The Z scale of Kosower

One other fact to be considered is that the interest initially aroused by this scale promoted attempts at overcoming the above-mentioned measurement problems by using correlations with other solvent-sensitive processes as a result, many of the Z values currently in use are not actually measured values but extrapolated values derived from previously established ratios. 

In 1951, Brooker suggested for the first time that solvatochromic dyes could be used to obtain measures of solvent polarity. This author constructed the Xr scale on the basis of the solvatochromism of the merocyanine dye (5), the electronic transition of which gives rise to a charge-transfer from the amine nitrogen to a carboxamide group at the other end of the molecule. Hence, the excited status is more dipolar than the ground state, and the resulting band is shifted bathochromically as solvent polarity increases. Xr values reflect the position of the maximum of the first band for the chromophore in kcal mol .  

The original scale encompassed 58 solvents spanning Xr values from 33.6 kcal mol for m-cresol to 50.9 kcal mol for n-heptane. 

Dubois and Bienveniie developed the 4 polarity solvent scale on the basis of the position of the n — Jt transition for eight selected aliphatic ketones that were studied in 23 solvents, using n-hexane as reference and the following equation for calculation  

The 4 values spanned by the 23 solvents studied range from -0.01 for carbon tetrachloride to 0.65 for formic acid. The values for DMSO and water are 0.115 and 0.545, respectively. 

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

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... 

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-... 

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-... 

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). 

One of the first examples of a polarity scale based on a solvatochromic probe was Kosower s Z values. " defined as the transition energy of the longest wavelength absorption band of dye 6. 

Kosower s Z ScMk. Another widely used dye indicator polarity scale is Kosower s Z index (168), based on energies (in kilocalories per mole) for the charge transfer band of N-ethyl-4-carbethoxypyridinium iodide (53), an electronic transition from a ground state like 53a to an excited state more closely resembling 53b. 

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. 

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. 

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. 

Electronic spectra of molecules are often strongly dependent upon the nature of the solvent. For example, the n- rc transition for C=0 groups shifts considerably to the red on going from protic to aprotic media. Such shifts have been used, in particular, in the construction of empirical solvent scales. Use has been made of systems with strongly dipolar transitions, as for example, the betaine (2) used by Dimroth and Reichardt [3] to construct the , scale and (3) used by Kosower [4] to give the Z-value scale. Much has been written about these scales and their utility [5]. However, they are empirical and do not provide direct structural information. 

The Z values for the 20 solvents originally examined by Kosower sparmed the range from 64.2 for dichloromethane to 94.6 for water. The scale was subsequently expanded to an overall 61 solvents by Marcus and the original range extended to 55.3 kcal mol (for 2-methyltetrahydrofuran). Further expansion of this scale was precluded by the fact that high-polar solvents shift the charge-transfer band at the shortest wavelength to... 

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... 

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