Acylation. Part XXVI. The kinetics and mechanism of the addition of carboxylic acids to ketens in diethyl ether and in dichlorobenzene solution

Abstract

The addition of carboxylic acids to dimethylketen in ether solution and to diphenyl- and mesitylphenyl-keten in o-dichlorobenzene solution has been studied kinetically at 25°. In ether the rate equation takes the form: –d[Keten]/dt=k[RCO₂H][Keten] and in dichlorobenzene it is normally –d[Keten]/dt=k[(RCO₂H)₂][Keten]. The reactivities of the acids, the substituent effect of the mesityl group, and the hydrogen isotope effect are all consistent with there being, in each solvent, two different paths for the addition. One path, taken by the conventionally weaker acids, involves a cyclic transition state and nucleophilic attack by the acid on the carbonyl carbon atom of the keten. The velocity of addition via this path is inversely related to acid strength. The other, non-cyclic, path, available to the strongest acids studied, involves a direct proton transfer from the acid either to the β-carbon atom or to the oxygen atom of the keten.