Esterification of Boric Acid with 1,2-Propanediol, 3-Amino-1,2-propanediol, and (±)-3-Dimethylamino-1,2-propanediol as Studied by 11B NMR Spectroscopy

Abstract

Abstract Ester formations of borate ion with 1,2-propanediol, 3-amino-1,2-propanediol, and (±)-3-dimethylamino-1,2-propanediol and stabilities of the formed esters were studied by 11B NMR spectroscopy. For each of the boric acid–diol systems, esters with the 1 : 1 mole ratio of borate ion and diol and with the 1 : 2 mole ratio were formed. In each of the amino–diol–containing systems the 1 : 1 ester was in fact composed of two esters; one from a borate ion and a diol molecule and the other from a borate ion and a protonated diol. Similarly, the 1 : 2 ester consisted of two esters; one from a borate ion and two diol molecules and the other from a borate ion and two protonated diols. The stability constants of the esters formed were calculated based on the peak areas of the signals of the boron species on the 11B NMR spectra. The stability of an ester was strongly correlated with the electronegativity of the anchor moiety of the diol; a higher electronegativity in general yielded a more stable ester.