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 k_Ψ = k_α + k_β increases with pressure while the equilibrium constant K_eq = k_α /k_β is nearly unchanged from the 1.7 found at 1 bar. From the experimental results, the individual rate constants k_α and k_β 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^‡.