Hughes–Ingold rules

In chemistry, solvent effects refers to the influence of a solvent on chemical reactivity or molecular associations. 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.

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.

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 decreases 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. (see also: Solvents - solvent classification)

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 interactions etc.

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