Solvent basics

The acid-base properties of isoxazole and methylisoxazoles were studied in proton donor solvents, basic solvents or DMSO by IR procedures and the weakly basic properties examined (78CR(Q(268)613). The basicity and conjugation properties of arylisoxazoles were also studied by UV and basicity measurements, and it was found that 3-substituted isoxazoles were always less basic than the 5-derivatives. Protonation increased the conjugation in these systems (78KGS327). 

The enthalpy of the H-bonds among the majority of the organic compounds is relatively low (usually within the range of about 20 kJ per one mol of hydrogen bonds) and therefore they can easily be disrupted. In order to demonstrate the presence of lateral interactions in chromatographic system, low-activity adsorbents are most advisable (i.e., those having relatively low specific surface area, low density of active sites on its surface, and low energy of intermolecular analyte-adsorbent interactions, which obviously compete with lateral interactions). For the same reason, the most convenient experimental demonstration of lateral interactions can be achieved in presence of the low-polar solvents (basically those from the class N e.g., n-hexane, decalin, 1,4-dioxane, etc.) as mobile phases. 

The a scale of solvent acidity (hydrogen-bond donor) and the (3 scale of solvent basicity (hydrogen-bond acceptor) are parameters derived from solvatochromic mea-siuements used in adsorption chromatography [51,54,55]. 

As BH dissociates into H+ and the uncharged base B, the dielectric constant can exert only a minor effect on the mutual coulombic attraction, so that even in water (e = 78.5) the pKa values of aliphatic amines do not differ much from the above picture of the influence of solvent basicity. That piiTa(water) lies between ptfa(m.cresoi) and p a(acetlc acid) instead of between the latter and p.Ka(pyridine) may be ascribed to effects of solvation however, the p a(Water) values of the aromatic amines are low owing to effects of mesomerism. 

Significant factor as same as in a case of swelling degree is the solvents basicity. With the solvents basicity increasing, the process rate is also increased. The less essential is a role the solvents ability to electrophilic solvation although this factor increases the process rate but it exclusion from the consideration decreases R till 0,928. The value IgQ calculated in accordance with the equation (15) is represented in Table 3. 

A third method of estimating solvent basicity is provided by the donor number concept 14 ). The donor number of a solvent is the enthalpy of reaction, measured in kcal per mole, between the solvent and a Lewis add such as antimony (V) chloride. (Other Lewis acids, such as iodine or trimethyltin chloride, may be used, but the scale most often reported is that for SbCl5.) Available values for the SbCls donor number have been included in Table 1. Plots of the Walden product versus solvent basicity (A//SbC1 ) for several solvents are shown for lithium, sodium, and potassium ions in Fig. 10 and for the tetraalkylammon-... 

As outlined in Section 1.3, the solvent acidity and basicity have a significant influence on the reactions and equilibria in solutions. In particular, differences in reactions or equilibria among the solvents of higher permittivities are often caused by differences in solvent acidity and/or basicity. Because of the importance of solvent acidity and basicity, various empirical parameters have been proposed in order to express them quantitatively [1, 2]. Examples of the solvent acidity scales are Kosower s Z-values [8], Dimroth and Reichard s Er scale [1, 9], Mayer, Gutmann and Gergefs acceptor number (AN) [10, 11], and Taft and Kalmefs a parameter [12]. On the other hand, examples of the solvent basicity scales are Gut-... 

The values of DN are listed in Table 1.5 in increasing order. The solvent basicity increases with the increase in the DN value. The DN value for DCE (reference solvent) is zero. 

Figure 4.8 shows the potential windows obtained at a bright platinum electrode, based on the Fc+/Fc (solvent-independent) potential scale. Because of the overpotentials, the window in water is 3.9 V, which is much wider than the thermodynamic value (2.06 V). The windows for other solvents also contain some overpotentials for the reduction and the oxidation of solvents. However, the general tendency is that the negative potential limit expands to more negative values with the decrease in solvent acidity, while the positive potential limit expands to more positive values with the decrease in solvent basicity. This means that solvents of weak acidity are difficult to reduce, while those of weak basicity are difficult to oxidize. This is in accordance with the fact that the LUMO and HOMO of solvent molecules are linearly related with the AN and DN, respectively, of solvents [8]. 

For another approach to solvent basicity scales, see Catalan G6mez Couto Laynez J. Am. Chem. Soc. 1990, 112. 1678. 

Similar arguments allow an explanation of the change of stereochemistry observed with Grignard reagents when increasing the solvent basicity (Table VIII). The addition of a basic solvent (THF, DME) implies a modification of the carbon-magnesium MO, in which the latter become more contracted on the carbon atom (Scheme 16). As a consequence the nucleophile is smaller in size and the stereochemistry is shifted to retention (Tables VI and VIII). 

When 2 equiv of HO" are available, the anodic process occurs at +0.14 V versus SCE with DTBCH- the electroactive species. Table 12.2f,g summarizes the effect of solvent basicity on the oxidation potentials for several catechols and benzohydroquinone. The basicity of the solvent as well as of the substituents on the aromatic ring of the catechols causes their oxidation potentials to shift to less positive values (Table 12.2). 

Solvent polarity influences the dissociation of the intimate ion pair, while the fate of the ion-radicals depends mainly on the solvent basicity. In non-basic solvents (benzene or dichloromethane), cationic polymerization of VCZ takes place. In moderately basic solvents (acetone or acetonitrile), VCZ cyclodimerizes. Radical polymerization occurs along with cycloaddition in strongly basic solvents, such as DMF and DMSO, while only radical polymerization takes place in the extremely basic solvent hexamethylphosphoric triamide (HMPA). 

For the strong donor monomer VCZ, the photoreaction depends on the solvent basicity and the molar ratio of the donor and the acceptor. In strongly basic solvents such as dimethyl formamide (DMF), the radical homopolymerization of VCZ occurs in the presence of catalytic amounts of FN or diethyl fumarate (DEF), but it is replaced by radical copolymerization in an equimolar amount of the monomers. The cationic homopolymerization of VCZ, which proceeds in less basic solvents, e.g., benzene, and the cyclodimerization of VCZ, which proceeds in moderately basic solvents, e.g., acetone, is accompanied by the radical copolymerization of VCZ with FN or DEF [6],... 

Shirota [5] has postulated a scheme involving such dimer cationic radicals as the intermediates for cyclodimerization and polymerization of VCZ. The ratio of the competing reactions depends mainly on the solvent basicity. In strong basic solvents only radical polymerization occurs, while it is accompanied by cyclodimerization of VCZ in moderate basic solvents and by cationic homopolymerization of VCZ in non-basic solvents. 

The influence of dicarboxylic acid ester plasticisers on the thermal degradation of PVC significantly depends on the physical state of the PVC-plasticiser system. If PVC retains the structure formed in the stage of suspension polymerisation, the additive produces inhibition of the process of thermal dehydrochlorination. In the case of true diluted PVC solutions in ester plasticisers, the polymer exhibits accelerated degradation, in accordance with a high value of the solvent basicity. 7 refs. 

Finally, the ambitious approach of Catalan et al. to introduce complete new comprehensive scales of solvent dipolarity/polarizabihty [SPP scale), solvent basicity SB scale), and solvent acidity SA scale) must be mentioned [296, 335-337]. These three UV/Vis spectroscopic scales are based on carefully selected positively solvatochromic and homomorphic pairs of probe dyes and include values for about 200 organic solvents for a recent review, see reference [296]. The molecular structures of the three pairs of homomorphic indicator dyes proposed are as follows ... 

A selection of SB values is collected in Table 7-5. Not unexpectedly, a satisfactory linear correlation exists between Catalan s SB values and Kamlet and Taft s y values (see Table 7-4) for 98 solvents (r = 0.928), with deviations for some aliphatic amines and ethers with long alkyl chains. For comparisons of the SB scale with further solvent basicity scales, see reference [336]. 

A more simplified but likewise sueeessful empirical two-parameter approach for the deseription of solvent effects has been proposed by Krygowski and Fawcett [113]. They assume that only specific solute/solvent interactions need to be eonsidered. These authors postulated that the solvent effeet on a solute property A can be represented as a linear funetion of only two independent but eomplementary parameters describing the Lewis aeidity and Lewis basicity of a given solvent. Again, for reasons already mentioned, the t(30) values were chosen as a measure of Lewis acidity. In addition, Gut-mann s donor numbers DN [26, 27] were chosen as a measure of solvent basicity cf. Table 2-3 and Eq. (7-10) in Sections 2.2.6 and 12, respectively). Thus, it is assumed that the solvent effect on A can be described in terms of Eq. (7-62) . 

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