The α-effect in SNar reaction of 1-fluoro-2,4- dinitrobenzene with hydrazine: Ground-state destabilization versus transition-state stabilization

Abstract

A kinetic study is reported on SNAr reaction of 1-fluoro-2,4-dinitrobenzene with a series of primary amines including hydrazine in H2O at 25.0 °C. The plots of kobsd vs. [amine] are linear and pass through the origin, indicating that general-base catalysis by a second amine molecule is absent. The Brønsted-type plot exhibits an excellent linear correlation with βnuc = 0.46 when hydrazine is excluded from the correlation. The reaction has been suggested to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs after the rate-determining step (RDS). Hydrazine is ca. 10 times more reactive than similarly basic glycylglycine (i.e., the α-effect). A five-membered cyclic intermediate has been suggested for the reaction with hydrazine, in which intramolecular H-bonding interactions would facilitate expulsion of the leaving group. However, the enhanced leaving-group ability is not responsible for the α-effect shown by hydrazine because expulsion of the leaving group occurs after RDS. Destabilization of the ground-state of hydrazine through the electronic repulsion between the nonbonding electron pairs is responsible for the α-effect found in the current SNAr reaction.