Proton transfer from heterocyclic compounds. Part 6. Detritiation rates of various xanthines

Abstract

Rates of detritiation from the C-8 position of xanthine, xanthosine, and a series of methylated xanthines (caffeine, theophylline, theobromine, and paraxanthine) have been measured over a pH range at 85°. In all cases the results can be interpreted in terms of rate-determining hydroxide ion attack on one or more of the different ionised species present in solution; the less the degree of methylation the greater the number of ionisable forms and the larger the number of potential mechanisms. In the case of theobromine and paraxanthine three forms (protonated, neutral, and monoanionic) are involved and in the case of xanthosine a further form (dianionic) makes a contribution to the overall rate. The possibility of zwitterionic contributions are also discussed.