Glucose and Fructose Hydrates in Aqueous Solution by IR Spectroscopy

In the mid-infrared attenuated total reflectance (MIR−ATR) spectra of aqueous d-glucose and d-fructose solutions, two hydrates were found by factor analysis (FA) for each sugar, d-glucose penta- and dihydrates and d-fructose penta- and monohydrates. We obtained the spectra and abundances for these hydrates as a function of carbohydrate concentrations. The biggest difference in these spectra lies in the CO stretch region. From the distribution of the species, the equilibrium between d-glucose pentahydrate and dihydrate is 3(H2O)2 + 2(C6H12O6·2H2O) ⇄ 2(C6H12O6·5H2O), with the equilibrium constant KG = (3.2 ± 0.6) × 10-5 L3 mol-3. For d-fructose, the equilibrium is between pentahydrate and monohydrate, 2(H2O)2 + C6H12O6·H2O ⇄ C6H12O6·5H2O, with the equilibrium constant KF = (7.1 ± 1.2) × 10-3 L2 mol-2. The four hydrates are present only in aqueous solutions and cannot be obtained in the solid state.