Raman spectroscopic study of dehydration reactions between boric acid and aliphatic dihydric alcohols in solution

Abstract

Boric acid (B(OH)3) esters in dihydric alcohol (R(OH)2) solutions were analyzed using Raman spectroscopy to investigate the sequential reactions between B(OH)3 and the lower dihydric alcohol molecules. The dihydric alcohols used in this study were ethane-1,2-diol, propane-1,2-diol, butane-1,2-diol, butane-2,3-diol, propane-1,3-diol, and butane-1,3-diol, which are classified into two types: alkyl-n,(n+1)-diol and alkyl-n,(n+2)-diol (n = 1 or 2). The Raman spectral changes caused by adding B(OH)3 or H2O were analyzed in terms of the formation of boric acid esters in the alcohol solutions. Multiple Raman bands were observed in the frequency range 900–700 cm−1 for each alcohol solution and most of the band intensities depended on the initial molar ratios of B(OH)3/R(OH)2 and B(OH)3/R(OH)2/H2O. The difference spectra, obtained using the difference spectrum method, suggested the formation of cyclic boric acid esters with a 5- or 6-membered ring for all the dihydric alcohol solution systems used in this study. Based on the Raman spectral changes, we examined the boric acid esters formed in the dihydric alcoholic solutions.