Solvation in Pure and Mixed Solvents: An Overview
|Omar A. El Seoud|
Instituto de Química, USP (IQUSP), Av. Lineu Prestes 748, Sao Paulo 05508-900, Brazil
Understanding solvation is a prerequisite to delineate the effects of solvents and their mixtures on reaction rates and equilibria. These effects cannot be satisfactorily described in terms of a single solvent property. For pure liquids, use of multiple-parameter equations has proved to be a more successful approach. The parameters employed include solvent acidity, basicity and dipolarity/polarizability. The success of these correlations has lead to a wide interest in studying the phenomena of solvatochromism and thermosolvatochromism (effects of temperature on solvatochromism). Solvation is a function of the properties of both probe and solvent, in particular, their pKa and hydrophobic/hydrophilic character. Increasing temperature leads to desolvation; the corresponding energy is ca. 1-2 kcal/mol, i.e., is not negligible compared to the enthalpy of activation of typical organic reactions. Solvation in binary solvent mixtures is more complex than in pure solvents because of “preferential solvation” of the probe by one component of the mixture. That is, the composition of the probe solvation micro-sphere does not match that of bulk solvent. This composition is given by the so-called solvent “fractionation factors”, j, whose value, relative to unity, indicates the solvent that is preferentially solvating the probe. Recently, a general model has been introduced to calculate j in which the formation of the hydrogen-bonded species (e.g., ROH--OH2) both in bulk solvent and in the probe solvation microsphere was explicitly considered. Application of this model to solvation in binary mixtures has shown that the probes are preferentially solvated by the hydrogen-bonded species. Solvation of a single probe in a series of aqueous alcohols depends on the pKa and hydrophobicity of the alcohol. Additionally, temperature increase affects the structure of alcohol more than that of water.
Conclusions and perspectives
The study of solvatochromism and thermo-solvatochromism has shown the complexity and interplay of the factors involved. This subject is generating increased interest because of the intense research on “green” solvents. The main conclusion of these studies is that the composition of the solvation microsphere of the species of interest (reagents and/or activated complexes) differs from that of bulk binary mixture. An example of application is the judicious choice of the organic component of the mixture (protic or aprotic solvent) in reactions where desolvation is important, e.g., SN2, acyl transfers, etc.
I would like to thank FAPESP and CNPq for financial support and a research productivity fellowship and my research co-workers for doing the work reported.
Presentation: invited lecture at 18th Conference on Physical Organic Chemistry, Plenary session, by Omar A. El Seoud
See On-line Journal of 18th Conference on Physical Organic Chemistry
Submitted: 2006-07-04 14:44 Revised: 2006-08-04 17:30