THE KINETICS OF THE MOLYBDATE-CATALYZED EPIMERIZATION OF D-GLUCOSE AND D-MANNOSE IN AQUEOUS-SOLUTIONS

The kinetics of the molybdate-catalyzed epimerization of D-glucose and D-mannose in aqueous solutions have been investigated. Batch experiments were carried out at initial sugar concentrations from 0.3 to 3 kmol m-3, molybdenum concentrations from 0.3 to 40 mol m-3, temperatures from 330 to 363 K and pHs from 1.9 to 3.1. Titrations of sugar/molybdate solutions with HCl could be described by the formation of dimolybdate complexes. The complexation and ionization equilibria are such that free molybdates can be neglected. The epimerization of D-glucose is faster than that of D-mannose. The rate of epimerization shows a maximum at a pH of 2.5. The kinetic observations are explained by a reaction sequence with a rate-determining step involving molybdate complexes of D-glucose and D-mannose as reactive intermediates and with the protonation or deprotonation of these intermediates leading to less active complexes.