Mechanisms for reactions of halogenated compounds. Part 1. Activating effects of fluorine in polyfluoropyridines in reactions with ammonia

Abstract

Rate constants have been determined for reactions of various polyfluoropyridines with ammonia in water–dioxan (60 : 40 v/v) at 25 °C. Comparison of these rate constants allows the separate effects of fluorine atoms ortho, meta, and para to the reaction centre to be established. The activating influence decreases in the series ortho > metapara(31 : 23 : 0.26 relative to a hydrogen atom at the same position). The hitherto puzzling orientation of nucleophilic substitution in polyfluoro-pyridines and -benzenes is, therefore, accounted for by the activating influence of fluoro-substituents being at a maximum. A synthesis of 2,3,4,5-tetrafluoropyridine is described.