Specific solvation effects

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  

In the absence of the polyether, potassium fluoride is insoluble in such solvents and unreactive toward alkyl halides. Similar enhancement of the reactivity and solubility of other salts are also observed in the presence of crown ethers. 

CHAPTER 4 STUDY AND DESCRIPTION OF ORGANIC REACTION MECHANISMS 

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The comparisons made by Parchment et al. [271] illustrate the importance of combining electronic polarization effects with corrections for specific solvation effects. The latter are accounted for parametrically by the explicit simulation, but that procedure cannot explicitly account for the greater polarizability of tautomer 8. The various SCRF models do indicate 8 to be more polarizable than any of the other tautomers, but polarization alone is not sufficient to shift the equilibrium to that experimentally observed. Were these two effects to be combined in a single theoretical model, a more accurate prediction of the experimental equilibrium would be expected. 

The specific processes discussed above are all special cases of the general process (9.2.1). In all of these cases we have seen the explicit modification of the equilibrium constant of the corresponding process. As indicated in Eq. (9.2.3), the general modification requires knowledge of the solvation Gibbs energies of all the components involved in the process. For macromolecules such as proteins or nucleic acid, none of these is known, however. Nevertheless, some specific solvation effects are examined in Sections 9.4 and 9.5. 

Before we examine some specific solvation effects on cooperativity we must first consider various aspects of the solvation Gibbs energy of a macromolecule a. We present here one possible decomposition of AG which will be useful for our purposes. Consider a globular protein a which, for simplicity, is assumed to be compactly packed so that there are no solvent molecules within some spherical region to which we refer as the hard core of the protein. The interaction energy between a and the fth solvent molecule (the solvent is presmned to be water, w) is written as... 

The nature of the medium may also have a strong influence on the complexation process via specific or non specific solvation effects on both the complexed and uncomplexed states. The solvent plays a very important role both on enthalpy and entropy of complexation. Stability and selectivity result from a subtle balance between solvation (of both L and S) and complexation (i.e. "solvation of S by L). 

The propagation rate constant and the polymerization rate for anionic polymerization are dramatically affected by the nature of both the solvent and the counterion. Thus the data in Table 5-10 show the pronounced effect of solvent in the polymerization of styrene by sodium naphthalene (3 x 1CT3 M) at 25°C. The apparent propagation rate constant is increased by 2 and 3 orders of magnitude in tetrahydrofuran and 1,2-dimethoxyethane, respectively, compared to the rate constants in benzene and dioxane. The polymerization is much faster in the more polar solvents. That the dielectric constant is not a quantitative measure of solvating power is shown by the higher rate in 1,2-dimethoxyethane (DME) compared to tetrahydrofuran (THF). The faster rate in DME may be due to a specific solvation effect arising from the presence of two ether functions in the same molecule. 

Unfortunately specific solvation effects are very difficult to understand, although molecular mechanics simulations have recently gone some way towards modelling complexation phenomena... 

The solubility parameter introduced by Hildebrand90, rather than the dielectric constant or dipole moment is a characteristic quantity of the solvent which appears appropriate (if no specific solvation effects have to be taken into account) to forecast the micellar solubility of the alkali dinonylnaphthalene sulfonates in the particular solvent. As the solubility parameter of the solvent is increased, the micelles tend to assume a smaller size (Fig. 14). This size reduction gives a looser packing of the DNNS tails and, thus, exposes the more interactive aromatic and polar parts in such a way as to reduce the difference between the solubility parameter of the solvent and the effective solubility parameter of the solvent-accessible portions of the lipophilic micelle. The automatic matching of the solubility parameter for micelle and solvent by reduction of micelle size and packing in solvents of high solubility parameters recalls the behavior of linear macromolecules in solvents of different solvent power. 

Section 2.4). It can be unequivocally concluded from a large amount of literature data, that the number of instances of deviation for reactions between dipolar molecules from the simple electrostatic relationship, given in Eqs. (5-87), (5-88), and (5-90), greatly exceeds the number of cases where the relationships hold. Thus, allowances must also be made for electrostatic, nonelectrostatic, and specific solvation effects in the general effect of the medium. 

Specific Solvation Effects on Reaction Rates 269 5.5.6 Solvent Influence on the Reactivity of Ambident Anions... 

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