Kinetic Study on Nucleophilic Displacement Reactions of 2-Chloro-4-Nitrophenyl X-Substituted-Benzoates with Primary Amines: Reaction Mechanism and Origin of the α-Effect

Abstract

The <TEX>${\alpha}$</TEX>-Effect; Ground state; Transition state; Intramolecular H-bonding; Yukawa-Tsuno plot; Second-order rate constants for aminolysis of 2-chloro-4-nitrophenyl X-substituted-benzoates (1a-h) have been measured spectrophotometrically in 80 mol % <TEX>$H_2O/20$</TEX> mol % DMSO at <TEX>$25.0^{\circ}C$</TEX>. The Br<TEX>${\emptyset}$</TEX>nsted-type plot for the reactions of 2-chloro-4-nitrophenyl benzoate (1d) with a series of primary amines curves downward, which has been taken as evidence for a stepwise mechanism with a change in rate-determining step (RDS). The Hammett plots for the reactions of 1a-h with hydrazine and glycylglycine are nonlinear while the Yukawa-Tsuno plots exhibit excellent linearity with <TEX>${\rho}_X=1.22-1.35$</TEX> and <TEX>${\gamma}= 0.57-0.59$</TEX>, indicating that the nonlinear Hammett plots are not due to a change in RDS but are caused by stabilization of substrates possessing an electron-donating group (EDG) through resonance interactions between the EDG and C=O bond of the substrates. The <TEX>${\alpha}$</TEX>-effect exhibited by hydrazine increases as the substituent X changes from a strong EDG to a strong electron-withdrawing group (EWG). It has been concluded that destabilization of hydrazine through the electronic repulsion between the adjacent nonbonding electrons is not solely responsible for the substituent dependent <TEX>${\alpha}$</TEX>-effect but stabilization of the transition state is also a plausible origin of the <TEX>${\alpha}$</TEX>-effect.