Proximate Charge Effects. 2. Enthalpies of Solvent Transfer in the Choline-Anhydrocholine Equilibrium

Article abstract of DOI:10.1021/ja00387a005

Equilibrium constants for the choline-anhydrocholine equilibrium, eq 1, ⁺(CH3)3NCH2CH2OH + OH^- --> ⁺(CH3)3NCH2CH2O^- + H2O, were measured conductometrically in water, aqueous alcohol, and aqueous dimethyl sulfoxide at several temperatures.On going to progressively less H-bonding media, there was a large shift of the equilibrium constant toward anhydrocholine (to the right) accompanied by an even larger exothermic shift of the enthalpy of reaction.Calorimetric measurement of the enthalpies of solvent transfer revealed that, in the case of the aqueous alcohol solvents, the exothermic shift was caused by enhanced solvation of anhydrocholine, ⁺(CH3)3NCH2CH2O^-, by the less polar media.In the comparison of aqueous ethanol with aqueous Me2SO, the dominant factor was found to be the very great desolvation of both hydroxide ion and anhydrocholine in the less H-bonding medium.