Highly selective aromatic chlorination. Part 3. Kinetics and mechanism of chlorination of electron-rich aromatic compounds by N-chloroamines in acidic solution

Abstract

The highly selective chlorination of electron-rich aromatic compounds with N-chloroamines in trifluoroacetic acid (TFA) is first order in both the aromatic substrate and the chlorinating agent. Kinetic and competitive kinetic studies show that electron-donating substituents on the substrate and electron-withdrawing substituents on the N-chloroamine have a marked rate-enhancing effect. Two mechanisms that fit the experimentally observed kinetics and that account for the high selectivity for 4-chlorination in terms of an electronic effect are proposed, namely an arenium-ion mechanism and an electron-transfer chain reaction. Evidence from chemical trapping experiments and from other studies suggest that for the majority of the substrates the chlorination proceeds by the arenium ion mechanism. However, for substrates, such as 1,4-dimethoxybenzene, that are very susceptible to one-electron oxidation chlorination may proceed at least in part by the electron-transfer chain reaction.