Lewis basicity measurement

When interpreting Lewis basicity measurements, it is expected that the adduct structure, in particular the site of interaction, can be identified experimentally or otherwise. This question, which seems trivial for simple monofunctional ligands, may be more difficult to answer when the molecular complexity increases, since several potential coordination... 

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... 

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... 

Fig. 10. Limiting single ion mobility-viscosity product for alkali metal ions vr. solvent Lewis basicity (as measured by enthalpy of reaction with SbCl5)...

The measurement units of each parameter give a preliminary indication of the nature of these parameters, but for a more precise idea of their chemical and physical significance the reader is referred to the literature7-9. In the present context it is sufficient to bear in mind that most of these empirical parameters can be subdivided into parameters which measure the Lewis acidity (hence, the electrophilic power) and Lewis basicity (hence, the nucleophilic power) of a solvent. 

Eor the former solvent log D = -1.92 (experimental) vs. -1.98 (calculated) and for the latter log D = 0.76 (experimental) vs. 0.88 (calculated). Obviously, succinic acid with two carboxylic groups that strongly donate hydrogen bonds (assigned a = 1.12 as for acetic acid) prefers the basic (in the Lewis basicity, hydrogen-bond-accepting sense) tri-n-butyl phosphate ( i = 0.82, measured for the wet solvent) over water (P = 0.47) and naturally also chloroform (P = 0.10). 

The thermodynamic tendency of a substance to act as a Lewis base. The strength of a Lewis base depends on the nature of the acid with which the Lewis base forms a Lewis adduct. Hence, comparative measures of Lewis basicities are given by equilibrium constants for the formation of the adducts by a common reference acid. See Lewis Base Nucleophilicity Hard Bases Soft Bases Donor Number... 

The ability of a solvent to donate a pair of electrons of one of its donor atoms towards the formation of a coordinate bond with an acceptor atom of a solute is a measure of its Lewis basicity. Several methods have been proposed over the years to express this donor ability or donicity, but only few of them have proved to be viable and of any real usefulness. 

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... 

Laurence, C., Gal, J. (2010). Lewis Basicity and Affinity Scales Data and Measurement. New York John Wiley Sons. Includes a discussion of the various acid-base theories and quantitative scales for judging the strengths of Lewis acids and bases. 

Table 2 also indicates that the nucleophiles effective for vinyl ethers are relatively mild, when compared with those for isobutene (cf., Section V.B.2). In fact, stronger bases lead to inhibition or severe retardation of polymerization [36,64] ketones aldehydes, amides, acid anhydrides, dimethyl sulfoxide (retardation) alcohols, aliphatic amines, pyridine (inhibition). The choice of nucleophiles is determined by their Lewis basicity (as measured by pKb, etc. [64,103]), and this factor determines the effic-tive concentrations of the nucleophiles. For example, the required amounts of esters and ethers decrease in the order of increasing basicity (i.e., a stronger base is more effective and therefore less is needed) [101,103] tetrahydrofuran < 1,4-dioxane ethyl acetate < diethyl ether. On the other hand, for amines not only basicity but also steric factors play an important role [142] thus, unsubstituted pyridine is an inhibitor, while 2,5-dimethylpyridine is an effective nucleophile for controlled/living polymerization, although the latter is more Lewis basic. 

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 remarkable Lewis basicity scale for 75 non-HBD solvents has been established by Gal and Maria [211, 212]. This involved very precise calorimetric measurements of the standard molar enthalpies of 1 1 adduct formation of EPD solvents with gaseous boron trifluoride, A//p gp, in dilute dichloromethane solution at 25 °C, according to Eq. (2-10a). 

Because of the rather localized negative charge at the phenoHc oxygen atom , the standard dye (44) is capable of specific HBD/HBA and Lewis acid/base interactions. Therefore, in addition to the nonspecific dye/solvent interactions, the betaine dye (44) predominately measures the specific HBD and Lewis acidity of organic solvents. On the other hand, the positive charge of the pyridinium moiety of (44) is delocalized. Therefore, the solvent Lewis basicity will not be registered by the probe molecule (44). If this solvent property is relevant for the system under study, other empirical measures of Lewis basicity should be used cf. Section 7.7. 

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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