Handbook of Solvents - George Wypych - ChemTech - Ventech!

13
Effect of Solvent on Chemical
Reactions and Reactivity
13.1 SOLVENT EFFECTS ON CHEMICAL REACTIVITY
Roland Schmid
Technical University of Vienna
Institute of Inorganic Chemistry, Vienna, Austria
13.1.1 INTRODUCTION
About a century ago, it was discovered that the solvent can dramatically change the rate of
chemical reactions. 1 Since then, the generality and importance of solvent effects on chemical
reactivity (rate constants or equilibrium constants) has been widely acknowledged. It
can be said without much exaggeration that studying solvent effects is one of the most central
topics of chemistry and remains ever-increasingly active. In the course of development,
there are few topics in chemistry in which so many controversies and changes in interpretation
have arisen as in the issue of characterizing solute-solvent interactions. In a historical
context, two basic approaches to treating solvent effects may be distinguished: a
phenomenological approach and a physical approach. The former may be subdivided further
into the dielectric approach and the chemical approach.
• Phenomenological approach
Dielectric
Chemical
• Physical approach
That what follows is not intended just to give an overview of existing ideas, but instead
to filter seminal conceptions and to take up more fundamental ideas. It should be mentioned
that solvent relaxation phenomena, i.e., dynamic solvent effects, are omitted.
13.1.2 THE DIELECTRIC APPROACH
It has soon been found that solvent effects are particularly large for reactions in which
charge is either developed or localized or vice versa, that is, disappearance of charge or
spreading out of charge. In the framework of electrostatic considerations, which have been
around since Berzelius, these observations led to the concept of solvation. Weak electrostatic
interactions simply created a loose solvation shell around a solute molecule. It was in
this climate of opinion that Hughes and Ingold 2 presented the first satisfactory qualitative
account of solvent effects on reactivity by the concept of activated complex solvation.