Mutarotation of D(+)-glucose. II. Volumes of activation for the uncatalysed and copper(II)-catalysed rates

Abstract

The effect of high pressure on the rate of mutarotation of α-and β-D(+)-glucose in aqueous sodium perchlorate solutions has been evaluated. The observed rate constant kΨ = kα + kβ increases with pressure while the equilibrium constant Keq = kα /kβ is nearly unchanged from the 1.7 found at 1 bar. From the experimental results, the individual rate constants kα and kβ and the corresponding activation volumes ∆V‡α and ∆V‡β can be evaluated. The values obtained, -10.7 � 0.1 and -10.0 � 0.1 cm3 mol-1, are discussed in relation to the reaction mechanism. In the presence of Cu(ClO4)2 the values of ∆V: form a continuous series, dependent upon the pH and the concentration of copper(II) catalyst, up to + 1.3 cm3 mol-1. The activation volumes for mutarotation by monomeric and dimeric copper(II) catalysts, ∆V‡(Cu2+) and ∆V‡ {CU((OH)2Cu)2+}, are -1.2 � 0.2 and + 3.6 � 0.2 cm3 mol-1 respectively. Reaction mechanisms are discussed to account for these values of ∆V‡.