The Mechanism of the Reaction of Diphenylketene with Bases in Aqueous Solution: Nucleophilic Attack versus General Base Catalysis of Ketene Hydration

Diphenylketene was generated in aqueous solution by flash photolysis, and rates of its decay accelerated by 30 bases of various structure were determined. The rate constants so obtained did not show the regular dependence on basic strength expected if the bases were serving as general base catalysts assisting the attack of water on the ketene, but they did vary with polarizability and steric bulk of the base in the way expected for direct nucleophilic attack of the base on the carbonyl carbon atom of the ketene. Assignment of a direct nucleophilic role to the bases is supported by the formation of amide products in addition to diphenylacetic acid in the reaction of diphenylketene accelerated by ammonia and morpholine, and quantitative analysis of the product ratios shows that these two bases serve only as nucleophiles and that the diphenylacetic acid is formed by uncatalyzed reaction of diphenylketene with solvent water. Ketenes are interesting and useful substances whose chemistry is receiving renewed attention,’ stimulated in part by the development of rapid, flash-photolytic techniques for studying these reactive molecules.2 This has led to a wealth of new information, among which is the discovery that the reaction of ketenes with wholly or partly aqueous solvents is accelerated by This acceleration could be due to general base catalysis of ketene hydration, through a transition state in which the base assists nucleophilic attack of a water molecule on the ketene by removing one of its protons, eq 1, or it could be the result of direct nucleophilic attack by the base itself, eq 2.