Mechanisms for reactions of halogenated compounds. Part 2. Orienting effects of chlorine substituents in nucleophilic aromatic substitution

Abstract

Rate constants and product analyses are provided for reactions of various chloropolyfluoropyridines with ammonia in dioxan–water (60: 40 v/v) at 25 °C. Comparison of these rate constants with the values for appropriate polyfluoropyridines allows the separate effects of chlorine atoms ortho, meta, and para to the reaction centre to be established. The activating influence of chlorine decreases in the series ortho > meta > para(86 : 24 : 6.9 relative to hydrogen atom at the same position). The rate constant ratios k_cl : k_F for chlorine orfluorine at the same position are: ortho ca. 3 : 1, meta ca. 1 : 1, and para ca. 2.6 : 1. These ratios have been measured for a number of different substitutions and are remarkably constant. The results help to explain the orientation and reactivity of nucleophilic substitution in polyhalogenoaromatic compounds.