Solvent effects on chemical reactivity

Roland Schmid Technical University of Vienna Institute of Inorganic Chemistry, Vienna, Austria 

That what follows is not intended just to give an overview of existing ideas, but instead to fdter seminal conceptions and to take up more fundamental ideas. It should be mentioned that solvent relaxation phenomena, i.e., dynamic solvent effects, are omitted. 

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 presented the first satisfactory qualitative account of solvent effects on reactivity by the concept of activated complex solvation. 

Picturing the solvent as a homogeneous dielectric continuum means in essence that the solvent molecules have zero size and that the molecules cannot move. The most adequate physical realization would be a lattice of permanent point dipoles fliat can rotate but cannot 

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The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Tapia, O. and Bertran, J. 1996. Solvent Effects on Chemical Reactivity, Kluwer Dordrecht. 

For many physical organic chemists, the Menschutkin reaction was a kind of guinea pig , which has been extensively used for the study of solvent effects on chemical reactivity. A comprehensive review of this reaction has been given by Abboud el al. [786], More recent theoretical treatments of the solvent influence on Menschutkin reactions can be found in references [787-789]. 

Thus, whenever a chemist wishes to carry out a chemical reaction he not only has to take into consideration the right reaction partners, the proper reaction vessels, and the appropriate reaction temperature. One of the most important features for the success of the planned reaction is the selection of a suitable solvent. Since solvent effects on chemical reactivity have been known for more than a century, most chemists are now familiar with the fact that solvents may have a strong influence on reaction rates and equilibria. Today, there are about three hundred common solvents available, nothing to say of the infinite number of solvent mixtures. Hence the chemist needs, in addition to his intuition, some general rules and guiding-principles for this often difficult choice. 

Swain, C.G. (1984). Substituent and Solvent Effects on Chemical Reactivity. J.Org.Chem., 49, 2005-2010. 

Swain CG, Swain MS, Powell AL, Alunni S (1983) Solvent effects on chemical reactivity evaluation of anion and cation solvation components. J Am Chem Soc 105 502-513... 

Supercritical carbon dioxide (SC-CO2) is proving to be a suitable environmentally benign solvent for free radical reactions, providing a unique alternative to many conventional solvents for these reactions which are either carcinogenic or damaging to the environment. Part 1 of this paper examines the implications associated with the use of SC-C02 with regard to issues of solvent effects on chemical reactivity. In Part 2, a new environmentally benign chemical process is described which effects the conversion RH + C=C-C-Br R-C-C=C + HBr via a free radical chain reaction. 

Since then, the generality and importance of solvent effects on chemical reactivity and physical properties of species in dilute solutions has been widely acknowledged. Solvent-solute interactions for reactants and for products account for observed shifts in chemical equilibria those involving reactants and transition states determine changes in the rates of elementary processes. Shifts of the absorption and/or fluorescence maxima originate in differential solvent-solute interactions of the ground and electronically excited states of a dissolved species. The perturbations induced by the solvents are reflected by concurrent variations of such physical properties of the solute as ir, nmr, and epr spectra and partial molar properties. 

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