Solvent chemical acceptor number

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. 

The acceptor number (AN) [10, 11] of solvent A (Lewis acid) is obtained by measuring the 31P-NMR chemical shift (AS, ppm) of triethylphosphine oxide (Et3P=0,... 

The use of the Gutmann41 donor and acceptor numbers for describing solvent effects on rates, equilibria and other physicochemical properties has met with some success in organic chemistry. 62 63 However, because the donor and acceptor numbers of mixtures of solvents can not be inferred from the values of the pure solvents but must be determined experimentally, and also because the relationships describing the effects of solvent on chemical reactions were found to apply to non-associated solvents of medium to high dielectric constant, there has been very little attempt to introduce this approach into inorganic systems where the commonly used solvents are protic, i.e. associated. However, one such reaction that has been studied was63 equation (34) ... 

The electron acceptor numbers (AN) can be used as a measure solvent ability to take a share in the electron pairs from suitable donors. They are defined by means of a different experimental procedure on the basis of the NMR chemical shifts of phosphorous which are produced on transfer of EtsPO through solvents. AN values are scaled from an arbitrarily chosen value of zero for the shift produced by hexane (or 1,2-dichloroethane) to one hundred for the shift produced by the 1 1 EtsPO—SbCls adduct in 1,2-dichloroethane. An estimation of the coordinate bond energy is possible by making use of the formula after Gutmann [155]... 

Triethylphosphine oxide contains a highly basic oxygen atom, which is easily accessible to solvent electrophilic attack. This causes a polarization of the P=0 bond and a downfield shift of the 31P NMR signal. The observed chemical shifts (5) referred to the reference solvent n-hexane and extrapolated to infinite dilution may be taken as a measure of the acceptor properties of the solvents. Hence, the acceptor number (AN) is defined as follows ... 

In conclusion, extensive work on solvent properties has revealed that simple physical properties, such as the dielectric constant or dipole moment, are inadequate measures for solvent polarity (which can correlate well with the influence of solvents on thermodynamic and kinetic reaction parameters in them). Better solvent parameters, which correlate well with the impact of the solvent chosen on electrochemical and chemical reactions, are donor and acceptor numbers or parameters based on solvatochromic effects, because these reflect not only pure electrostatic effects but rather the entire electronic properties of a solvent. 

These quantities have been termed acceptor number AN (or acceptivity) and they were obtained from the relative P NMR chemical shift values corr (n-hexane as reference solvent) with respect to that of the 1 1 adduct EtsPO—SbCls dissolved in 1,2-dichloroethane, which has been arbitrarily taken to have the value of 100. The acceptor numbers are dimensionless numbers expressing the acceptor property of a given solvent relative to those of SbCb, which is also the reference compound for assessing the donor numbers. A compilation of organic solvents in order of increasing acceptor number is given in Table 2-5. 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

AN solvent acceptor number, based on P NMR chemical shift of Et3PO (Gutmann and Meyer) ... 

The acceptor number (AN) was defined as dimensionless number related to the relative chemical P-NMR shift in triethylphosphine oxide (C2H5)3PO in the particular acceptor solvent ... 

Another scale for measuring solvent acidity was formulated by Mayer et al. [43]. It is called the solvent acceptor number (AN) and is based on the relative values of the NMR chemical shifts produced by a given solvent with a strong Lewis base, triethylphosphine oxide (fig. 4.13). The data were normalized so that the acceptor number of hexane is zero and that for the 1 1 adduct with the strong Lewis acid, SbCls, 100 when dissolved in 1-2 dichloroethane. The attractive feature of this scale is that it varies over a wide range for the polar solvents con-... 

Polar molecular solutes have been used to probe the donor-acceptor properties of polar solvents. chemical shifts have been measured for interaction between trifluoroiodomethane and the solvent molecule as electron pair donor [24]. As interaction between the donor molecule and the iodine atom in this molecule increases, electron density at the fluorine atoms increases with a resulting positive chemical shift in the NMR signal. An excellent correlation between these shifts and the Gutmann donor number was reported [24]. 

The chemical shifts in triethylphosphine oxide measured in polar solvents is used to define the acceptor number scale for solvent acidity [25]. In this case, the oxygen atom in the P=0 bond acts as an electron pair donor to the solvent as a Lewis acid. The resulting inductive effect lowers the electron density at the phosphorus atom and results in a chemical shift which depends on solvent acidity. 

The chemical shifts have been correlated satisfactorily with the solvent parameters AN (acceptor number (ref. 28)), DN (donor number (ref. 29)) and e (dielectric constant) for a set of nine solvents (acetone, acetonitrile, DMF, DMAc, nitrobenzene, sulfolane, HMPT, benzonitrile, methanol) (ref. 17) (Fig. 4). The predominant weight of AN indicates clearly the basic character of solvated fluorides which, however, is strongly modulated by HF-solvation and can be quantified in that way. Thus, the correlation between the chemical shift and the reactivity of soluble fluoride anions could, in principle, allow to predict their fluorination efficiency in any solvent. 

Various attempts have been made to classify solvents, e.g. according to bulk and molecular properties empirical solvent parameter scales hydrogen-bonding ability and miscibility >. In table I solvents are divided into classes on the basis of their acid-base properties which can be used as a general chemical measure of their ability to interact with other species. Detailed information on these and other solvents, their symbols, fusion and boiling pointe and Gg), bulk properties (6,Ti, q), and currently-used correlation parameters DN (donor number), Ej-value, and AN (acceptor number) is given in Appendix A-1. 

A cobalt-59 NMR procedure was elaborated by Laszlo and Stockis [La 80] for the determination of H-bond donation of protic solvent. The Co chemical shifts were found to be in linear correlation with the Gutmann acceptor number of the solvents studied. 

The Gutmann-Mayer acceptor number is determined with the aid of phosphorus-resonance spectroscopy. Triethylphosphine oxide is used as reference donor. If the solvent dependences of the P-NMR chemical shifts measured in the various solvents are referred to the chemical shift of the complex formed between antimony pentachloride and the reference donor (the latter value is taken as 100),. the acceptor number values are obtained. A description of the methodology can be found in the original publication by Mayer [Ma 75]. 

Although the success of the empirical solvent parameters has tended to downgrade the usefid-ness of the dielectric approach, there are correlations that have succeeded as exemphfied by Figure 13.1.1. It is commonly held that the empirical solvent parameters are superior to dielectric estimates because they are sensitive to short-range phenomena not captured in dielectric measurements. This statement may not be generahzed, however, since it depends strongly on the chemical reaction investigated and the choice of solvents. For instance, the rate of the Menschutkin reaction between tripropylamine and methyl iodide in select solvents correlates better with the log e function than with the solvent acceptor number. ... 

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). 

The Gutmann-Mayer acceptor number scale AN [49] uses the NMR chemical shift 5 of the atom of triethylphosphine oxide as the electron-pair donor probe in dilute solution in the solvent to be smdied relative to the shift in n-hexane. The AN values are normalized to make AN=2.348(5/ppm), the 5 values being corrected for the diamagnetic susceptibility / of the solvent The AA scale does include solvent polarity effects besides its ability to donate hydrogen bonds, as is seen in its being proportional to the Ej values AN = 56.SE [37]. In fact, both scales are sensitive to both the solvent polarity and its acidity. The acceptor numbers of solvents are Usted in Table 3.9. 

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