Fig. 4

Fructose dehydration reaction structures. Structures along the reaction path for fructose dehydration carried out in water and HCl mixtures (a–c) and in 90% GVL/10% H₂O (w/w) and HCl mixtures (d–f). The structure in the first (a, d), second (b, e), and third (c, f) columns refers to fructose along with H⁺ and Cl⁻ in their reactant state in solution, the resulting oxocarbenium ion from the initial protonation of the C2 hydroxyl group and the elimination of water, and the transition state for the deprotonation of the oxocarbenium ion to form the enol product, respectively. Red, gray, green, and white spheres refer to the oxygen, carbon, chlorine, and hydrogen atoms, respectively. The yellow and blue highlighted circles refer to the reactive proton and oxocarbenium ion centers, respectively (additional structures along the path are presented in Supplementary Figure 6 and Supplementary Figure 9)