A comparison of the mechanisms of hydrolysis of diphenylketene and dimethylketene in diethyl ether solution

Abstract

The spontaneous hydrolysis of diphenylketene in diethyl ether solution at 25°C is first order in the ketene concentration and third order in the stoicheiometric water concentration; unlike the hydrolysis of dimethylketene, it is not auto-catalytic and hydrolysis is negligibly catalysed by added carboxylic acids. Added boron trifluoride strongly inhibits the hydrolysis of diphenylketene in ether but powerfully catalyses that of dimethylketene; the formation of the strong Brønsted acid H₂OBF₃(association constant K= 120 ± 20 l mol^–1 at 25°C) underlies these effects. A pK_a value of ca. 0.5 is deduced for this acid and the results as a whole point to the marked inability of diphenylketene, compared with dimethylketene, to accept protons from Brønsted acids. Predominantly nucleophilic attack on the ketene is, however, easier for the diphenyl-derivative whose spontaneous hydrolysis varies between ca. 5- and ca. 40-fold faster over the range of stoicheiometric water concentrations used.