Hydrolysis of Trimethyl Orthocyclopropanecarboxylate: A Change in Rate-Determining Step

Article abstract of DOI:10.1021/ja00377a023

The rate constant for the hydronium ion catalyzed hydrolyis of trimethyl orthocyclopropanecarboxylate measured in dilute aqueous HCl, k_H+=5300 M^-1s^-1, was found to be different from that measured in buffer solutions at pH 6-8, k_H+=81 00 M^-1s^-1.This difference is similar to that observed for cyclic ortho esters and is taken as evicence for a change in the reaction mechanism from rate-determining conversion of ortho ester to a dialkoxycarbonium ion intermediate at high pH to rate-determining decomposition of the hydrogen ortho ester formed by hydration of this ion at low pH.Discovery of this mechanistic change in this acyclic system suggests that this is a general phenomenon common to all ortho esters substituted with carbication-stabilizing groups at their pro-acyl carbon atoms.