Ionization constants of weak acids and bases in organic solvents

A discussion of the influence of organic solvents on pKa values is presented. Enthalpy and entropy of ionization in organic solvents are compared with aqueous systems. The impact of the solvent on the ionization constants is interpreted based on the free energy of transfer applied to all particles involved in the ionization reaction of acids and bases, and the concept of the 'medium effect' on these species. The limitation of Born's approach (which takes into account only electrostatic effects on the ionization equilibrium) is demonstrated and the importance of solute-solvent interactions on the change of the pKa values emphasized.