A polarimetric and 11B and 13C nuclear magnetic resonance study of the reaction of the tetrahydroxyborate ion with polyols and carbohydrates

Abstract

The reaction of the tetrahydroxyborate ion, B(OH)^–₄, with 31 polyols and carbohydrates in aqueous solution has been studied by complementary investigations involving polarimetry and ¹¹B and ¹³C n.m.r. spectroscopy. The calculation of equilibrium constants for the complexation of the carbohydrates with B(OH)^–₄ from the polarimetry results, using a previously derived equation, was only partially successful owing to the presence of multiple equilibria in a number of cases. The presence of several complexed species in these reactions is demonstrated by ¹¹B n.m.r. studies. A calibration method devised to relate peak area in ¹¹B n.m.r. spectra with concentration has been used to calculate equilibrium constants for the various equilibria present, and an attempt has been made to rationalise the equilibrium constants with the molecular structures of the substrates. In the majority of cases the ¹³C n.m.r. spectra confirm the polarimetric and ¹¹B n.m.r. studies, and in most cases allow a more specific identification of the reaction sites in the polyols/carbohydrates to be made.