Solvent property scales Reichardt

To account for the involvment of a solvent in a chemical reaction it is therefore necessary to use multivariate methods and in this context the principal properties are useful. Many attempts have been made to derive various "scales" of solvent properties to account for solvent-related phenomena. There are ca. 30 different empirical solvent scales described in the book by Reichardt[22] but few of these descriptors are available for a sufficiently large number of solvent to be practically useful as selection criteria. The principal properties described here were determined from the following property descriptors ... 

Over the last few years, the development of solvents of desired properties with a particular use in mind has been challenging. To evaluate the behaviour of a liquid as solvent, it is necessary to understand the solvation interactions at molecular level. In this vein, it is of interest to quantify its most relevant molecular-microscopic solvent properties, which determine how it will interact with potential solutes. An appropriate method to study solute-solvent interactions is the use of solvatochromic indicators that reflect the specific and non-specific solute-solvent interactions on the UV-Vis spectral band shifts. In this sense, a number of empirical solvatochromic parameters have been proposed to quantify molecular-microscopic solvent properties. In most cases, only one indicator is used to build the respective scale. Among these, the E (30) parameter proposed by Dimroth and Reichardt [23] to measure solvent dipolarity/polarisability which is also sensitive to the solvent s hydrogen-bond donor capability. On the other hand, the n, a and P (Kamlet, Abboud and Taft)... 

Several solvent polarity scales vere proposed to quantify the polar effects of solvents on physical properties and reactivity parameters in solution, such as rate of solvolyses, energy of electronic transitions, solvent induced shifts in IR, or NMR spectroscopy. Most of the polarity scales vere derived by an empirical approach based on the principles of the linear free energies relationships applied to a chosen reference property and system vhere hydrogen bonding effects are assumed negligible [Reichardt,1965, 1990 Kamlet, Abboud et al., 1981, 1983]. 

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... 

The Reichardt scale provides another set of parameters that are related to solvent polarity and basicity. This parameter has been used to correlate the properties and reactivity of Co(CO)3(L)2 systems. ... 

Polarity is one of the most important properties of the solvent as it can significantly influence the course of the reaction. Although different scales could be used to estimate the polarity of a solvent, the most commonly used is the electronic transition energy of a probe dye (e.g. Reichardt s Dye 30, Ej(30)). In this method, the polarity is calculated fi-om the wavelength (run) of maximmn absorbance of the standard solvatochromic dye 30 in solvents of different polarity at 25 C, 1 bar of pressure and using the following equation ... 

---