Electron pair donation solvents

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, by ion-dipole or ion-induced dipole interactions and by more direct interactions, such as hydrogen bonding to anions or electron-pair donation to cations. Lewis acidity and basicity of the solvents affect the latter. The redox properties of the ions at an electrode depend on their being solvated, and the solvation effects electrode potentials or polarographic half-wave potentials. 

Some other classification schemes are provided in a work by Kolthoff (Kolthoff, 1974). It is according to the polarity and is described by the relative permittivity (dielectric constant) e, the dipole moment p (in 10 ° C.m), and the hydrogen-bond donation ability Another suggested classification (Parker, 1969) stresses the acidity and basicity (relative to water) of the solvents. A third one (Chastrette, 1979), stresses the hydrogen-bonding and electron-pair donation abilities, the polarity, and the extent of self-association. A fourth is a chemical constitution scheme (Riddick et al., 1986). The differences among these schemes are mainly semantic ones and are of no real consequence. Marcus presents these clearly (Marcus, 1998). 

The reaction between dimercury(I) salts and molecules with an electron-pair-donating atom normally destroys the metal-metal bond of the dimercury(I) ion Hg+—Hg+ by disproportionation, forming metallic mercury and a mercury(II) compound, but the use of nonpolar solvents, weak Lewis bases and dialytic crystallization methods has contributed to the successful preparation of single crystals of several dimercury(I) coordination compounds in the past 25 years.9,30,31 The myth that the dimercury(I) species Hg + forms few coordination compounds has been exploded. 

On the assumption that the solutions are dilute and that there is no direct association of the solute with the solvent, then the only way that the solvent affects the solubility is via its solubility parameter, 5r For a given solute, and since the term involving 8, is negative, the more the solubility parameter of the solvent differs from that of the solute, the lower the solubility. Since the term in the solubility parameters is squared, the difference may be either positive or negative to provide the same effect. As already specified above, this behaviour is observed (Shinoda 1978) when only dispersion forces between solute and solvent are operative. If, however, electron-pair donation and acceptance between them come into play, as with C02 as the solute in aromatic solvents or tetrachloromethane, then a somewhat higher solubility than Eq. (2.10) predicts is observed. 

Those solutes for which the solvent shifts are particularly large have been used in the specification of solvent properties, such as electron-pair donation ability, Lewis basicity, or softness. For the former property, the solvent shifts of deuteromethanol or of phenol have served as suitable scales. For the latter property the solvent shifts of the symmetrical stretch of Hg-Br in the Raman spectrum of HgBr2 and of I-CN in the infrared spectrum of ICN have been so employed (see Chapter 4). 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

Of the many empirical polarity parameters or indexes that have been proposed, only a few remain viable, in the sense that they are currently more or less widely used to describe the polarity of solvents for various purposes. Some such parameters that are commonly used describe better other, more specific, properties than polarity e.g., hydrogen bond or electron-pair donation ability. Thus, only two polarity parameters have been employed in recent years Dimroth and Reichardt s A T(30) (Dimroth et al. 

Many other scales of electron-pair donation abilities have been proposed over the years, which are in general in good correlation with DN e g., the heat of complexation of the solvent molecules with boron trifluoride in dichloromethane (Maria and Gal 1985), and P e.g., SB, the solvatochromism of 5-nitroindoline compared with l-methyl-5-nitroindoline in neat solvents (Catalan el al. 1996) scales. The latter, the SB scale, has the advantage that the N-H acid function of the 5-nitroindoline probe has only a single hydrogen atom, contrary to the nitroanilines used for the P scale, that have two. It was devised quite recently for... 

Another property that characterizes solvents is their softness, in terms of the HSAB concept (Pearson 1963), according to which the interactions of soft solvents are strongest with soft solutes, of hard solvents with hard solutes, but are weaker for hard solvents with soft solutes and vice versa. The applicability of the softness property takes into account that it is superimposed on the more general electron pair donation property discussed above. In fact, it can replace (Marcus 1987) the notion of the family dependence of the P scale, expressed by the , parameter (Kamlet etal. 1985). A few quantitative scales have been... 

Solvents can be differentiated according to the availability of dissociable hydrogen atoms attached to a more electronegative atom. Protic solvents readily form hydrogen bonds and can stabilize cations by electron pair donation and anions via hydrogen bonding. Examples include water, alcohols, carboxylic acids, ammonia, and amines. 

Kamlet-Taft electron pair donation ability of solvent mean ionic molal activity coefficient of electrolyte chemical shift of NMR signal (ppm)... 

The chemical properties of solvents that are relevant to their dissolution abilities for electrolytes and the ionic dissociation of the latter include their structuredness or self-association and their donor (electron pair donation, basicity) and acceptor (hydrogen bonding ability, acidity) properties as well as their softness. The mutual solubility with other solvents, in particular water, is also of importance as are the windows for making spectroscopic and electrochemical measurements on solutions of ions in the solvents. 

P Kamlet-Taft electron pair donation ability of solvent... 

Taft et al. [28] proposed the solvatochromic parameters, tt, a, and p, which describe three solvent s abilities, respectively, to stabilize a charge or a dipole by virtue of its dielectric effect, to donate a proton (or accept an electron pair), and to accept a proton (or donate an electron pair). It was shown that the AN for nonprotonic solvents correlates... 

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 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 ... 

The dissolution of a solute in a solvent always affects the solvent-solvent interactions in the vicinity of the solnte particles in addition to the solnte-solvent interactions that take place (Marcus, 1998b). This may be viewed in several stages. First, a cavity in the solvent is formed, to accommodate the solute, which breaks down the cohesive forces of the solvent. Next dispersion forces take effect. They apply to nonpolar and hardly polarizable solutes and solvents, as well as to polar and polarizable ones. Other forces that become active provide contributions from interactions of polar molecnles with polar or polarizable ones and from donor acceptor interactions, such as electron-pair or hydrogen-bond donation and acceptance, whether from or to the solute, the solvent, or both. 

Solutes that, whatever their state of aggregation, undergo solvation interactions with the solvent beyond those due to dispersion forces, exhibit specific contributions to the solubility, that can be attributed to dipole-dipole and dipole-induced dipole interactions and electron-pair or hydrogen-bond donation by the solvent to the solute or vice versa. 

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