Structures of borate–aldohexose and borate–chetohexose complexes in aqueous solution. A thermodynamic study

Abstract

Heats of complex formation of the borate ion with the carbohydrates D-galactose, D-glucose, D-fructose, D-mannose, and L-sorbose have been determined by direct calorimetry. By means of the equilibrium constants the corresponding Gibbs functions and entropies were also obtained. The present data refer to the aqueous medium, at T= 25 °C and I= 0.1 mol dm^–3. Examination of the results indicates a leading role, in several cases, of solute–solvent external factors (e.g. desolvations of the borate ion) in determining the different stabilities of the complexes. Amongst various factors for the ligands, some appear of minor importance, while the contribution of others (such as the strength of the borate—hexose C–O–B bonds) is fairly constant throughout one series. The present data seem also to confirm the presence of the furanose cyclic form of the carbohydrate in the complex.