Competition between Solute‐Solvent and Solvent‐Solvent Hydrogen Bonds: Pyridines in Alcohols and in Water

Partial enthalpies of mixing of pyridines in methanol, ethanol, 1‐butanol, water and cyclohexane were determined calorimetrically at 25°C. From these data their molar enthalpies of transfer, at infinite dilution, from the inert solvent to the H bonded solvents were calculated. They were compared with the enthalpies of H bond formation between the pyridines and the alcohols in CCl4 and the enthalpies of the selfassociation bonds in the pure liquids. Although in the pure alcohols more than 95% of the molecules are at a given time involved in H bonds with an energy of about 25 kJmol−1 which is markedly larger than the energy of the pyridine‐alcohol bonds in CCl4, the molar enthalpy of transfer of the pyridines from an inert solvent to an H bonded one remains exothermic. A quantitative treatment of the competition between the two types of H‐bonds is presented. It is based on the thermodynamics of mobile order that was recently developed. It leads to an equation that reasonably predicts the transfer enthalpies. According to this treatment the competition reduces the complexation constant by a factor K0AB/(K0AB K0AA), K0AB and K0AA being the stability constants of the H bonds in the absence of competition. In water the exothermic effect is enhanced because the second O ‐ H group of the molecule forms much weaker selfassociation H‐bonds than the first one.