Y values for some solvent systems

The Y values determined in this way are empirical measures of the solvent s ability to accommodate formation of the dipolar solvolysis transition state. Table 4.5 lists the y values for some alcohol-water mixtures and for some other solvent systems. 

Solvents that fall in the nonpolar aprotic class are not effective at stabilizing the development of charge separation. These molecules do not contain polar groups, and they do not have hydrogen atoms capable of forming hydrogen bonds. Reactions that 

CHAPTER 4 STUDY AND DESCRIPTION OF ORGANIC REACTION MECHANISMS 

In aprotic solvents, no hydrogen atoms capable of hydrogen bonding are present, and this type of solvation cannot occur. As a result, the electrons of the anion are more easily available for reaction. The polarity of the aprotic solvent involved is important, because if the solvent has a low dielectric constant, dissolved ionic compounds are likely to be present as ion pairs or ion aggregates. Reactivity in this case is greatly 

Particularly striking examples of the effect of specific solvation can be cited from the study of the crown ethers. These are macrocyclic polyethers that have the property of specifically solvating cations such as Na and K . For example, in the presence of 18-crown-6, potassium fluoride is soluble in benzene or acetonitrile and acts as a reactive nucleophile  

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Although much evidence has been presented in support of the Ito-Yamashita interpretation, several things cause us to be cautious about accepting it. First of all, the a value (0.5) obtained for S — M placements in styrene-MMA copolymers is much larger than the corresponding values estimated for M —M (0.2) and S — S (0.3) placements in polyMMA and polystyrene. Secondly, our work indicates that y" values for some copolymer systems are definitely not zero, especially when the spectra are obtained from copolymers in aromatic solvents. In addition, I-Y plots of methoxy resonance patterns recorded at 100 MHz are not super-imposable. An example of this is shown in Fig. 7. Finally, the chemical shifts between the A, B and C areas are very large (0.5 ppm) to be attributed to stereoregularity effects. 

Values of Yots and Nots for selected solvents are shown in Table 8.3. Many other Y scales have been developed for particular reaction systems. As noted in Chapter 6, however, writing a linear free energy relationship implies the modeling of one reaction on another. The ability of the linear free energy equation to correlate experimental data thus depends to some extent on the degree to which the model reaction is appropriately chosen. 

Clearly eq. [5.5.24] possesses the functional flexibility to describe the data. (In some systems a 1-step (2-state) equation is adequate. To transform eq. [5.5.24] to a 1-step version, set Kj = 0 and let y = 72 - 7i) The next step is to examine the parameter values for their possible physical significance. It seems plausible that K, and Kj should be larger than luiity, but not very large, on the basis that the solutes are organic and so are the cosolvents, but the cosolvents are water-miscible so they are in some degree water-like. In fact, we find that nearly all K, and K2 values fall between 1 and 15. Likewise the gA values seem, in the main, to be physically reasonable. Earlier estimates of g (reviewed in ref. ) put it in the range of 0.35-0.5. A itself can be estimated as the solvent-accessible surface area of the solute, and many of the gA values found were consistent with such estimates, though some were considerably smaller than expected. Since gA arises in the theory as a hydrophobicity parameter, it seemed possible that A in the equation represents only the nonpolar sxuface area of the... 

Polystyrene was employed in GLC as solvent for the following solutes benzene, cyclohexane and n-pentane [3], n-hexadecane [88], normal, branched and cyclic alkanes, single and dinuelear aromatic hydrocarbons, ds- and ira -decaline [60], n-octane, n-decane, chloroform, carbon tetrachloride, benzene, nitromethane and ethanol [89], methanol, carbon sulphide, 1,2,4-trimethylbenzene, pentane and tri-isopropylbenzene [90]. Some of the polymers-solute systems mentioned are encountered in other contributions [31, 43, 91 — 93]. The results differ in respect of y" and Xj2 values and their temperature variation. The possible somces... 

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