The Influence of Solvation on the Acid-Base Properties of Organic Compounds in Various Media

The influence of non-specific (electrostatic) and specific (with formation of a hydrogen bond) solvation of ions in water on the strength of different organic acids is considered by comparing their deprotonation constants (pK_d) in water, nitromethane (a solvent which serves as a model for electrostatic solvation), and in the gas phase. The decisive role of the electrostatic solvation of ions in the variation of pK_d and in the weakening of the influence of substituents on the pK_d for ammonium, anilinium, and pyridinium cations, benzoic acids, and phenols on passing from the gas phase to water and also in the differentiation of the basicities of primary, secondary, and tertiary amines in water is demonstrated. The pK_d for organic OH-, O⁺H-, NH-, N⁺H-, PH-, and CH-acids in water, dimethyl sulphoxide, and nitromethane are compared, which permits the determination of the relation between the localisation of the charge in organic ions and dipolar species, on the one hand, and their solvation via hydrogen bonding and dispersion interaction in solution, on the other. The relation between the capacity of bridge groups to transmit electrons and the medium is considered.
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