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

Abstract

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 °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 σ- constants than with σ° or σ constants, indicating that the leaving group departure occurs at the rate determining step (RDS). On the contrary, σ- constants result in poorer Hammett correlation than σ° 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 ρX value (1.7 ± 0.1) obtained for the reactions of 1a-l with the anionic nucleophiles supports the proposal that the reactions proceed through an addition intermediate with its formation being the RDS. © The Royal Society of Chemistry 2006.