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 basi c solution using flash photolytic techniques, and rate constants for their uncatalyzed, k(uc), and hydroxide-ion catalyzed, k(HO), reactions were dete rmined. 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 keten es. This good correlation implies that uncatalyzed and hydroxide-ion cataly zed ketene hydraton occur by similar reaction mechanisms, which for the hyd roxide-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 m echanistic 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 ef fects are largely, if not entirely, secondary in nature and that they are c onsistent with both a reaction mechanism in which nucleophlic attack of a s ingle water molecule on the ketene carbonyl carbon atom produces a zwitteri onic intermediate and also a mechanism that avoids this intermediate by pas sing through a cyclic transition state involving several water molecules.