α-effect nucleophiles and azide ion:: effective charge studies of displacement reactions at esters

The bimolecular reactions of azide, 2-iodosobenzoate, and acetohydroxamate ions and methoxyamine nucleophiles with substituted phenyl acetates possess Yukawa-Tsuno type correlations with substantial resonance interaction. The rate constants for hydroperoxide ion and the oxyanion of N,N-diethylhydroxylamine obey simple Hammett equations. The reactivities of azide, acetohydroxamate, hydroperoxide, and 2-iodosobenzoate ion nucleophiles exhibit modest a-effects when the reactivity in substitution at 4-nitrophenyl acetate is compared with that of phenoxide ions of similar pK(a) values. The N,N-diethylhydroxylamine oxyanion is less reactive than its phenoxide ion analogue due to steric requirements, and the reactivities of the anionic nucleophiles exhibit changes in effective charge on the leaving oxygen similar to those for regular nucleophiles of similar pK(a). Methoxyamine possesses only a small enhancement compared with the reactivity of a normal primary amine of similar pK(a). Values of the Leffler parameters for the bond fission process in the reactions of alpha-nuclcophiles with phenyl acetates do not exhibit any special differences apart from those due to the pK(a) of the nucleophile. The causes of the a-effect are considered to be due to steric and solvation requirements of the reactions.