Lewis basicity of solvents

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

In ion solvation, the solvent molecules approach a cation with their negative charge and approach an anion with their positive charge (Fig. 2.1). Therefore, cation solvation is closely related to the electron pair donor capacity or Lewis basicity of solvents and tends to become stronger with the increase in donor number (DN). On the other hand, the anion solvation is closely related to the electron pair acceptability or Lewis acidity of solvents and tends to become stronger with the increase in acceptor number (AN). 

Physical organic chemists have tended to examine parameters based on shifts in the absorption peaks in the spectra of various dyes or indicator molecules. The a and P scales of Taft and Kamlet, the ET(30) scale of Dimroth and Reichardt, the 7t scale of Taft and co-workers and the Z value of Kosower are all examples of this type of parameter. The definitions and measurement means for these parameters, as well as important references, are shown in Table 5. An alternative definition of the Dimroth-Reichardt parameter is the dimensionless, ETN, which is now preferred by some organic chemists (for a discussion see Ref. 15). The Z value is important in that it led to the scale of Dimroth and Reichardt, which overcomes many of the limitations of the earlier scale. Several workers have shown that relationships exist, with good correlation coefficients, between similar parameters. Thus, DN is linearly related to p, both parameters being designed to measure the donor properties (or Lewis basicity) of solvent molecules. Also, Lr(30) is related to a as well as to AN all three parameters purport to measure the electron acceptor properties (or Lewis acidity) of solvent molecules. It has been found that different solvent types have different coefficients in linear relationships between n and the dipole moment. The Taft and Dimroth-Reichardt parameters, in particular, have been found to correlate with free energies and... 

Reduction of 3-trimethy1s11yl-2-propyn-l-ol exemplifies the problem of stereoselectivity in hydride reduction of acetylenic alcohols to E-allyl alcohols, Early reports that lithium aluminum hydride stereoselectively reduced acetylenic alcohols gave way to closer scrutiny which revealed a striking solvent dependence of the stereochemistry, Specifically, the percentage of trans reduction is seen to increase with increasing Lewis basicity of solvent. Similarly, the addition of less Lewis acidic cations to the reducing mixture leads to improved trans/cis ratios, Sodium b1s(2-... 

The DN scale has historical merits. It was among the first attempts to establish Lewis base strength quantitatively. It was widely used in the field of solution, coordination and organic chemistry to describe the Lewis basicity of solvents. However, after 40 years of use, convergent critical analyses have pointed out the serious limitations of DN. They concern (i) the choice of SbCls as a reference Lewis acid, (ii) the sample of Lewis bases studied, (iii) the quality of calorimetric measurements and (iv) the domain of validity of the scale. 

Finally, the total red shift of 5380 cm of the solvatochromic band of 4-nitrophenol, on going from the gas phase to acetonitrile solution, has been resolved into a 785 cm hydrogen-bond shift (15% of the total shift) and a 4595 cm shift caused by nonspecific (van der Waals) interactions. With triethylamine as solvent, the hydrogen-bond shift amounts to 42% of the total shift. These examples show the importance of quantifying the Lewis basicity of solvents for a quantitative description of solvent effects in chemistry. 

The Lewis basicity of the solvents was found to be more important for the solvatochromism than the solvent polarity, since such Si... solvent and Si... F interactions are competitive in the presence of solvents with donor atoms. In contrast, the analogous non-fluorinated polysilane, poly(methyl-/z-propylsilylene), revealed a disordered conformation in both coordinating and non-coordinating solvents. Additionally, the UV spectra of various molecular weight fractions of 87 showed an unusual molecular weight dependency an isosbestic point is apparent, suggesting an equilibrium between globule- and rod-like conformations at room temperature, which was also evident from the... 

The donor number, DN, of a solvent, proposed by Gutmann, is a measure of the Lewis basicity of the solvent, i.e. its ability to donate a pair of electrons [16]. The DN is determined by measuring the negative enthalpy for the reaction of equimolar quantities of the solvent with the standard Lewis acid, SbCls, at room temperature in 1,2-dichloroethane (Scheme 1.1), and reflects the ability of the solvent to solvate Lewis acids. SbCls reacts with protic solvents such as alcohols... 

Complexes 21 can only be isolated as their THF adducts. Attempts to synthesize 21 by using no THF as solvent failed. The Lewis basicity of the Ph O ligands appears to be insufficient to compensate for the electron deficiency at the tungsten atom, and coordination of an additional THF molecule becomes necessary. The high stability of the THF adducts was furthermore confirmed by DFT studies of these compounds. 

The QSC value of the lithium phenolate tetramer in THF solution is between 40 and 46 kHz at —67 and +30 °C, respectively. For other tetramers the QSC ranges from 35 to 57 kHz, depending on substitution pattern and solvent. It was observed that the magnitude of the observed QSC values correlates with the Lewis basicity of the solvent. 

As a rule, lanthanide bromides, and more especially the iodides, are more reactive because of their often higher solubility (Table 5), and also show enhanced thermodynamic lability (Scheme III). Moreover, reactivities different from those of the chloride analogues should be expected because of, for example, the softer Lewis basicity of the iodide anion and different solubility properties of the eliminated alkali salts. Table 5 gives an arbitrary sample of solubilities for lanthanide halides in various standard-laboratory donating solvents [97f]. 

Hydrogen bonds can be either intermolecular or intramolecular. Both types of hydrogen bonds are found in solutions of 2-nitrophenol, depending on the Lewis basicity of the solvent [298]. The intramolecularly hydrogen-bonded form exists in nonhydrogen-bonding solvents e.g. cyclohexane, tetrachloromethane). 2-Nitrophenol breaks its intramolecular hydrogen bond to form an intermolecular one in electron-pair donor (EPD) solvents e.g. anisole, HMPT). 

The donor number of 38.8 kcal moP for HMPT was given by Gutmann [67]. It should be mentioned, however, that a much higher DN value of 50.3 kcal moP was subsequently measured for this solvent by Bollinger et at. [214]. This shows that serious problems arise in measuring the Lewis basicity of this EPD solvent towards SbCls. 

Another example is the y9-ketonitrile (6a,b). Because of the linearity of the cyano group, a cyclic structure with an intramolecular hydrogen bond is impossible. As predicted, it is found that the enol content is greater in polar than in apolar solvents [53], In general, for the protomer pairs in which the enol cannot form an intramolecular hydrogen bond, such as (5a) (5c),ihc tautomeric equiUbrium seems to be controlled almost completely by the hydrogen-bond acceptor property (Lewis basicity) of the solvent. EPD solvents enhance the enol content strongly cf. (5a) in Table 4-2. 

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) . 

Equation (63) was applied to the analysis of the dependence of the rate of cadmium, lithium and sodium deposition on the solvent parameters [4, 239]. These correlations with solvent basicity parameters show, in agreement with the preceding analysis, that all these reactions decrease in rate when the Lewis basicity of the solvent increases. This behavior is common for all the systems analyzed. 

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. 

Zincation of iodide 48 was performed with a two- to three-fold excess of activated zinc [48] in Et20 (since decreasing the Lewis basicity of the solvent considerably speeds up the... 

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