Solvate ionization

Substrate organization in membrane mimetic systems leads to altered solvation, ionization and reduction potentials and, hence, to altered reaction rates, paths and stereochemistries. These properties have been advantageously exploited, in turn, for reactivity control, catalysis, drug delivery and artificial photosynthesis (8). There are only limited examples of the utilization of membrane mimetic systems in permeability control. In order to gain insight into this important area, we have initiated a research program in BLMs. A status report of our activities in this area will be summarized in the next section. 

Selectivity is generally not affected by the eluent composition or temperature unless these parameters modify the analyte nature (solvation, ionization, tautoermization, etc.). However, the type of solvent methanol versus acetonitrile, for example, may affect the selectivity between critical pair of components (i.e., isomers). 

Three typical electron-ion configurations are shown in Fig. 2 for f = 0.5, 4.0 and 8.0 A which correspond, respectively, to the minimum energy state of Fig. Id, the transition state, and to the fully solvated (ionized) state. 

Photoelectron spectroscopic studies show that the first ionization potential (lone pair electrons) for cyclic amines falls in the order aziridine (9.85 eV) > azetidine (9.04) > pyrrolidine (8.77) >piperidine (8.64), reflecting a decrease in lone pair 5-character in the series. This correlates well with the relative vapour phase basicities determined by ion cyclotron resonance, but not with basicity in aqueous solution, where azetidine (p/iTa 11.29) appears more basic than pyrrolidine (11.27) or piperidine (11.22). Clearly, solvation effects influence basicity (74JA288). 

Fig. 4.11. Solvation changes during ionization of f-butyl chloride.

Stereochemical analysis can add detail to the mechanistic picture of the Sj l substitution reaction. The ionization mechanism results in foimation of a caibocation intermediate which is planar because of its hybridization. If the caibocation is sufficiently long-lived under the reaction conditions to diffirse away from the leaving group, it becomes symmetrically solvated and gives racemic product. If this condition is not met, the solvation is dissymmetric, and product with net retention or inversion of configuration may be obtained, even though an achiral caibocation is formed. The extent of inversion or retention depends upon the details of the system. Examples of this effect will be discussed in later sections of the chapter. 

It has been possible to obtain thermodynamic data for the ionization of alkyl chlorides by reaction with SbFs, a Lewis acid, in the nonnucleophilic solvent S02C1F. It has been foimd that the solvation energies of the carbocations in this medium are small and do not differ much from one another, making comparison of the nonisomeric systems possible. As long as subsequent reactions of the carbocation can be avoided, the thermodynamic characteristics of this reaction provide a measure of the relative ease of carbocation formation in solution. 

Aromatic denitrocyclizations have been used for many years in some well-known synthetic reactions. Probably the best known example is the Turpin synthesis of phenoxazines and similar synthesis of phenothiazines. The classical setup used usually base-catalyzed reactions in polar protic solvents, very often alcohols. In many cases using polar aprotic solvents was found advantageous. Besides the mentioned influence of the H-bonding, better ionization and lower solvation of the nucleophile are also important. Sf Ar reactions proceed through strongly polarized complexes, which are well soluble and highly polarized in polar aprotic solvents. 

Let us discuss now the conditions required for the electron transfer process. This reaction requires, of course, a suitable electron donor (a species characterized by a low ionization potential) and a proper electron acceptor, e.g., a monomer characterized by a high electron affinity. Furthermore, the nature of the solvent is often critical for such a reaction. The solvation energy of ions contributes substantially to the heat of reaction, hence the reaction might occur in a strong solvating solvent, but its course may be reversed in a poorly solvating medium. A good example of this behavior is provided by the reaction Na -f- naphthalene -> Na+ + naphthalene". This reaction proceeds rapidly in tetrahydrofuran or in dimethoxy... 

C—X, Cf, X- and C+ fX (see Fig. 2), the solvation energy increasing the driving force of these dissociations. It is possible that a coordination catalyst is not active in the C—X state but only in one or other of the ionized states. Such behavior blurs the distinction between ionic and coordination polymerization. 

The first step is a slow ionization of the substrate and is the rate-determining step. The second is a rapid reaction between the intermediate carbocation and the nucleophile. The ionization is always assisted by the solvent, since the energy necessary to break the bond is largely recovered by solvation of R" " and of X. For example, the ionization of f-BuCl to f-Bu" and Cl" in the gas phase without a solvent requires ISOkcalmol" (630kJmol" ). In the absence of a solvent such a process simply would not take place, except at very high temperatures. In water, this... 

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