Nucleophilic substitution in nitroarenes: a general corrected mechanism

Abstract

Nucleophilic substitution in electron-deficient arenes is one of the fundamental processes in organic chemistry; however, its mechanism as presented in textbooks does not adequately describe the process. According to this generally accepted mechanism, it is limited to substitution of halogens via nucleophilic addition at positions occupied by halogens. The possibility of addition at positions occupied by hydrogen is totally ignored. Research papers have shown that nucleophilic addition at positions occupied by hydrogen is a fast and reversible process, and σH adducts are initially formed as intermediates. These σH adducts can be converted into products of nucleophilic substitution of hydrogen in a few different ways or dissociate so that substitution of halogen can proceed. This general picture is confirmed by many examples presented.