-bonds

A new differentiating medium for the study of prototropic equilibria is proposed: acetonitrile with a small addition of DMSO. From the dependence of the picric acid pK _a on the content of DMSO in this medium the optimum composition of the mixture was found ensuring the stability of the pK _a values with minimal addition of DSMO: 4 wt %. It was experimentally confirmed that the addition of water in an amount up to 0.1 wt % has virtually no effect on the measured pK _a value in a mixture with the optimum composition.     

Protolytic stability of palladocycles

Protonolysis of ortho-palladated asymmetric metallacycles

(3: Z_A = NMe₂, Z_B = PPh₂; 4: Z_A = NMe₂, Z_B = AsPh₂; 5: Z_A = AsPh₂, Z_B = PPh₂) by acetic acid leads to the departure of PhCH₂NMe₂ (in the case of 3 and 4) and PhCH₂AsPh₂ (for 5) and formation of corresponding dimeric acetato bridged complexes 6 and 7, respectively. The relative stability of the palladocycles is determined by the nature of the donor atom and decreases in the order: P > As > N.     

Insoluble mixed monolayers : II. Protolytic equilibria and the influence of the pH on the collapse pressure

A general thermodynamic treatment is given for the protolytic equilibria in an insoluble monolayer, containing surfactant molecules with n ionizable protons and able to accept m more protons, and being spread at the liquid/gas interface. The correlation between the pH of the subphase liquid and the collapse pressure of the monolayer is discussed. By using the approximation of perfect solutions and of binary surface systems (protonated and deprotonated molecular species of the surfactant) several methods are proposed for deriving apparent surface acidity constants from experimental collapse pressure vs pH curves, in the case of both miscible in monolayer miscible in collapsed bulk phase and miscible in monolayer immiscible in collapsed bulk phase, type systems. Some of these methods are based on a complete perfect solution approximation (CA) taking into account the molar fraction of the subphase liquid in the monolayer and the others use “surfactant” approximation (SA), neglecting this molar fraction. The methods proposed are tested on monolayers of carotenoid pigments spread at aqueous solution/air interfaces. Results obtained by the different methods are rather close to each other, but the approximation CA is better than SA. The apparent surface acidity constants of different carotenoids are compared with each other and discussed in terms of molecular structure and electronic effects.     

Protolytic properties of 2-acylthioacetamides

Concentration acidity of 2-acylthioacetamides dissolved in DMSO-H₂O was measured by the method of pH-metric titration. The data obtained were extrapolated for the determination of pK _a values of 2-acylthioacetamides in water and DMSO. We found that in aqueous solutions the acidity of thioacetarylamide (pK _a = 5.75?C6.01) was almost independent of the nature of substituents in their N-phenyl rings, whereas in DMSO solution the nature of substituents significantly affects their ionization constants (pK _a = 7.88?C11.70). The graphs were displayed and the acidity of N-aryl-3-oxobutanethioamides dependence on the Hammett constants of meta- and para-substituents in the phenyl rings derived. Water is shown to be a leveling, and DMSO a differentiating solvent for 2-acylthioacetamides.     

Protolytic defluorination of trifluoromethyl-substituted arenes

A series of trifluoromethyl-substituted arenes were studied in their reactions with Br?nsted superacids. The products from these reactions suggest the formation of reactive electrophiles, such as carbocations, acylium cations or equivalent electrophilic species. As such, Friedel-Crafts-type reactions occur between these species and arene nucleophiles. NMR studies were done, and the results suggest the formation of an acyl group from the trifluoromethyl groups in the superacid.     

Medicinal chemistry of tetrazoles

Properties of responsible for biological activity tetrazoles are considered. Examples are given of active pharmaceutical ingredients of modern drugs containing the tetrazole ring in the molecular structure. New publications on the synthesis and investigations of biological activity of promising tetrazole-containing compounds are cited.     

Protolytic Opening of Two Diastereoisomeric Cyclopropanols

Cob(I) alamin ( 1(I) )-catalyzed reduction of the aldehyde 2 led to the two crystalline cyclopropanols 3 and 4 (see Scheme 2). The protolytic ring-opening starting from 3 produced the saturated aldehydes 6 and 7;8 was formed in traces only (see Scheme 3). The protolysis starting from 3 led, therefore, mainly to retention of configuration at the spiro C-atom ( 7 ); ring-opening with inversion was observed in traces only ( 8 ). Starting from 4 , the protolysis produced 9 and 7 ; the absence of 8 showed this protolysis to proceed 9 and 7 ; the absence of 8 showed this protolysis to proceed exclusively with inversion of configuration at the spiro center. Of the p-bromobenzoate 5 (cf. Scheme 2) the structure has been determined by X-ray analysis.     

大庆蜡油催化裂化过程中的质子化裂化反应

在小型固定流化床装置上采用酸性催化剂进行了多系列不同反应深度的催化裂化实验,对大庆VGO催化裂化过程中发生的质子化裂化反应进行了初步的研究。在重质油的催化裂化过程中会出现二次质子化裂化反应。二次质子化裂化反应主要是由于汽油中烷烃重新在酸性催化剂上形成五配位正碳离子随后分解所造成,其产生的原因主要是由于反应后期催化剂对反应中间产物的选择性吸附改变所致。二次质子化裂化反应对温度不敏感。大庆VGO在500℃下反应时二次质子化裂化反应约占整个质子化裂化反应的60%。     

Protolytic equilibrium of certain annelated azoloazines

The first and second protonations of annelated azoloazines have been investigated quantitatively in aqueous solution. Compounds investigated were pyrazolo[1,5-a]pyrimidine (pK_BH ^– 0.03±0.02, pK_BH ²⁺ –7.87±0.30), 1,2,4-triazolo[4,3-b]-1,2,4-triazine (pK_BH ⁺ –0.04±0.02, pK_BH ²⁺ –8.00±0.10), 1,2,4-triazolo[1,5-a]pyrimidine (pK_BH ⁺ 0.21±0.03, pK_BH ²⁺ –9.00±0.09) and its 6R derivatives (R=NO₂, Br, Cl). The annelated azoloazines studied are weaker bases than their unannelated analogs. According to quantum chemical calculations (AM1), protonation of these heterocycles may occur both at the azole and the azine fragments of the molecule.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 816–824, June, 2000.     

Strategies and prospects in functionalization of tetrazoles

The review summarizes the methods of functionalization of tetrazoles, which were reported from 2001 to mid 2005, and analyzes main strategies in the development of this field of tetrazole chemistry.     

Synthesis of (difluoroamino)tetrazoles and [(difluoroamino)alkyl]tetrazoles

(Difluoroamino)tetrazoles were obtained by direct fluorination of 5-aminotetrazoles with gaseous fluorine. Reactions of 2-(oxoalkyl)- and 2-(hydroxyalkyl)tetrazoles with difluoroamine yielded [(difluoroamino)alkyl]tetrazoles. Deceased Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 950–952, May, 2000.     

Protolytic Properties of Thiofluorescein and Its Derivatives

Ionization constants of thiofluorescein, 4-nitro-, 4,5-dinitro-, 2,4,5,7-tetrabromo-, and 2,4,5,7-tetraiodothiofluoresceins, and also of 4,5-dibromofluorescein and N-ethyl-4,5-dinitroazafluorescein in 50 wt % aqueous ethanol have been determined. The electronic absorption spectra of their ionic and molecular forms have been recorded and analyzed. The behavior of these dyes can be described in terms of the scheme of prototropic equilibria proposed previously for fluorescein and its halo derivatives.