Thermodynamic Insights into MgBr2-Mediated Glucose Interconversion to Fructose Undertaking Multiple Reaction Pathways by Applying the Macro- and Micro-Kinetic Principles

Abstract

While many reports on the kinetics and mechanism of glucose isomerization to fructose via a chemical pathway have been published, thermodynamic insights into the interconversion reaction are rarely reported. Here, we report the temperature-dependent characteristics of the cation- and anion-mediated glucose conversion to fructose. The counter ions of MgBr2 enabled the reaction via different pathways in water; e.g., Mg2+ influenced the reaction by undertaking the 1,2-hydride shift mechanism and accounted for up to 50% of the conversion. However, the counter ion (Br–) promoted the conversion via the proton transfer mechanism, which contributed to the remaining 50%, based on the results of the isotopic labeling experiments. This selective transformation (32% wt fructose yield and 76% selectivity) aided by MgBr2 is attributed to the formation of a weak water shell around Mg2+ (due to a lower ratio of MgBr2 to water), which permitted the cation to expose its catalytic activity and affected the administering activity by Br–, which induces maximum side reactions. By applying the principles of transition-state Eyring and Marcus theories to the elementary steps involving a proton transfer and electron transfer, respectively, the temperature-dependent characteristics of the corresponding pathways were determined. The Eyring model exhibited a linear trend in the ln(k/T) vs 1/T plot with an activation energy barrier of 70.25 kJ/mol (comparable to the value of the collision-based Arrhenius model). The semi-classical Marcus model disclosed that the hydride shift is a normal electron transfer rate based on the localization of kET in the λ< -ΔGo >0 region.