The relationship between nonaqueous hydrogen bonding and aqueous acidities

Abstract

The relationship between formation of hydrogen bonds in nonaqueous solvents and the aqueous acidities of the donor and the acceptor conjugate acid is described by a multi-step phase-transfer pathway in which the reactants are transferred to water for formation of the hydrogen bond followed by transfer back to the nonaqueous phase. The explicit dependence of the hydrogen bond formation constant on aqueous acidity K_a values and nonaqueous–aqueous partition coefficients is tested with a large body of literature data. The slope of the resultant linear free energy plots is related to how change in acidity free energy is divided between nonaqueous hydrogen bond free energy and the free energy for aqueous hydrogen bond formation from ionized hydrogen donor and acceptor. Analysis of the intercept leads to a proposed new constant, the standard hydrogen bonding constant, which permits comparison of hydrogen bonding tendencies for donor–acceptor classes with no common members, with quite different acidities, and which were studied in different solvents.