The effect of spatial orientation of hydroxyl groups on the cleavage of the C3–C4 bond in typical monosaccharides

YCl₃ is found to be effective in catalyzing the conversion of various monosaccharides to lactic acid (LaA) in water, and rhamnose conversion has been discovered to occur at lower temperature than glucose conversion. In the present work, DFT calculations were performed to study the conversion of rhamnose and glucose comparatively, emphasizing the effect of spatial orientation of hydroxyl groups on the reaction. The results showed that [Y(OH)₂(H₂O)₂]⁺ species was active for the conversion. At 160 °C, the TOF value for the conversion of rhamnose to LaA was higher than that for the conversion of glucose, indicating the kinetic advantage of the rhamnose conversion. It was found that compared to rhamnose, when glucose was isomerized to ketose, the hydroxyl groups on C2 and C4 could form strong H-bonds in the reaction region, which caused the 1,2-isomerization of glucose to need to overcome a higher energy barrier, suppressing the conversion to LaA, whereas such H-bonds did not exist in rhamnose conversion. The TOF value of mannose was predicted according to the reaction mechanism, and the rationality of the reaction mechanism was confirmed by kinetic experiments.