Basicité de solvants dipolaries aprotoniques

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Abstract
The basicities of five dipolar aprotic solvents, sulfolane, propylene carbonate, acetonitrile, dimethylformamide, dimethyl sulfoxide, and of water have been characterized by using as parameter the solvation enthalpy of the gaseous proton. The corresponding values of ΔH(g → s)(H+) are deduced from experimental calorimetric data on the reaction HCl(g) → H+(s) + Cl−(s) and the known enthalpy changes for H+(g) → H+(aq) and Cl−(aq) → Cl−(s). The transfer enthalpies of Cl− from water to solvent are estimated on the basis of an extrathermodynamic assumption. The values obtained for ΔH(g → s)(H+) are (kcal mol−1): TMS(−252.6), AN(−256.6), PC(−259.5), H2O(−270.0), DMF(−276.7), DMSO(−276.1). These values are compared to other parameters currently used to define the basicity of these solvents whether as molecules or as media. Thus, proton affinities are such that in the gas phase, acetonitrile is more basic than water by 17 kcal mol−1, while in the liquid phase our results indicate that acetonitrile is less basic than water by 13 kcal mol−1. This variation of 30 kcal mol−1 is attributed to differences in the nature of the solvated proton H+(s) in both solvents: several indications suggest that H+(s) is present as H+(AN)2 in AN, an aprotic solvent, while it is stabilized as H+(H2O)n in water, a protic solvent because of the formation of hydrogen bonds of a cooperative nature. Solvent basicity orders as defined with respect to ΔH(g → s)(H+) and ΔH(g → s)(HA) are shown to differ and the consequences as to the solvent effect on the dissociation of acids HA are considered. Further, there is a nearly linear relationship for the five dipolar aprotic solvents between the values of ΔH(g → s)(H+) and the enthalpy of protonation in fluorosulfonic acid.

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