Acceptor numbers

This scale provides a useful indication of solvating ability towards Lewis acids. Noteworthy are the high values for the amide solvents and dimethylsulphoxide, which are sharply distinguished from ketones, esters and nitriles. 

A complementary scale, the acceptor number [6], is a measure of the ability of the solvent to accept an electron pair, and is closely related to hydrogen bonding ability. It is the normalised NMR chemical shift of the complex of triethylphosphine oxide with the test compound relative to that with the strong acceptor antimony pentafluoride (equation 12.3). 

A selection of acceptor numbers is shown in Table 12.4. There are fewer surprises in this scale. It is noteworthy that chloroform interacts strongly with the phosphine oxide. 

---

Desimoni et al. initially advocated the Acceptor Number (AN) as the dominant solvent parameter" The AN describes the ease with which a solvent can act as an electron pair acceptor (Lewis acid) and... 

Table 1,5, Donor scales (Dg, DN and DNbulk) of some selected solvents, as well as acceptor number (AN) and hydrogen bond donor capacities (a).

The second important influence of the solvent on Lewis acid - Lewis base equilibria concerns the interactions with the Lewis base. Consequently the Lewis addity and, for hard Lewis bases, especially the hydrogen bond donor capacity of tire solvent are important parameters. The electron pair acceptor capacities, quantified by the acceptor number AN, together with the hydrogen bond donor addities. O, of some selected solvents are listed in Table 1.5. Water is among the solvents with the highest AN and, accordingly, interacts strongly witli Lewis bases. This seriously hampers die efficiency of Lewis-acid catalysis in water. 

The dimensionless acceptor number, AN, ranked the acidity of a solvent and was defined for an acidic solvent A as the relative P NMR downfield shift (A3) induced in triethyl phosphine when dissolved in pure A. A value of 0 was assigned to the shift produced by the neutral solvent hexane, and a value of 100 to the shift produced by SbClj. Gutmann suggested that the enthalpy of acid-base adduct formation be written as ... 

Using a method suggested by Saint-Flour and Papirer [100], Schultz and Lavielle obtained A// -values for the interaction of several vapors of differing donor numbers and acceptor numbers with various treated and untreated carbon fibers used in the preparation of carbon fiber-epoxy matrix composites. was expressed as ... 

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 acceptor number (AN) for a solvent A is determined by comparing the electron-pair accepting ability of the solvent with that of SbCls, from the oxygen atom of triethyl-... 

Acceptor number (AN), 23 87 Accident prevention, safer chemistry for, 12 805 Accidents... 

Table 1.6 Dipole moments (p.), dielectric constants (er), normalized donor numbers (DNn) and acceptor numbers (AN) for some common solvents [1,2]...

The acceptor number, AN, of a solvent is a measure of the power of the solvent to accept a pair of electrons [18], Experimental evaluation of AN involves observing the frequency changes induced by a solvent on the 31P NMR spectrum when triethylphosphine oxide, Et3P=0, is dissolved in the solvent. Donation of an electron pair from the oxygen atom of Et3P=0, as shown in Scheme 1.2, reduces the electron density around the phosphorus, causing a deshielding effect which leads to an increase in chemical shift. Hexane (AN = 0) and SbCls (AN = 100) were used as fixed points to define this scale. 

Gutmann introduced3 the concepts of donor number (donicity) and acceptor number (acceptivity), as dimensionless numbers, for the characterization of donor properties of bases independently of the solvent. 

There are numerous attempts to correlate solvent parameters with the reaction rate of Diels-Alder reactions122. Examples are the Brownstein Polarity Parameter S123, the Solvophobicity Parameter Sp124,125 the D-it parameter (based on the solvent effect on the reaction of tetracyanoethylene and diazodiphenylmethane with benzene as the reference solvent)126 or the Acceptor Number / /V127, l2X (a parameter which describes the ability of a solvent to act as an electron pair acceptor)129. These examples included either reactions that were next to insensitive to solvent effects (like that in Table 9) or reactions in which the reactants mainly interact with the electron pair on the donor atom of the solvent130. 

Molecular weight is the most significant parameter for predicting the BBB permeability, followed byTPSA, log I , number of hydrogen-bond acceptors, number of hydrogen-bond donors, P-gp substrate probability, and finally the number of rotatable bonds. 

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. ... 

It was shown earlier that the acceptor number can be treated as a measure of disorder of the solvent structure around the ion, reflecting the changes of caused by the solvent-solvent interaction. The contact... 

Figure 3. Comparison of the Volta potential differences at the Hg/solvent interface vs. (a) donor number and (b) donor and acceptor numbers. Jt=0.007 AN-0.011DN-0.485 ...

---