The Rates of Hydrolysis of Some Substituted Phenyl 2-Acetamido-2-deoxy-α-and-β-D-glucopyranosides

Abstract

The rates of hydrolysis of some substituted phenyl 2-acetamido-2-deoxy-α-and-β-D-glucopyranosides have been measured in acidic, neutral and alkaline aqueous solutions. In acidic solution, the β-glycosides hydrolyze 2-6 times as fast as the corresponding α-glycosides, and the rates of hydrolysis of the β-glycosides exhibit almost linear ρ-σ relationships (ρ=-0.52 at 61°, -0.54 at 78.2°) whereas those of the α-glycosides show seemingly convex ρ-σ curves, and the tentative calculation on linear relationship shows small ρ values (ρ=-0.16 at 61°, -0.12 at 68.9° and -0.08 at 78.2°). The kinetic parameters for the acid hydrolysis were calculated and discussed in connection with the participation of 2-acetamido group in intramolecular catalysis. The hydrolysis of the glycosides at 120° in neutral buffer solution (pH 6.8) is facilitated by electron-withdrawing substituent (ρ=+0.47 for the α-glycosides, +1.62 for the β-glycosides). The pH-log k_obs profile at 120° for the p-chlorophenyl β-glycoside indicates that the glycoside hydrolyzes at a spontaneous hydrolysis rate. The rate of alkaline hydrolysis of the β-glycosides increases with σ (ρ=+1.69 at 100°, +1.65 at 110°), but the α-glycosides are abnormally alkali-stable.