Kosower Z scale

The scale frequently used to characterize the solvating power (the acceptor strength) of an acceptor solvent is the Kosower Z scale [Ko 58]. The procedure is based on the fact that the cation of the l-ethyl-4-carbomethoxypyridinium iodide ion pair used as the model system is not an electron pair acceptor, while the iodide anion is capable of hydrogen bonding. Consequently, the extent of ion pair formation between the l-ethyl-4-carbomethoxypyridinium cation and the iodide ion in solutions prepared with various solvents depends on the solvation of the iodide ion. 

Nevertheless, the solvatochrome effect has served as the basis of a number of scales characterizing the solvating powers of solvents. These include the Kosower Z scale and the Dimroth-Reichardt Ej scales, dealt with in Chapter 4. 

Kosower in 1958 was the first to use solvatochromism as a probe of solvent polarity. The relevant Z-scale is based on the solvatochromic shift of 4-methoxycarbonyl-1-ethylpyridinium iodide (1). Later, Dimroth and Reichardt suggested using betain dyes, whose negative solvatochromism is exceptionally large. In particular, 2,6-... 

The Z Scale. It was developed by Kosower (34/i) in 1958. The property chosen as a standard is the energy of the electronic transition... 

Kosower initially reported Z values for 21 solvents and 35 solvent mixtures (interpretation of the data relating to the solvent mixtures is, however, often uncertain because of the overlapping of the individual effects of the components). The Kosower scale was later extended to 45 solvents [Le 66, Be 62, Go 65, Fo 69], and Griffiths and Pugh [Gr 77] determined the Kosower Z values of a further 40 solvents. 

In actual fact, this consideration is only true for systems in which one of the many factors influencing the solvent effect is predominant and the others are negligible in comparison. The reason why this correlation was suitable for a description of the Kosower Z values was that the spectral shifts serving as its basis are determined predominantly by the acceptor strength of the solvent. The experiments of Brownstein can actually be regarded as a confirmation of the Kosower acceptor scale. 

Another solvent polarity scale, Kosower s Z-scale, is based on the highly solvent-dependent charge transfer absorption of the para-substituted pyridin-ium salt (Scheme 4). The ground state of this dye is best described as a highly polar tight ion-pair, whereas the less polar charge-transfer form is more important in the excited state. Thus, the solvatochromism of the pyridinium salt is negative, Uke Reichardt s dye. In fact, the Ex(30)- and the Z-scale... 

As outlined in Section 1.3, the solvent acidity and basicity have a significant influence on the reactions and equilibria in solutions. In particular, differences in reactions or equilibria among the solvents of higher permittivities are often caused by differences in solvent acidity and/or basicity. Because of the importance of solvent acidity and basicity, various empirical parameters have been proposed in order to express them quantitatively [1, 2]. Examples of the solvent acidity scales are Kosower s Z-values [8], Dimroth and Reichard s Er scale [1, 9], Mayer, Gutmann and Gergefs acceptor number (AN) [10, 11], and Taft and Kalmefs a parameter [12]. On the other hand, examples of the solvent basicity scales are Gut-... 

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... 

Solvent polarity parameters — use solvatochromic dyes (dyes whose electronic transitions are strongly dependent on the nature of the solvent) as indicators of solvent polarity. A comprehensive solvent polarity scale was first proposed by Kosower who defined the polarity parameter, Z, as the molar transition energy, Ej, for the charge transfer band of 1-ethyl-(methoxycarbonyl)pyridynium iodide in a given solvent as... 

The Kosower parameter (Z = //v) is derived from the effect of the solvent on the charge transfer spectrum of the pyridinium iodide (5) excitation. A high value of Z (the energy of the charge transfer transition) corresponds to highly polar solvents. Similar parameters have been obtained for other reporter molecules and the charge transfer system of 6 generates the Dimroth-Reichardt scale. 

Z polarity scale. A solvent polarity scale proposed by Koso ver [Kosower 1958a, 1958b] based on the energy of the electronic transition of the 1 -ethyl-4-carbomethoxypyridinium iodide that is strongly solvent-dependent. This is a measure of an internal charge transfer process. The original set of Z values being quite small, it was successively extended by means of other indicators (Table L2). 

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