Mixed solvent basicity

In contrast molecular interaction kinetic studies can explain and predict changes that are brought about by modifying the composition of either or both phases and, thus, could be used to optimize separations from basic retention data. Interaction kinetics can also take into account molecular association, either between components or with themselves, and contained in one or both the phases. Nevertheless, to use volume fraction data to predict retention, values for the distribution coefficients of each solute between the pure phases themselves are required. At this time, the interaction kinetic theory is as useless as thermodynamics for predicting specific distribution coefficients and absolute values for retention. Nevertheless, it does provide a rational basis on which to explain the effect of mixed solvents on solute retention. 

PSS columns for medium polar or mixed solvents PSS HEMA and PSS SUPREMA Basic were designed to allow SEC separations in polar media such... 

As already indicated, ion exchange resins are osmotic systems which swell owing to solvent being drawn into the resin. Where mixed solvent systems are used the possibility of preferential osmosis occurs and it has been shown that strongly acid cation and strongly basic anion resin phases tend to be predominantly aqueous with the ambient solution predominantly organic. This effect (preferential water sorption by the resin) increases as the dielectric constant of the organic solvent decreases. 

Assuming Ks = 5 mM in 60% (v/v) aqueous DMSO (since it is 2 mM in 50% aqueous DMSO), Xxs = 9.8 ju.m, as compared to 0.37 /am in water transition state binding is 26 times weaker in the mixed solvent. More surprising, however, k2 (= kc/Ks) is the same in both media (Table 3). Thus, the much faster cleavage of the ester by /3-CD in 60% aqueous DMSO originates from two factors (i) the enhanced nucleophilicity and basicity of anions in the mixed medium (Reichardt, 1988) and (ii) substantially weaker substrate binding (1/50) in 60% aqueous DMSO while transition state binding is weakened less (1/26). Of the two, the first factor is much more important. The virtual equality of k2 in the two media arises because the 48-fold increase in kc is matched by the 50-fold increase in Ks (Table 3). 

The SP procedure of water-soluble vitamins from multivitamin tablets is particularly challenging due to the diverse analytes of varied hydrophobicities and pfC. Water-soluble vitamins (WSVs) include ascorbic acid (vitamin C), niacin, niacinamide, pyridoxine (vitamin B ), thiamine (vitamin Bj), folic acid, riboflavin (vitamin B2) and others. While most WSVs are highly water soluble, riboflavin is quite hydrophobic and insoluble in water. Folic acid is acidic while pyridoxine and thiamine are basic. In addition, ascorbic acid is light sensitive and easily oxidized. The extraction strategy employed was a two-step approach using mixed solvents of different polarity and acidity as follows ... 

The acid-base properties of a mixed solvent is also an important factor influencing the behavior of solutes. Thus, the parameters of the acidity and basicity of mixed solvents have been studied to some extent [35], Figure 2.10 shows the donor numbers of mixtures of nitromethane and other organic solvents. Because ni-tromethane has very weak basicity (DN= 2.7), the addition of small amounts of basic solvents (HMPA, DMSO, pyridine) increase the donor number remarkably. 

Figure 2.11, on the other hand, shows the acceptor numbers of mixtures of water and aprotic solvents. Because water is protic and selectively interacts with Et3P = 0 (strong Lewis base), many of the relations curve upward. However, with HMPA, the relation curves downward, because HMPA is a strong base and easily interacts with water to weaken the interaction between water and Et3P = O. The acidity and basicity of mixed solvents are influenced not only by the acidity and basicity of the constituent solvents but also by the mutual interactions between the molecules of constituent solvents. At present, however, this cannot be treated theoretically. 

Among amphiprotic solvents of high permittivities, there are water-like neutral solvents (e.g. methanol and ethanol), more acidic protogenic solvents (e.g. formic acid), and more basic protophilic solvents (e.g. 2-aminoethanol). There are also amphiprotic mixed solvents, such as mixtures of water and alcohols and water and 1,4-dioxane. The acid-base equilibria in amphiprotic solvents of high permittivity can be treated by methods similar to those in aqueous solutions. If the solvent is expressed by SH, the acid HA or BH+ will dissociate as follows ... 

The reactions of 160 in H20-DMS0 mixed solvents (0.5M ionic strength, Me4N+CT) follow the same pattern as in water and have been treated by a similar kinetic analysis.224 On transfer from water to mixed solvent a considerable increase was found in the equilibrium constants and rates of formation of 161 in acidic (Ku kt) and in basic conditions (K2, k2), as expected from the known tendency of DMSO to solvate polarizable anions. Under the stated acidic conditions the H20 molecule actually acts as the nucleophile. 

Thus hydrobromic acid is more strongly stabilized in monoglyme + water mixtures than in water. Moreover, the negative values of AG for the experimental mixed solvent compositions support the view that water is less basic than the mixed solvents, if it is assumed that the hydration of a larger bromide ion in aqueous solution is negligible, although our data indicates that the hydration number of chloride in aqueous solution might not be zero (22). 

J- for Hydroxide Solutions in Aqueous Ethanol. From the pK2(H20) values and values of log CArCH(OH)cr/CArCHO a a given Coh- in a given solvent mixture, it is possible to calculate J- values for the solvent mixture under consideration using Equation 1 where pKw is the autoprotolytic constant of water and pK2(H20) is inserted for pK2- This definition expresses J values with reference to a standard state in pure water, and therefore basicities of sodium hydroxide solutions in mixed solvents can be compared to basicities of sodium hydroxide solutions in water by J values. 

It was found that basic alumina worked well as the basic catalyst for the in situ dipole generation from the AT-acyl pyridinium salt. A three-component mixture of phenacyl bromide (1 mmol), pyridine (1.2 equiv.) and the acetylene (1.2 equiv.) was thoroughly mixed in basic alumina (1 g) and then irradiated for 8 min at 80% power in a domestic microwave. The products were formed in 87-94% yields when running the reaction under solvent-free conditions and in 60-71% yields when using anhydrous toluene as the solvent. 

Heteropoly acids and salts that undergo partial hydrolytic degradation in water to produce hydrogen ion can be stabilized in mixed solvents, such as water dioxane, water acetone, water alcohol, etc.ls 114 Thus, when HafPMo O ] is potentio-metrically titrated with base in aqueous solution, it behaves as a six to seven basic acid, however, when similar measurements were carried out in 1 1 water-acetone or water dioxane, the acid was found to be tribasic15,114 ... 

Reactive absorption processes occur mostly in aqueous systems, with both molecular and electrolyte species. These systems demonstrate substantially non-ideal behavior. The electrolyte components represent reaction products of absorbed gases or dissociation products of dissolved salts. There are two basic models applied for the description of electrolyte-containing mixtures, namely the Electrolyte NRTL model and the Pitzer model. The Electrolyte NRTL model [37-39] is able to estimate the activity coefficients for both ionic and molecular species in aqueous and mixed solvent electrolyte systems based on the binary pair parameters. The model reduces to the well-known NRTL model when electrolyte concentrations in the liquid phase approach zero [40]. 

HC1. The aqueous extracts were pooled, washed with CH2CI2, and made basic by the addition of 25% NaOH. The precipitate that formed was extracted into several small portions of CH2CI2 which were pooled and dried with anhydrous Na2S04. After removal of the drying agent, the solvent was removed under vacuum. To the residue there was added a 1.0 M solution of HC1 in anhydrous Et20 until the mixture was neutral, as determined by external, damp pH paper. The resulting solid was removed by filtration and twice recrystallized from aMeOH/acetone mixed solvent. There was thus obtained N,N-diethyl-2-methyltryptamine hydrochloride (2-Me-DET) as white crystals with a mp 214-216 °C. 

The other point that was discovered was that some reaction rates were accelerated by operating in a mixed solvent rather than in pure water. The one that was examined most carefully was the acetyl transfer from bound ra-f-butylphenyl acetate to /3-cyclodextrin with buffers that in water give a pH of 9.5. It was observed that the reaction was almost 50-fold accelerated in a 60% DMS0-H20 solvent compared with the reaction rate in pure water. Part of this acceleration came from an increase in the apparent basicity of the medium, since relative pK s are solvent dependent part of it was also a solvent effect on the reaction rate of the cyclodextrin anion with the substrate. Thus, in 60% DMSO-H20 the /3-cyclodextrin reaction with this substrate was 13,000-fold faster than was the rate of hydrolysis of the substrate in an aqueous buffer of the same composition. Of this approximately 50-fold acceleration over cyclodextrin in water, about 10-fold was caused by changes in the pK s in the system and about 5-fold was caused by a change in the reaction rate of the cyclodextrin. 

The concept of acidity and basicity in mixed solvents is discussed and a method for analyzing differential solvation effects is described. This enables the free energy of transfer of the proton between water and the mixed solvent, AGt°(H+), to be calculated, and thereby AGt°fi) for i = X" and M+, using values for AGt°(HX) and AGt°(MX). The pKa values for acids are combined with AGt°(H+) to calculate proton affinities in mixed solvents, and these are used as measures of free energies of transfer of the charges on the molecular species, AGt°(i)e. Values of AGt°(i) and AGt°(i)e are compared for a range of co-solvents and the factors influencing the way these quantities vary with solvent composition are discussed. 

The problems involved in the measurement of acidities or relative acidities of weak acids are illustrated by the widely different estimates which have been given for the acidity of substituted acetylenes. Two different approaches have been used for measuring the equilibrium acidity of carbon acids which do not ionize in the pH range. In one approach, the ionization of a carbon acid is studied in mixed solvents containing base. Some of these solutions are more basic than aqueous solutions and by varying the solvent mixture the ionization of acids with pK values in the range 12—25 can be studied. Values at the low end of the pK range are directly compared with aqueous p/iC values. It is assumed that ratios of the activity coefficients (f) for the ionized (S-) and unionized acids (SH) are the same for all the acids studied and an acidity function (86)... 

As an example we mention the dependence of the free energy of transfer of man-ganese(II) ions from water to water-acetonitrile mixtures on the parameter [85]. It is a complicated nonlinear dependence, which suggests that the interaction of /7-nitroaniline (used in determination of nd manganese(II) with aqueous solutions of acetonitrile is totally different. One may expect that complicated dependences of AG,r or E° EI/2) potentials on parameters of the Lewis basicity or acidity of mixed solvents will be observed for other ions also. Such relationships of the solvation abilities of water-organic solvent mixtures in respect to different cations are determined by several factors ... 

The change from inhibition to acceleration of the rate of electrode reaction occurs exactly at the mixed solvent composition at which the process of reactant resolvation begins in the bulk of the solution. It appears that even partial resolvation of vanadium(III) ions (Avv(in)>0) initiates the increase of the rate of reaction in the surface phase, when the concentration of the organic solvent is considerably higher there than in the bulk. Such behavior is observed in mixtures of water with solvents of Lewis basicity lower (AN) and also higher (HMPA) than water. 

In the case when resolvation already occurs at a very low concentration of the added second solvent, in which the reaction being studied is faster than in water, an increase of the rate constant is observed instead of a minimum (H2O-DMF mixtures in Fig. 16). The composition of the mixed solvent at which the change from inhibition to acceleration is observed does not follow the order of Lewis basicity of the solvents involved, but is a function of the affinity of the added solvent for the electrode surface and for the reactant. 

Neutral solvents The term neutral solvent applies here to solvents not predominately either acidic (protogenic) or basic (protophilic) in character. Some are weakly basic but not appreciably acidic (ethers, dioxane, acetone, acetonitrile, esters), some aprotic (benzene, carbon tetrachloride, 1,2-dichloroethane), and some amphiprotic solvents (ethanol, methanol). Aprotic solvents are used mainly in mixed solvents to alter the solubility characteristics of the reactants. 

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