HYDROLYSIS OF ORTHO ESTERS: FURTHER INVESTIGATION OF THE FACTORS WHICH CONTROL THE RATE-DETERMINING STEP.

Article abstract of DOI:10.1021/ja00361a017

Comparison of rates of hydronium ion catalyzed hydrolysis measured at low and at high pH reveals that the cyclic ortho ester 2-methyl-2-methoxy-1,3-dioxolane undergoes a change in reaction mechanism from rate-determining generation of a dialkoxycarbonium ion (stage 1) to rate-determining decomposition of a hydrogen ortho ester intermediate (stage 3), but it shows that its acyclic counterpart, trimethyl orthoacetate, does not. This difference is not caused by the dimethyleneoxy structure of the cyclic ortho ester, inasmuch as an acyclic ortho ester with a similar structure, tris(2-methoxyethyl) orthoacetate, also does not undergo a change in rate-determining step. This difference in behavior is inconsistent with a hypothesis advanced before involving differential substituent effects on stage 1 and stage 3, and analysis of previously published data shows that the substituent effects on these two stages are in fact quite similar.