Solvents relative basicities

The relative basicity and acidity of isothiazole and its methyl derivatives have been compared by IR spectroscopy (77MI41702). The isothiazoles, dissolved in inert solvents (e.g. CCI4, CS2) containing traces of butanol (a proton donor), interact with the butanol OH... 

The QSAR technique, widely developed by Kamlet, Taft and coworkers38,98 for the prediction of specific solute-solvent interactions, has been used to predict the different solute-solvent contributions to property variations of compounds. The influence of solvent on the relative basicity of dipolar trimethylamines has been recently studied a descriptor was developed to describe a unique solute-solvent interaction involving dipolar amines99. 

The multiparametric equation 14 has been also applied to estimate solvent effects on the relative basicities (SAG) of propylamines97 ... 

TABLE 9. Solvent solvatochromic parameters0 and the relative basicities (SAG) of propylamines in the gas phase and various solvents (values are in kcal mol-1 at 298 K and relative to diethylamine)97. Reprinted with permission from Reference 97. Copyright (1995) American Chemical Society... 

Table 9 shows the basicity variations of propylamines in the gas phase and in different solvents, relative to diethylamine the equilibrium is given by equation 15. 

Also the value reported by Haney and Franklin of 186 1 kcal/mol is in agreement. The latter workers determined the proton affinity of ammonia as 207 kcal/mol which is approximately 21 kcal/mol larger than that for phosphine. The greater basicity of ammonia as compared to phosphine is showi by the difference of about 20 pH units in their relative basicities in aqueous solutions. The difference in the basicities of the aqueous solutions of 23-32 kcal/mol, which is comparable to that in the gas phase, leads to the somewhat surprising conclusion that solvent effects appear to play an unessential part in the relative basicities of PH3 and NH3 in aqueous solutions. The proton affinities of HjO and H2S, 164 and 170 kcal/mol, respectively, are in the reverse order. 

Figure 6-17 Solvent-Corrected 6-31G vs. Experimental Aqueous-Phase Relative Basicities of Amines...

It becomes complex if we try to compare anions having nucleophilic centres from different parts of the periodic table. In such a case, relative nucleophilicity does not match relative basicity. This is because the solvent used in a reaction has an important effect. In protic solvents like water or alcohol, the stronger nucleophiles are those which have a large nucleophilic centre, i.e., an atom lower down the periodic table (e.g. S" is more nucleophilic than O but is less basic). This is because protic solvents can form hydrogen bonds to the anion. The smaller the anion, the stronger the solvation and the more difficult it is for the anion to react as a nucleophilic. 

The results for the free energies of transfer of cations do not present as clear a picture. They suggest that solvents such as DMSO which contain a relatively basic oxygen atom solvate cations better than does water. To account for these solvation differences, cation-dipole interactions and solvent structure-making or -breaking effects could be invoked. Some cations (e.g., R3NH+) could also be stabilized in DMSO because of their ability to act as hydrogen bond donors. 

A characteristic feature of some of these reactions is the dependence of their efficiency on the basicity of the radical anion [108], The differences are especially manifested in non-polar solvents, where the CIP are expected to dominate. Some of these cleavage processes are more efficient than expected, based on the thermodynamic evaluations of the unassisted fragmentation (Sect. 3.2). Also a stereochemical preference for cleavage is observed for erythro isomers as compared to the threo isomers (Scheme 6). In benzene erythro/threo selectivity is high, being highest for the relatively basic radical anion of dicyanonaphthalene and lowest for the relatively nonbasic radical anion of thioindigo. The stereochemical preference disappears in acetonitrile if biphenyl is used as a co-sensitizer [108]. 

Solvent effects may influence strongly differences between pATa values of amines and sometimes even reverse them216,217. A well-known example is the relative basicity of tertiary amines which, in water, are weaker bases than secondary amines, but in the gas phase this situation is reversed270,283,284. 

Solvent designation Relative acidity Relative basicity Examples... 

The data plots of Fig. 15b (silica) are differentiated for the use of methyl- -butyl ether (MTBE) or acetonitrile (ACN) as localizing solvent C in the mobile phase. It is seen that for some solute pairs (Fig. 15a and c) the open squares (MTBE) fall on a different curve than the closed squares (ACN). This implies that the constant in Eq. (31a) is solvent-specific, rather than being constant for all solvents (as first-order theory would predict). A similar behavior is observed for alumina as well. Figure 15a plots data for 18 different polar solvents B or C, and some scatter of these plots of log a versus m is observed here, as in Fig. 15b for silica. The variation of Q with the localizing solvent C used for the mobile phase has been shown (18) to correlate with the relative basicity of the solvent, or its placement in the solvent classification scheme of Refs. 40 and 41. Thus, for relatively less basic solvents (groups VI or VII in Refs. 40 and 4/),... 

---