General Solvent Effects

Because the key operation in studying solvent effects on rates is to vary the solvent, evidently the nature of the solvation shell will vary as the solvent is changed. A distinction is often made between general and specific solvent effects, general effects being associated (by hypothesis) with some appropriate physical property such as dielectric constant, and specific effects with particular solute-solvent interactions in the solvation shell. In this context the idea of preferential solvation (or selective solvation) is often invoked. If a reaction is studied in a mixed solvent. 

The site-, regio-, and stereochemistry of some of these DA reactions has been investigated by the density functional theory, including solvent effects. Generally, these cycloadditions proceed by a concerted but asynchronous reaction mechanism. The endo stereochemistry is in most case preferred. 

Percus-Yevick, and the mean spherical approximations. The last of these assumes that the solvent consists of hard spheres with a long-range attractive force. It is widely applied to the modeling of solvent effects. Generalizations to multi-component fluids are straightforward. ... 

Several alternative attempts have been made to quantify Lewis-acid Lewis-base interaction. In view of the HSAB theory, the applicability of a scale which describes Lewis acidity with only one parameter will be unavoidably restricted to a narrow range of struchirally related Lewis bases. The use of more than one parameter results in relationships with a more general validity ". However, a quantitative prediction of the gas-phase stabilities of Lewis-acid Lewis-base complexes is still difficult. Hence the interpretation, not to mention the prediction, of solvent effects on Lewis-add Lewis-base interactions remains largely speculative. 

In a few cases, where solvent effects are primarily due to the coordination of solute molecules with the solute, the lowest-energy solvent configuration is sufficient to predict the solvation effects. In general, this is a poor way to model solvation effects. 

NITRATIONS WITH NITRONIUM IONS THE GENERAL CASE 6.2.1 Evidence from solvent effects... 

The solvent effects are generally less than 1 ppm, which are well within the error bounds of the standard deviations of the calculated shifts. It is possible, however, that under extreme conditions larger deviations may be observed. 

Entries 4 and 5 point to another important aspect of free-radical reactivity. The data given illustrate that the observed reactivity of the chlorine atom is strongly influenced by the presence of benzene. Evidently, a complex is formed which attenuates the reactivity of the chlorine atom. This is probably a general feature of radical chemistry, but there are relatively few data available on solvent effects on either absolute or relative reactivity of radical intermediates. 

Aromatic steroids are virtually insoluble in liquid ammonia and a cosolvent must be added to solubilize them or reduction will not occur. Ether, ethylene glycol dimethyl ether, dioxane and tetrahydrofuran have been used and, of these, tetrahydrofuran is the preferred solvent. Although dioxane is often a better solvent for steroids at room temperature, it freezes at 12° and its solvent effectiveness in ammonia is diminished. Tetrahydrofuran is infinitely miscible with liquid ammonia, but the addition of lithium to a 1 1 mixture causes the separation of two liquid phases, one blue and one colorless, together with the separation of a lithium-ammonia bronze phase. Thus tetrahydrofuran and lithium depress the solubilities of each other in ammonia. A tetrahydrofuran-ammonia mixture containing much over 50 % of tetrahydrofuran does not become blue when lithium is added. In general, a 1 1 ratio of ammonia to organic solvents represents a reasonable compromise between maximum solubility of steroid and dissolution of the metal with ionization. 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

To date, most studies of ionic liquids have used a small set of ionic liquids and have been based on the idea that, if the response of a particular probe molecule or reaction is like that in some known molecular solvent, then it can be said that the polarities of the ionic liquid and the molecular solvent are the same. This may not necessarily be the case. Only systematic investigations will show whether this is tme, and only when a wide range of ionic liquids with a wide range of different solvent polarity probes have been studied will we be able to make any truly general statements about the polarity of ionic liquids. Indeed, in our attempts to understand the nature of solvent effects in ionic liquids, we will probably have to refine our notion of polarity itself However, it is possible to draw some tentative general conclusions. 

For sparingly soluble salts of a strong acid the effect of the addition of an acid will be similar to that of any other indifferent electrolyte but if the sparingly soluble salt MA is the salt of a weak acid HA, then acids will, in general, have a solvent effect upon it. If hydrochloric acid is added to an aqueous suspension of such a salt, the following equilibrium will be established ... 

Solvent effects on the reactions of small radicals have been discussed in general terms in Chapter 2 (see 2.3.6.2 2.4.5). Small, yet easily discernible, solvent effects have been reported for many reactions involving neutral radicals. These effects on the rates of radical reactions often appear insignificant when... 

While the VB approach is more effective than the MO approach in treating reactions in solutions, it is useful to be familiar with both approaches. Here we outline a simple and general procedure that incorporates solvent effects into MO calculations. 

After the discovery of the remarkable acceleration of some Diels Alder reactions performed in water, a number of polar non-aqueous solvents and their salty solutions were investigated as reaction medium. This revolutionized the concept that the Diels-Alder reaction is quite insensitive to the effect of the medium and emphasized that a careful choice of the solvent is crucial for the success of the reaction. The polarity of the reaction medium is an important variable which also provides some insights into the mechanism of the reaction. If the reaction rate increases by using a polar medium, this means that the transition state probably has polar character, while the absence of a solvent effect is generally related to an uncharged transition state. 

The diradical mechanism b is most prominent in the reactions involving fluorinated alkenes. These reactions are generally not stereospecificand are insensitive to solvent effects. Further evidence that a diion is not involved is that head-to-head eoupling is found when an unsymmetrical molecule is dimerized. Thus dimerization of F2C=CFC1 gives 106, not 107. If one pair of electrons moved before the other, the positive end of one molecule would be expeeted to attack the negative end of the other. 

Solvent effects also play an important role in the theory separating enthalpy and entropy into external and internal parts (134-136) or, in other terms, into reaction and hydration contributions (79). This treatment has been widely used (71, 73, 78, 137-141). The most general thermodynamic treatment of intermolecular interaction was given by Rudakov (6) for various states of matter and for solution enthalpy and entropy as well as for kinetics. A particular case is hydrophobic interaction (6, 89, 90). 

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