Correlation of rates of uncatalyzed and hydroxide-ion catalyzed ketene hydration. A mechanistic application and solvent isotope effects on the uncatalyzed reaction

Rates of hydration of a number of ketenes were measured in neutral and basic solution using flash photolytic techniques, and rate constants for their uncatalyzed, k(uc), and hydroxide-ion catalyzed, k(HO), reactions were determined. These results, plus additional data from the literature, were found to provide the remarkably good correlation log k(uc) = -3.21 + 1.14 log k(HO), which spans 10 orders of magnitude in reactivity and includes 31 ketenes. This good correlation implies that uncatalyzed and hydroxide-ion catalyzed ketene hydraton occur by similar reaction mechanisms, which for the hydroxide-ion catalyzed process is known to involve nucleophilic attack on the carbonyl carbon atom of the ketene. Rate constants for phenylhydroxyketene, on the other hand, do not fit this correlation, which suggests that the mechanistic assignment upon which these rate constants are based may not be correct. Solvent isotope effects on these uncatalyzed ketene hydrations are weak; most are less than k(H)/k(D) = 2. It is argued that these isotope effects are largely, if not entirely, secondary in nature and that they are consistent with both a reaction mechanism in which nucleophlic attack of a single water molecule on the ketene carbonyl carbon atom produces a zwitterionic intermediate and also a mechanism that avoids this intermediate by passing through a cyclic transition state involving several water molecules.