The α-Effect in S N Ar Reaction of Y-Substituted-Phenoxy-2,4-Dinitrobenzenes with Amines: Reaction Mechanism and Origin of the α-Effect

• Published in: Bulletin of the Korean Chemical Society Vol. 35; no. 8; pp. 2448 - 2452
• Main Authors: Cho, Hyo-Jin, Kim, Min-Young, Um, Ik-Hwan
• Format: Journal Article
• Language: Korean
• Published: 2014
• Subjects: S_NAr$ reaction; 1-Aryloxy-2,4-dinitrobenzenes; Hydrazine; Hammett plot; The ${\alpha}$-effect
• ISSN: 0253-2964; 1229-5949

Second-order rate constants ($k_N$) have been measured spectrophotometrically for $S_NAr$ reactions of Y-substituted-phenoxy-2,4-dinitrobenzenes (1a-1g) with hydrazine and glycylglycine in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. Hydrazine is 14.6-23.4 times more reactive than glycylglycine. The magnitude of the ${\alpha}$-effect increases linearly as the substituent Y becomes a stronger electron-withdrawing group (EWG). The Br${\o}$nsted-type plots for the reactions with hydrazine and glycylglycine are linear with ${\beta}_{lg}=-0.21$ and -0.14, respectively, which is typical for reactions reported previously to proceed through a stepwise mechanism with expulsion of the leaving group occurring after rate-determining step (RDS). The Hammett plots correlated with ${\sigma}^{\circ}$ constants result in much better linear correlations than ${\sigma}^-$ constants, indicating that expulsion of the leaving group is not advanced in the transition state (TS). The reaction of 1a-1g with hydrazine has been proposed to proceed through a five-membered cyclic intermediate ($T_{III}$), which is structurally not possible for the reaction with glycylglycine. Stabilization of the intermediate $T_{III}$ through intramolecular H-bonding interaction has been suggested as an origin of the ${\alpha}$-effect exhibited by hydrazine.