KINETICS AND MECHANISM OF THE ACID-CATALYZED HYDROLYSIS OF 1,1-DIMETHOXYETHENE (KETENE DIMETHYL ACETAL) AND TRIMETHOXYETHENE IN AQUEOUS-SOLUTION

Rates of acid-catalyzed hydrolysis of 1,1-dimethoxyethene and trimethoxyethene were measured in wholly aqueous solutions of perchloric acid, sodium hydroxide, and a series of carboxylic acid buffer solutions at 25-degrees-C. The reactions show general acid catalysis and give substantial solvent isotope effects in the normal direction (k(H)/k(D) > 1). This indicates that hydrolysis Occurs via proton transfer from catalyst to substrate followed by hydration of the dialkoxy cation so formed and subsequent decomposition of the hydrogen ortho ester hydration product; deuterium incorporation experiments show that the initial proton transfer is not reversible, even in sodium hydroxide solutions. Comparison of Bronsted exponents for these and other ketene acetal hydrolysis reactions with Bronsted exponents for vinyl ether hydrolysis suggests that ketene acetal hydrolysis is the intrinsically slower reaction, despite the fact that the presently available examples show it to be occurring more rapidly.