Mutarotation of D(+)-glucose. I. Evidence of electrophilic attack by hydrated monomeric and dimeric copper(II) species

Abstract

The rate of mutarotation of α-D(+)-glucose is markedly affected by the addition of electrolytes. At 293 K the rate constants for reaction with H₂O, HClO₄, NaClO₄ and CU(ClO₄)₂ are 4.6 × 10^-6, 5.9 × 10^-3,1.2 × 10^-4 and 4.6 × 10^-3 dm³ mol^-1 s^-1 respectively. In the pH range 3.5 - 4.4 the rate of mutarotation increases sharply in solutions of Cu(ClO₄)₂ because of the presence of small concentrations of Cu((OH)₂CU)²⁺ for reaction with which the second-order rate constant is 4.0 dm³ mol^-1 s^-1.

A model for mutarotation is proposed which involves a specifically oriented aquation sheath about the charged ion which then acts as an enhanced electrophile in the rate-determining step to form the aldehyde intermediate. In the presence of mixed cationic catalysts, additivity rather than competition is observed between the effect of two catalysts. This result and the observed increase in the entropy of activation in the presence of aquated copper(II) support this model.