Solvation, physical organic models

Solvent effects on chemical equilibria and reactions have been an important issue in physical organic chemistry. Several empirical relationships have been proposed to characterize systematically the various types of properties in protic and aprotic solvents. One of the simplest models is the continuum reaction field characterized by the dielectric constant, e, of the solvent, which is still widely used. Taft and coworkers [30] presented more sophisticated solvent parameters that can take solute-solvent hydrogen bonding and polarity into account. Although this parameter has been successfully applied to rationalize experimentally observed solvent effects, it seems still far from satisfactory to interpret solvent effects on the basis of microscopic infomation of the solute-solvent interaction and solvation free energy. 

Continuum solvation models have also been used to rationalize the Hammett p+ parameters determined [200] from SN1 solvolysis rate constants [201] of cumyl chlorides. In particular the SM5.42R/AM1 [112] model reproduces the experimental p+ within 24%. The use of continuum models for placing the empirical correlations of physical organic chemistry on a firmer basis is in its infancy. 

We draw upon each of these notions as necessary to explain various aspects of bonding, reactivity, and structure. We will add solvation effects later in the book. For particular reactions or structures, we may have to refine and/or combine these notions to get the optimal model for the molecule. In some cases we may even need to completely re-think and modify these foundations. However, there is often a tendency in physical organic chemistry to become quite focused on the "exceptions to the rule". We should keep in mind that the vast majority of organic structures are well described by this simple model. 

Cramer and Truhlar are currentiy developing SM7. We now have a handle on how to treat solvation free energies for ions in organic solvents. We believe we have a more physical way to construct the model. This is probably the last thing that we will do in terms of SM development. Where solvation models will move to is to non-homgeneous solutions liquid crystals, interfaces, etc., Cramer predicts. 

The data presented above demonstrate that the total solvation free energy of organic compounds in aqueous solution can be calculated with some confidence using a minimum number of parameters (the dielectric constant and the solute cavity size contibution for the solvent) and provided that appropriate quantum chemical and statistical physical models are used for the description of the reaction field and dispersion interactions, and the cavity formation in solution. 

Modeling of the Baylis-Hillman reaction, using parameters from standard formulae and the Peng-Robinson equation of state reproduces the general experimental trends observed for the pressure dependence of a particular Baylis-Hillman reaction. The calculations show that the effect has a physical explanation, which is that the product stabilisation by solvation is reduc at hi er pressures, reducing its equilibrium amount. The work has led to a general method, which can be used to predict conditions for optimum yield for any Baylis-Hillman reaction and by adaptation to any organic equilibrium reaction. 

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