The Donor Number

These findings lead to the definition of the donor-number for each of the solvents. The donor number is defined as the numerical quantity of the —/IHd sbcu value26. 

The donor number is nearly a molecular property of the solvent, which is easily determined by experiment. It expresses the total amount of interaction with an acceptor molecule, including contributions both by dipole-dipole or dipole-ion 

Gutmann, V., and E. Wychera Inorg. Nucl. Chem. Letters 2, 267 (1966). 

Thus the donor number is considered a semiquantitative measure of solute-solvent interactions. It is recognised that no allowance has been made for specific interactions between certain individual donor-acceptor species. Such refinements are currently impossible owing to a lack of knowledge, and indeed of experimental data, as to the precise nature of such specific interactions. 

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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... 

On the basis of the results in acetonitrile, it might be reasonable to assume that the values for A//het(R-R ) and AG°het(R-R ) are apparently close to each other also in sulfolane, since the dielectric constant (43.3) and the donor number (14.8) of this solvent are close to those of acetonitrile (37.5 and 14.1, respectively). On the basis of this assumption, Arnett s equation (28) was examined for reactions of type (23). For these reactions, except for [3-2], only the AGhet(R-R ) values are avtiilable. As shown in Fig. 3, the values for this system are about 10 kcal moP less than predicted from (28). The negative deviation can also be ascribed to steric congestion in these hydrocarbon molecules. The large negative deviations, similar to those observed in sulfolane, are also seen in Fig. 3 for the values of AGSet(R-R ) in DMSO. 

The best-known solvent parameters are the donor number [21] and acceptor number [22] proposed by Gutmann and coworkers. The donor number (DN) for a donor solvent D is defined as the positive value of the enthalpy difference AH (kcalmol ) for the reaction of D with an acceptor-halide SbCls (D + SbCls D SbCls) in an inert medium such as 1,2-dichloroethane. DN is a fair measure for the donor properties of a solvent. The correlations of DN with the solvation energies are known to be good particularly for solvation of cations. A typical example [19] is shown in Fig. 3. 

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... 

The donor number has been defined as the negative AH values for the formation of 1 1 adduct between solvents and antimony pentachloride, in 1,2-dichloroethane, at 25 °C8. In equilibrium 1, S is the solvent (with an unshared electron pair). 

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-... 

The metal-solvent interaction is expected to depend on the donicity of the solvent the higher the donor number of the solvent the stronger the solvent-metal interaction should be. Hence, a correlation between the contact potential difference A>// (a = 0) and the donor number of the solvent should be observed. However, this correlation for the Hg electrode is rather poor, with the most deviant point having been found for water, that is, for the case of the strongest dipole-dipole interaction in the bulk. The correlation is better when acceptor numbers of solvents are taken into account. ... 

The major disadvantage of the HSAB principle is its qualitative nature. Several models of acid-base reactions have been developed on a quantitative basis and have application to solvent extraction. Once such model uses donor numbers [8], which were proposed to correlate the effect of an adduct on an acidic solute with the basicity of the adduct (i.e., its ability to donate an electron pair to the acidic solute). The reference scale of donor numbers of the adduct bases is based on the enthalpy of reaction. A//, of the donor (designated as B) with SbCb when they are dissolved in 1,2-dichloroethane solvent. The donor numbers, designated DN, are a measure of the strength of the B—SbCb bond. It is further assumed that the order of DN values for the SbCb interaction remains constant for the interaction of the donor bases with all other solute acids. Thus, for any donor base B and any acceptor acid A, the enthalpy of reaction to form B A is ... 

Gutmann acceptor numbers were determined in the "usual" way via the chemical shift variation of triphenylphosphine oxide by Osteryoung et al. [26]. While, again, the donor numbers were concentration- and composition-independent for basic melts, the acidic melts showed a strong composition dependence. Nonetheless, the acidity range was comparably small and was found around 100 (which compares to the acidity of trifluoroacetic acid). The donor number for basic melts was found to be 98, which was, of course. 

An important chemical measure of solvent polarity is the donor number, DN. It is a measure of the Lewis base donor power of the solvent. DN is defined as the negative enthalpy for the reaction of the solvent with the standard Lewis acid SbCls (eqn. 3.8). The enthalpy measurements are carried out in an inert solvent, 1,2-dichloroethane, which has a donor number of zero. The units are kcal/mol. 

The converse of the donor number is the acceptor number, AN, of a solvent. This is a measure of its power to accept electron pairs as a Lewis acid. There are a number of ways of assessing the acceptor ability. One convenient way is to use NMR to measure the interaction between a standard reference Lewis base, (C2H5)3PO, and the solvent by looking at the shift (eqn. 3.9). 

The donor number (DN) of a solvent is defined as the enthalpic change at 25°C for formation of the adduct between SbCIs and the solvent in 1,2-dichloroethane. 

The donor number, DN [11, 13], of solvent D (Lewis base) is determined calori-metrically as the negative value of the standard enthalpy change, -AH° (in kcal mol-1), for the 1 1 adduct formation between solvent D and antimony pentachlor-ide (SbCls), both being dilute, in 1,2-dichloroethane (DCE) at 25 °C [Eq. (1.5)] ... 

Fig. 2.3 Standard Gibbs energies of transfer of the potassium ion from AN to other solvents and standard potentials of the hydrogen electrode, both plotted against the donor number of solvents [13].

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. 

Fig. 4.4 Relation between the half-wave potentials of K+ (V vs BCr+/BCr) in various organic solvents and the donor number of solvents [3]. For solvents, see Fig. 4.2.

Fig. 8.4 Relation between the halfwave potentials of metal ions (V vs BCG/BCr) and the donor number of solvents O Na+ O TT Ba2 ...

Thus this model is capable of explaining the variation of redox potential with solvent in terms of the coordinative interactions between the ions of the redox couple and the solvent, i.e. redox potentials are determined by the donor or acceptor properties of the solvents. An important point of this model is that the nature of the binding forces between the solute and solvent are not specified. It should therefore be possible, once the linear relationship between the redox potential and the donor number or acceptor number of the solvent is established, to predict the value of the redox potential of that same couple in another solvent. 

Solvents with a large donor number tend to associate with hydrogen donors such as water or chloroform. This may explain the finding19 that the stability of the anthracene and naphthalene anion radicals in the presence of small amounts of added water is greater in dimethylformamide (DMF) and dimethyl sulfoxide (Me2SO) than in acetonitrile. This stability order parallels the donor numbers of the solvents and reflects the greater tendency of DMF and Me2SO to associate with water. The latter causes the protons on water to be less... 

Cu(acac)(tmen)]BPh4, is known to provide a good correlation between the donor number (DN) of the solvent and the /,max corresponding to the lowest energy of d-d transition [84], In spite of the small number of experiments, there is a certain relation between anion species and T.max, as shown in Table 3.10. 

The donor number (DN), introduced by Gutmann and Mayer [15-19], represents a measure of the donor properties of solvents and is defined as the numerical value of the heat of adduct formation between the donor molecule and the reference acceptor SbCl5 in dilute 1,2-dichloroethane solution ... 

The association constants of the above salts are the highest in nitromethane (low DN), lower in acetonitrile (medium DN), and negligible in DMF (high DN). These results are well understood in light of the meaning of the donor number, as they reflect the ability of a solvent to interact with cations, acids, and other... 

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