Mechanisms for reactions of halogenated compounds. Part 3. Variation in activating influence of halogen substituents in nucleophilic aromatic substitution

Abstract

Rate constants are reported for reactions of polyhalogeno-pyridines and -benzenes with sodium methoxide in methanol. Relative activating effects of individual fluorine and chlorine atoms at positions ortho,meta, and para to the reaction site are determined and compared with orders determined from reactions involving ammonia in aqueous dioxan. The results are remarkably similar. Additional support is provided for earlier explanations of the activating effects of ortho-fluorine and -chlorine. Activation parameters, determined for reactions of polyhalogenopyridines with ammonia in aqueous dioxan, clearly demonstrate that differences in reactivity along the series arise mainly from changes in activation energy.