Kinetics and Mechanism of Hydration of Alkylketenes

Article abstract of DOI:10.1021/ja00243a034

The hydration reactivities of CH2=C=O (1), t-Bu2C=C=O (5), Et2C=C=O (7), (CH2)4C=C=O (8), (CH2)5C=C=O (9), and t-BuCH=C=O (10) in H2O or H2O/CH3CN mixtures have been examined, including acid and base catalysis and solvent and structural isotope effects.These results provide the first systematic comparison of structural effects on the hydration of aliphatic ketenes, as well as the first measurements of base-induced hydration and pH-rate profiles for this process.The significant steric and electronic effects of the substituents observed lead to the interpretation that the acid-catalyzed reaction involves rate-limiting proton transfer to C_β perpendicular to the ketene plane, while the H2O- and OH^--induced reactions involve nucleophilic attack in the ketene plane.These results resolve the many conflicting previous reports and interpretations regarding ketene hydration.