Kinetics of the reactions between 1‐fluoro‐2,6‐dinitrobenzene and pyrrolidine and piperidine in binary solvent systems: Influence of the nucleophile structure

Abstract

AbstractWe studied the kinetics of the reaction between 1‐fluoro‐2,6‐dinitrobenzene and pyrrolidine or piperidine in ethyl acetate–chloroform or acetonitrile and acetonitrile–chloroform binary solvent mixtures. The kinetic response of these reactions was compared with that of the reactions with homopiperidine. The aim of this work was to evaluate the influence of the nucleophile structure and of solvent effects on those reactive systems. The amine structure has a great influence on second‐order rate constants, especially on the rate constants related to the catalyzed step. In mixtures with chloroform the amine structure is also responsible for the change in the reaction mechanism. Theoretical quantum mechanics calculations confirm that the origin of these results lies in stereoelectronic effects due to the conformational difference between the amino moieties in the intermediate σ adducts as they release the nucleofuge. Solvation effects are dominated by non‐specific interactions. The order of incidence of the molecular–microscopic solvent properties on the second‐order rate coefficient kA is dipolarity/polarizability>hydrogen‐bond donor ability (HBD)>hydrogen‐bond acceptor ability (HBA). The (HBA+HBD) and (HBA+HBA/HBD) solvent systems have similar solvation mechanisms: the critical state is preferentially solvated by the structure formed by intersolvent hydrogen‐bonded species. Mixtures of the type (HBA/HBD+HBD) manifest a particular solvation behavior. Among the reactive systems selected there is only one example in which preferential solvation is not operative. Copyright © 2004 John Wiley & Sons, Ltd.Additional material for this paper is available in Wiley Interscience