Nonspecific solvation

This relationship proves that the influence of solvents (decane, benzene, acetic acid, p-dichlorobenzene) on the rate constant is due to the nonspecific solvation of the reactants and the transition state. The values of the dipole moments of the TS calculated from experimenntal data within the scope of Equation (18.2) have the following values [87] ... 

Another factor influencing the reactivities of polar particles is their nonspecific solvation. Since both the individual particles, namely phenol and peroxyl radicals and their complex are polar, rate constants must depend on the polarity of the medium, its permittivity s, in particular. This was confirmed in experiments with mixtures of benzene and methylethyl-ketone, which showed that kq diminishes as the concentration of methylethylketone decreases provided the hydrogen bonding between the benzene and methylethylketone molecules are taken into account [10]. The dependence of ogkq on the medium permittivity s is described by the formula... 

Their approach in looking into the problem further was to find structures in which specific covalent bonding to the back side of the carbon undergoing substitution is difficult or impossible. As models for reactions at tertiary carbon they chose bridgehead substitutions. We have seen in Section 5.2 that rates in these systems are retarded, in some cases by many powers of ten, because of the increase in strain upon ionization. But the important point in the present context is that it is impossible for a solvent molecule to approach from the back side of a bridgehead carbon the only possibilities are frontside attack, known to be strongly disfavored (Section 4.2), or limiting SW1 solvolysis with nonspecific solvation. 

X-ray analysis showed that isoquinolines 125 (R1 = H, R2 = H, Me R1 = OMe, R2 = Me) exist in the solid state as NH-tautomers 125a. The tautomeric equilibrium does not shift noticeably in a solution, and no solvent effects were observed. The MNDO calculations, however, predict the OH-tautomer 125b to be the most stable, the experimentally observed predominance of 125a being explained by nonspecific solvation effect (93IZV288, 95KG922). 

How nucleophilic substitution occurs has been one of the preeminent problems in mechanistic organic chemistry for more than 50 years (1-3), and the importance of this process more than justifies this sustained interest. An area that has received particularly close scrutiny is the borderline or combat zone (2) region where potential competition between mechanisms of the SN1 and SN2 type exists. These processes involve formation of a nonspecifically solvated carbocation intermediate in the former case and a direct displacement of the leaving group by a solvent molecule in the latter and are designated kc and ks processes for solvolysis, respectively. 

The reaction of peroxy radicals with ketone is that between two dipolar particles in a polar medium. The role of the medium in methyl ethyl ketone oxidation has been studied in detail [152—157]. The rate coefficient, ftp, decreases with dilution of methyl ethyl ketone by a non-polar solvent (benzene, n-decane, etc.). The change of fep is caused by the nonspecific solvation of reacting particles and activated complexes. The relationship between ftp and the dielectric constant, e, is expressed by the Kirkwood equation... 

The interaction that occurs between the solvent and the transition state is sometimes described in terms of specific and nonspecific, depending on the nature of the interaction. Specific interaction refers to hydrogen bonding or charge transfer complexation. Nonspecific interaction is the result of general attraction due to van der Waals forces. Some of the correlations that have been devised are restricted to only nonspecific solvation of the transition state by the solvent. 

There are two type of solvation interactions a specific solvation, and a nonspecific solvation. In both cases there is some interaction between the solvent and the solute. 

In nonspecific solvation there s a weaker interaction that arises due to either van der Waals forces (see Chapter 2), or dipole-dipole interactions between the solute and the solvent. This is what happens when you dissolve salt into water. 

It is worth to mention that a shift in the value for a redox couple, in an aprotic solvents or an anhydrous ionic liquids, can be caused by changes in the polarity of the solvent, nature of the electrolyte or the presence of acidic additives these changes are associated with nonspecific solvation energies, ion pairing and protonation equilibria, respectively [13, 14]. 

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