NMR studies of the tautomer distributions of d‑fructose in lower alcohols/DMSO‑d6

Abstract

The tautomer distributions of d‑fructose, a popular model compound for biorefinery, in various lower alcohols and DMSO‑d6 (ROHs/DMSO‑d6) mixtures at 298 K were investigated by quantitative 13C NMR. The tautomer distributions of d‑fructose in ethanol/DMSO‑d6 and methanol/DMSO‑d6 with different volume ratios of alcohols to DMSO‑d6 were also examined. For these normal alcohols (methanol, ethanol, n‑propanol and n‑butanol)/DMSO‑d6, the tautomer distribution of d‑fructose in methanol/DMSO‑d6 exhibited the shortest equilibration time (46 h) and the lowest concentration of β‑furanose (39.0%) at equilibrium. The fraction of fructofuranose (α‑furanose and β‑furanose) at equilibrium and time needed to reach equilibrium in branched chain alcohols/DMSO‑d6 obey the following order: 2‑propanol/DMSO‑d6 (62.2%, 320 h) > tert‑butanol/DMSO‑d6 (61.6%, 307 h) > 2‑butanol/DMSO‑d6 (60.6%, 216 h) > isobutanol/DMSO‑d6 (57.8%, 134 h). For the diols/DMSO‑d6, the time for the tautomer distribution of d‑fructose to reach the equilibrium was found to be: 1,4‑butanediol/DMSO‑d6 (113 h) > 1,2‑propanediol/DMSO‑d6 (38 h) > ethylene glycol/DMSO‑d6 (36 h). For ethanol/DMSO‑d6 and methanol/DMSO‑d6 with respectively different volume ratios, the experimental results showed that the high concentration of ethanol or methanol increases the conversion rate of tautomers.