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‑ d 6 (ROHs/DMSO‑ d 6) mixtures at 298 K were investigated by quantitative 13C NMR. The tautomer distributions of d ‑fructose in ethanol/DMSO‑ d 6 and methanol/DMSO‑ d 6 with different volume ratios of alcohols to DMSO‑ d 6 were also examined. For these normal alcohols (methanol, ethanol, n ‑propanol and n ‑butanol)/DMSO‑ d 6 , the tautomer distribution of d ‑fructose in methanol/DMSO‑ d 6 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‑ d 6 obey the following order: 2‑propanol/DMSO‑ d 6 (62.2%, 320 h) > tert ‑butanol/DMSO‑ d 6 (61.6%, 307 h) > 2‑butanol/DMSO‑ d 6 (60.6%, 216 h) > isobutanol/DMSO‑ d 6 (57.8%, 134 h). For the diols/DMSO‑ d 6 , the time for the tautomer distribution of d ‑fructose to reach the equilibrium was found to be: 1,4‑butanediol/DMSO‑ d 6 (113 h) > 1,2‑propanediol/DMSO‑ d 6 (38 h) > ethylene glycol/DMSO‑ d 6 (36 h). For ethanol/DMSO‑ d 6 and methanol/DMSO‑ d 6 with respectively different volume ratios, the experimental results showed that the high concentration of ethanol or methanol increases the conversion rate of tautomers. Graphical abstract Unlabelled Image Highlights • Tautomer distributions of d ‑fructose in ROHs/DMSO‑ d 6 were investigated by 13C qNMR. • The effects of volumetric ratios between ethanol or methanol and DMSO‑ d 6 on tautomerization of d ‑fructose were examined. • Significant differences in the tautomer distributions of d ‑fructose in lower alcohols/DMSO‑ d 6 have been observed. • High concentration of ethanol or methanol increases the conversion rate of tautomers. [ABSTRACT FROM AUTHOR]