Solvent electron-donor number

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 ... 

Solvatation, solvolysis and ionic dissociation phenomena, in both aqueous and nonaqueous solutions are subsumed by the Lewis definitions. In addition to the previous discussion of the dual polarity character of Lewis acids and bases, it should be noted that many of them are amphoteric, by definition. Donor number, DN, was developed in order to correlate the behavior of a solute in a variety of donor solvents with a given basicity or donicity. A relative measurement of the basicity of a solvent D is given by the enthalpy of its reaction with an arbitrarily chosen reference acid (SbCls in the Gutmann s scale). Latter Mayer introduced an acceptor number, AN, as the relative P NMR shift induced by triethylphosphine, and relative to acidic strength (AN=0 for hexane and 100 for SbCls). In 1989, Riddle and Fowkes modify these AN numbers, to express them, AN ", in correct enthalpic unit (kcaLmol). Table 10.2.3 gathers electron acceptor number AN and AN " and electron donor number DN for amphoteric solvents. 

Oxidation rates depend very strongly on the solvent electron-donor ability. Oxidation is the fastest in DMSO, which is the best electron donor among those investigated [the rate acceleration factor 2x10" (relative to the rate in water where = 1.3x 10 s , at 35 °C, and at pH = 6.0), the electron-donor number, = 29.8, relative permittivity, = 48.9, at 20 °C], see Table... 

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 these studies we decided to carry out a series of AMI and IMA experiments (2) with the TMPCl/EtAlCl2/DtBP combination. Figures 1 and 2 show the results. The M versus Wp (g of poly(P-PIN) formed) plots and the N (number of moles of poly(P-PIN) formed) versus Wp plots (insets) indicate increasing deviation from the theoretical values (calculated for Ieff = 100%). According to these results chain transfer proceeds in these polymerizations, i.e., the systems are nonliving. Further experimentation would be necessary to develop satisfactory living conditions, in particular to investigate the effect of solvent polarity, temperature and electron donors on the mechanism. 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

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). 

By single electron transfer from an electron donor, e.g. a transition metal ion, a trivalent phosphorous derivative or a base, followed by dissociation of the intermediate diazenyl radical in an aryl radical and dinitrogen. The aryl radical reacts with the solvent or with added reagents in various ways, as shown by the relatively large number of classical named reactions (Sandmeyer, Pschorr, Gomberg-Bachmann, Meerwein reactions). 

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). 

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). 

An approach to quantifying the interaction between solute and solvent and hence to solvent effects on redox potentials is that developed by Gutmann.41 Interactions between solvent and solute are treated as donor-acceptor interactions, with each solvent being characterized by two independent parameters which attempt to quantify the electron pair donor properties (donor number)... 

Nucleophilic aromatic photosubstitution reactions have been divided into five mechanistic categories17 and each of these mechanistic types has its representatives in the class of aryl halides. Which reaction pathway is followed in any particular case depends on a number of factors such as the nature of the leaving group, the presence of electron-donating or electron-withdrawing substituents on the aromatic ring, the solvent, the multiplicity and the lifetime of the reactive excited state and the presence or absence of electron donors or acceptors in the reaction medium. This renders it rather difficult to make predictions about the mechanistic course that will be followed under a given set of circumstances. 

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