A temperature-dependent change in the mechanism of acid catalysis of the hydrolysis of p-nitrophenyl β-D-glucopyranoside indicated by oxygen-18 and solvent deuterium kinetic isotope effects

Abstract

The ¹⁸O kinetic isotope effect for hydrolysis of p-nitrophenyl [1-¹⁸O]-β-glucopyranoside in 2.0M-HCl, measured by the isotopic quasi-racemate method, is 1.025₇ at 65.5 °C and 1.023 at 75.1 °C; there is a literature value of 1.0355 ± 0.0015 at 50.0 °C, measured mass spectrometrically. That this apparent discrepancy arises from a change in the mode of acid catalysis as the temperature is lowered is shown by the strongly temperature-dependent solvent deuterium isotope effect [log (k_D2O/k_H2O)= 1.84 – 0.595 × 10³/T], and by the greater effect of trifluoroacetate buffers at 45.0 than at 85.0 °C: a rough catalytic constant for general acid catalysis by trifluoroacetic acid of ca. 10^–7.5 I mol^–1 s^–1 can be estimated at the former temperature.