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 ρ_X 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.