Potentially bifunctional reactants. Aromatic nucleophilic substitution by imidazole or pyrazole

Abstract

Quantitative N-2,4-dinitrophenylation of imidazole by 1-fluoro-(FDNB) or 1-chloro-2,4-dinitrobenzene (CDNB), or of pyrazole by FDNB occurs in benzene. The kinetics of the reaction of FDNB with imidazole are third-order overall (second-order with respect to imidazole) at low imidazole concentration, while levelling off of the rate occurs at higher imidazole concentration. Kinetics for the other reactions fit an equation containing terms both first- and second-order in the nucleophile. These results are interpreted in terms of an addition–elimination mechanism, with decomposition of the intermediate into products being fast for CDNB and slow (involving bifunctional reactivity for pyrazole and monofunctional reactivity for imidazole) for FDNB.