Methanol acidity constant

Acid hydrolyses are usually carried out by refluxing in 6 N aqueous hydrochloric acid (constant boiling) or 5 to 10% solutions of HC1 in methanol (to promote solubility) for 4 to 30 hr, depending on the lipid in question. Most glycerophosphatides are hydrolyzed by acid to fatty acids, glycerophosphate, and the free base, just as with alkali. However, inositol phosphatides initially yield inositol phosphate and diglycerides on acid hydrolysis. Hydrochloric acid is easily removed by vacuum, which makes chromatographic examination of the hydrolysis products easier. 

Nitration. Add 0.4 mL of concentrated sulfuric acid to 100 mg of the aryl halide (or aromatic compound) and stir. Add 0.4 mL of concentrated nitric acid dropwise with stirring and shaking while cooling the reaction mixture in water. Then heat and shake the reaction mixture in a water bath at about 50°C for 15 min, pour into 2 mL of cold water, and collect the product by filtration. Recrystallize from methanol to constant melting point. 

Kaminski, G.A. Accurate prediction of absolute acidity constants in water with a polarizable force field Substituted phenols, methanol, and imidazole. J. Phys. Chem. B 2005, 109(12), 5884-90. 

The data (Table 8 and Scheme 3) illustrate again the possible length selectivity, with a maximum affinity for (gly)3 with receptor 21, and for (gly>4 with receptor 22, in accordance with computer-aided molecular modeling (see also our website). Host 21 shows a pronounced sequence selectivity for aromatic amino acids in position 1 of the tripeptides which as in the case of receptor 15 systematically decreases for aliphatic amino acids with in the order val>leu>ile>ala>gly. Notably, proline in position 1 leads to particular affinity drop, likely due to geometric mismatch. A preference for aromatic amino acids in position 1 is observed also with host 22, more pronounced with trp than with phe. In methanol, large constants of nearly lO M are found, but the sequence selectivity disappears almost entirely (see Table 8) this highlights the importance of hydrophobic side-chain interactions that are also visible in computer simulated structures (see website). 

The acidity of these products cannot be determined by means of direct pH determination. However, the virtual acidity constant, AT l, is accessible from optical and electrochemical measurements. Comparison of the pX i values of selected Knoevenagel products and the pX values of some typical carboxylic acids in methanol establishes that they exhibit comparable acidity (Scheme 7). Using the o -values, the pX i values of benzylidene derivatives of Meldrum s acid are in good accordance with the Hammett equation. The Lewis acidity of Knoevenagel products is due to the formation of labile pseudobase adducts, the so-called anbadons (10), upon reaction with nucleophiles. Structures of the anbadons have... 

Reactives of Side-Chains of Monocyclic Thiophens. - The rate constants for the esterification of some 3-, 4-, and 5-substituted thiophen-2-carboxylic acids and of some 2- and 4-substituted thiophen-3-carboxylic acids with diazodiphenylmethane in methanol solution have been measured, and linear correlations gave information about the transmission of substituent effects. The rates of alkaline hydrolysis of ethyl thiophen-2-carboxylate in ethanol-water and DMSO-water media have been measured and compared with those of other heterocyclic esters. The kinetics of iodination of 2-acetylthiophen in methanol-water, using different carboxylate buffers, have been studied.Basicity constants have been measured for j3-(2-thienyl)-acrylamides and compared with those of the corresponding benzene and furan derivatives. The acidity constants of ( )-a-phenyl-j3-(2-thienyl)-acrylic acids and analogous furan-, selenophen-, and pyridine-substituted compounds have been measured, and have been rationalized by an equation involving separate contributions of polar, conjugative, and steric effects of the heterocycles. ... 

Conductance measurements have been used to investigate the ionisation of hydrochloric and acetic acids in pure methanol and methanol-water mixtures. Hydrochloric acid is not completely dissociated in mixtures containing more than 40wt% methanol in pure methanol its acidity constant is 0.059 mol 1". Values of the constant were obtained from the slope of a graph of 1 / A S against cA.Sy according to the relation ... 

The derivation of p a values from pH measurements is too familiar an exercise to be repeated here. Since pH values refer to a standard state for activities in methanol, the acidity constants will likewise have this reference state, i.e. they will be Ka values. When measurements are made in very dilute solutions (ca. 10" AO, not only are corrections to zero ionic strength small but ion-pairing of buffer salts becomes negligible. Some results of Juillard s measurements, based on the de Ligny standard buffers, are given in Appendix 3.5.5. 

Conductance, .fho-112 spectrophotometric, solubility, and electromotive force measurements can be adapted to give formation constants of the conjugation complexes as well as acidity constants of the acids. The reproducibility of measurement is often lower than that found for water or methanol solutions, and it is common practice to simplify the calculation by introducing various approximations. For a given acid the results of different methods or different investigators seldom indicate a precision of better than 0.1 pK unit. Greater precision can of course be achieved in measuring relative values of acidity constants. ... 

Three main concepts influence acidity when comparing methanol with formic acid (1) the carbonyl group polarizes the 0-H bond, (2) the OH of carboxylic acids hydrogen bonds to a greater extent, and (3) the conjugate base (formate, 9) is more stable than methoxide (2), driving the equilibrium to the right. These concepts are the same as those introduced in Chapter 2 to explain acid-base reactions and the importance of the acidity constant, K. ... 

Srour, R. K. and McDonald, L. M. Determination of the acidity constants of methyl red and phenol red indicators in binary methanol- and ethanol-water mixtures. J. Chem. Eng. Data, 53, 116-127 (2008). 

Garrido G, Rafols C, Boseh E (2006) Acidity constants in methanol/water mixtures of poly-carboxylic acids used in drug salt preparations. Potentiometric determination of aqueous pKa values of quetiapine formulated as hemifumarate. Eur J Pharm Sci 28 118 127... 

K. Sarmini, E. Kenndler, J. Chromatogr. A, 806 (1998) 325. K. Sarmini, and E. Kenndler, Capillary zone electrophoresis in mixed aqueous-organic media Effect of organic solvent on actual ionic mobilities, acidity constants and separation selectivity of substituted aromatic acids. I. Methanol. J. Chromatogr. A, 806 (1998) 325-335. 

Hydrolysis of TEOS in various solvents is such that for a particular system increases directiy with the concentration of H" or H O" in acidic media and with the concentration of OH in basic media. The dominant factor in controlling the hydrolysis rate is pH (21). However, the nature of the acid plays an important role, so that a small addition of HCl induces a 1500-fold increase in whereas acetic acid has Httie effect. Hydrolysis is also temperature-dependent. The reaction rate increases 10-fold when the temperature is varied from 20 to 45°C. Nmr experiments show that varies in different solvents as foUows acetonitrile > methanol > dimethylformamide > dioxane > formamide, where the k in acetonitrile is about 20 times larger than the k in formamide. The nature of the alkoxy groups on the siHcon atom also influences the rate constant. The longer and the bulkier the alkoxide group, the lower the (3). 

Physical properties of the acid and its anhydride are summarized in Table 1. Other references for more data on specific physical properties of succinic acid are as follows solubiUty in water at 278.15—338.15 K (12) water-enhanced solubiUty in organic solvents (13) dissociation constants in water—acetone (10 vol %) at 30—60°C (14), water—methanol mixtures (10—50 vol %) at 25°C (15,16), water—dioxane mixtures (10—50 vol %) at 25°C (15), and water—dioxane—methanol mixtures at 25°C (17) nucleation and crystal growth (18—20) calculation of the enthalpy of formation using semiempitical methods (21) enthalpy of solution (22,23) and enthalpy of dilution (23). For succinic anhydride, the enthalpies of combustion and sublimation have been reported (24). 

In this definition ko is the rate constant for CH3COOR and k is the constant for RCOOR thus = 0 for R = CH3. Table 7-11 lists some values. Taft s Es steric constants are in some instances based on averages of several different reactions, so MacPhee et al. have defined a steric constant Es by Eq. (7-52) for a single reaction, namely, the acid-catalyzed esterification of carboxylic acids in methanol at 40°C. Es values are also given in Table 7-11. Additional Es and Es values are available. 

To a suspension of 25.0 g of 11/3,17a,21-trihydroxy-6,16a-dimethyl-4,6-pregnadiene-3,20-dione in 1.5 liters of alcohol-free chloroform cooled to about 5°C in an ice bath is added with constant stirring 750 ml of cold, concentrated hydrochloric acid and then 750 ml of formalin (low in methanol). The mixture is removed from the ice bath and stirred at room temperature for 7 hours. The layers are separated and the aqueous phase is back-extracted twice with chloroform. The combined organic layers are washed twice with a 5% solution of sodium bicarbonate, and twice with a saturated salt solution. The solution is dried over magnesium sulfate and evaporated to dryness under reduced pressure. 

IB) Preparation of the Chioromercuri Acid Sodium Sait Soiution 50.6 g (= 0.100M) of the chioromercuri acid (dried over CaClj at 0.1 mm and room temperature overnight) is dissolved in 100 ml of warm methanol. To this solution 6,0 g (= 0.111 M) of sodium methylate is added in small portions with constant stirring, so that the temperature of the solution does not rise over 30°C. The solution is centrifuged, and the glass is rinsed with 10 ml of methanol. The final pH of the combined solutions is 8.5. 

The Dissociation Constant of Nitric Add. Alodcrately Weak Acids. The Variation of J with Temperature. Proton Transfers between Solute Particles. A Proton Transfer in Methanol Solution. Proton Transfers with a Negative Value for. / . The Hydrolysis of Salts. Molecules with Symmetry. Substituted Ammonium Ions. Deuteron Transfers in D2(). The Dissociation of Molecular Ions. 

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