The hydrolysis of 8-hydroxyquinoline sulphate and 8-hydroxyquinoline glucopyranoside

Abstract

The hydrolysis of 8-hydroxyquinoline β-D-glucopyranoside has been investigated over a wide range of acidities. In the pH range 1·0 to 5·2 reactions of both free base and N-protonated glucoside contribute to the total rate. Application of the Bunnett and Olsen criterion, the solvent deuterium isotope effect, and the entropy criterion indicates that in more strongly acidic media the hydrolysis mechanism is considerably A-2 in character. Di-positive metal ions have been observed to exert specific catalytic effects on the hydrolysis in the pH range 5·5 to 6·2; the relative catalytic effectiveness of copper(II), nickel(II) and cobalt(II) being 1380:5 :1. For these metal ions a plot of log k cat against the logarithm of the stability constants of the corresponding 1:1 metal ion-8-hydroxyquinoline complexes is linear. This result has been rationalized by assuming the transition state for the reaction involves extensive glucosyl-oxygen bond fission. The hydrolysis of 8-hydroxyquinoline sulphate is also significantly catalysed by copper(II) while catalysis by nickel(II) is less effective. On the basis of the solvent isotope effect (k D2O/k H2O = 1·9), the value of the parameter ø for the reaction, and the effect or dioxan on the rate, the acid catalysed hydrolysis of the sulphate ester is viewed as occurring by an A-1 pathway.