Structure-reactivity correlations in nucleophilic substitution reactions of Y-substituted phenyl X-substituted benzoates with anionic and neutral nucleophiles

摘要

A kinetic study is reported for the reactions of 4-nitrophenyl X-substituted benzoates (1a-l) and Y-substituted phenyl benzoates (2a-l) with two anionic nucleophiles (OH- and CN-) and three amines (piperidine, hydrazine, and glycylglycine) in 80 mol% H2O-20 mol% dimethyl sulfoxide (DMSO) at 25.0 +/- 0.1 degrees C. Each Hammett plot exhibits two intersecting straight lines for the reactions of 1a-l with the anionic nucleophiles and piperidine, while the Yukawa-Tsuno plots for the same reactions are linear. The Hammett plots for the reactions of 2a-l with hydrazine and glycylglycine demonstrate much better linear correlations with sigma(-) constants than with sigma(o) or sigma constants, indicating that the leaving group departure occurs at the rate determining step (RDS). On the contrary, sigma(-) constants result in poorer Hammett correlation than sigma(o) constants for the corresponding reactions with OH- and CN-, indicating that the leaving group departure occurs after the RDS for the reactions with the anionic nucleophiles. The large rho(x) value (1.7 +/- 0.1) obtained for the reactions of 1-al with the anionic nucleophiles supports the proposal that the reactions proceed through an addition intermediate with its formation being the RDS.