Solvents, acceptor properties dipole moment

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. 

Physical organic chemists have tended to examine parameters based on shifts in the absorption peaks in the spectra of various dyes or indicator molecules. The a and P scales of Taft and Kamlet, the ET(30) scale of Dimroth and Reichardt, the 7t scale of Taft and co-workers and the Z value of Kosower are all examples of this type of parameter. The definitions and measurement means for these parameters, as well as important references, are shown in Table 5. An alternative definition of the Dimroth-Reichardt parameter is the dimensionless, ETN, which is now preferred by some organic chemists (for a discussion see Ref. 15). The Z value is important in that it led to the scale of Dimroth and Reichardt, which overcomes many of the limitations of the earlier scale. Several workers have shown that relationships exist, with good correlation coefficients, between similar parameters. Thus, DN is linearly related to p, both parameters being designed to measure the donor properties (or Lewis basicity) of solvent molecules. Also, Lr(30) is related to a as well as to AN all three parameters purport to measure the electron acceptor properties (or Lewis acidity) of solvent molecules. It has been found that different solvent types have different coefficients in linear relationships between n and the dipole moment. The Taft and Dimroth-Reichardt parameters, in particular, have been found to correlate with free energies and... 

Concerning the nature of interaction between a delocalized electron and polar solvents and the dependence of this interaction on the structure and properties of the solvent only a tentative conclusion can be made at present. Vg does not show correlation either with the dimensions of the solvent molecule or with its characteristics such as acceptor and donor numbers, optical and static permittivity, and the molecule s dipole moment. It is obvious that the electron-medium electrostatic interaction alone cannot explain the results obtained. 

Taft and coworkers described the formulation of three scales of solvent properties which were used to unravel and rationalize solvent effects on many types of physico-chemical properties. A tt scale of polarity/polarizabilities describes the solvent s ability to stabilize a charge or a dipole by virtue of its dielectric effect. The n values have been shown to be generally proportional to molecular dipole moments. The a scale of hydrogen bond donor acidities provides a measure of the solvent s ability to donate a proton. The jS scale of hydrogen bond acceptor basicities quantifies the solvent s ability to donate an electron pair (accept a proton). 

In spite of the difficulties mentioned, an increasing number of workers have made use of dielectrometric methods in the investigation of solution structures [Ja 77b, Wi 82a, b]. All of these methods are based on the fact that intermolecular interactions between the solvent and solute also change the dielectric properties of the system. In the studies of the solvent effect, changes in the dipole moment resulting from the formation of hydrogen-bonded associates and other electron-pair donor-acceptor complexes are of particular importance [So 76]. 

The complications due to ion pairing in non-aqueous solvents can be avoided to some extent by the use of dipolar aprotic solvents which have been much investigated recently. These solvents are unable to act as hydrogen bond donors (i.e., they have at most very weak acid properties), but they have large dipole moments and polarizabilities, and hence moderately high dielectric constants. Examples are acetonitrile (e = 38), dimethyl sulphoxide (s = 49), sulpholane s = 38), nitrobenzene (e = 35), nitromethane (6 = 36), dimethylformamide (e = 38), and propylene carbonate (6 = 65). Some of them can act as bases and hydrogen bond acceptors, but this is not true of nitrobenzene and nitromethane. Unlike the solvents which we have considered so far, they show little specific tendency to solvate anions, which has important consequences for the reactivity of many anions towards organic compounds. ... 

The choice of solvent is of particular importance. First, it has to be chosen such that the solubility is in a suitable range for the selected type of crystallization experiment (reasonably high solubility for cooling experiments, very low solubility in solvents used for precipitation, etc.). Second, it is important to use solvents with diverse physical properties in order to explore the whole parameter space of possible environments. In addition to molecular solvent-solute interactions, bulk properties of solvents such as viscosity may play a role. Gu et al. [19] examined 96 solvents in terms of 8 relevant solvent properties hydrogen bond acceptor propensity, hydrogen bond donor propensity, polarity/dipolarity, dipole moment, dielectric constant, viscosity, surface tension, and cohesive energy density (calculated from the heat of vaporization). Based on all 8 properties, the 96 solvents were sorted into 15 groups... 

---