Solvent effects is the group of effects that a solvent has on chemical reactivity. Solvents can have an effect on solubility, stability and reaction rates and choosing the appropriate solvent allows for thermodynamic and kinetic control over a chemical reaction.
Effects on solubility
A solute dissolves in a solvent when it forms favorable interactions with the solvent. This dissolving process all depends upon the free energy change of both solute and solvent. The free energy of solvation is a combination of several factors.
Solvation of solute by solvent
First, a cavity must be created in the solvent. The creation of the cavity will be entropically and enthalpically unfavorable as the ordered structure of the solvent increases and there are fewer solvent-solvent interactions. Second, the solute must separate out from the bulk solute. This is enthalpically unfavorable as solute-solute interactions are breaking but is entropically favorable. Third, the solute must occupy the cavity created in the solvent. This results in favorable solute-solvent interactions and is also entropically favorable as the mixture is more disordered than when the solute and solvent are not mixed. Dissolution often occurs when the solute-solvent interactions are similar to the solvent-solvent interactions, signified by the term like dissolves like. Hence, polar solutes dissolve in polar solvents, whereas nonpolar solutes dissolve in nonpolar solvents. There is no one measure of solvent polarity and so classification of solvents based on polarity can be carried out using different scales.
Effects on stability
Different solvents can affect the equilibrium constant of a reaction by differential stabilization of the reactant or product. The equilibrium is shifted in the direction of the substance that is preferentially stabilized. Stabilization of the reactant or product can occur through any of the different non-covalent interactions with the solvent such as H-bonding, dipole-dipole interactions, van der waals ineteractions etc.
Effects on reaction rates
Often, reactivity and reaction mechanisms are pictured as the behavior of isolated molecules in which the solvent is treated as a passive support. However, solvents can actually influence reaction rates and order of a chemical reaction.
Solvents can affect rates through equilibrium-solvent effects that can be explained on the basis of the transition state theory. In essence, the reaction rates are influenced by differential solvation of the starting material and transition state by the solvent. When the reactant molecules proceed to the transition state, the solvent molecules orient themselves to stabilize the transition state. If the transition state is stabilized to a greater extent than the starting material then the reaction proceeds faster. If the starting material is stabilized to a greater extent than the transition state then the reaction proceeds slower. However, such differential solvation requires rapid reorientational relaxation of the solvent (from the transition state orientation back to the ground-state orientation). Thus, equilibrium-solvent effects are observed in reactions that tend to have sharp barriers and weakly dipolar, rapidly relaxing solvents.
Frictional solvent effects
The equilibrium hypothesis does not stand for very rapid chemical reactions in which the transition state theory breaks down. In such cases involving strongly dipolar, slowly relaxing solvents, solvation of the transition state does not play a very large role in affecting the reaction rate. Instead, dynamic contributions of the solvent (such as friction, density, internal pressure, or viscosity) play a large role in affecting the reaction rate