Participation of an elimination mechanism in alkaline hydrolyses of alkyl N-phenylcarbamates

Abstract

Second-order rate constants for alkaline hydrolysis of alkyl N-phenylcarbamates and alkyl and aryl N-methyl-N-phenylcarbamates have been measured. Together with results from an earlier paper the alkaline hydrolysis of aryl and alkyl N-phenylcarbamates is related linearly with pK_a(7–16) of the leaving alcohol and phenol (β=–1·15); the fully substituted carbamates have β=–0·25. These results are interpreted to mean that alkyl N-phenylcarbamates hydrolyse in alkali via an elimination type of process. It is estimated that a changeover from an elimination (E1cB) to a bimolecular substitution (S_N2) mechanism occurs when the pK_a of the leaving alcohol exceeds ∼17. A discussion of the factors favouring S_N2 or E1cB mechanisms in alkaline hydrolysis is given.