Base-catalysed mutarotation of α-D-tetramethylglucose in mixed H2O–D2O solvents

Abstract

The kinetics of the general acid–base-catalysed mutarotation of α-D-tetramethylglucose have been studied at 25° with the base catalysts pyridine, 2-picoline, 4-picoline, and 2,6-lutidine in mixtures of H₂O and D₂O and the pure waters. The ratios k_D : k_H of the rate coefficients in pure D₂O and H₂O are 0·35₆, 0·33₈, 0·42₂, and 0·35₄ for pyridine, 2-picoline, 4-picoline, and 2,6-lutidine respectively. An analysis in terms of the Brönsted catalysis law reveals the existence of steric hindrance to proton abstraction, the magnitude of the steric effect depending on the number of α-methyl substituents in the base molecule. The experimental data are best fitted by the generalized Gross–Butler equations for the transition state with no water molecules; inclusion of solvent water molecules leads to poorer fits, but seems to be required in a cyclic transition state containing the base in the ring, to explain the existence of the steric effect.