KINETICS AND MECHANISM OF AROMATIC ELECTROPHILIC SUBSTITUTION-REACTIONS .4. DIVISION OF METHYLBENZENE REACTIONS INTO BASICITY-VERSUS IONIZATION POTENTIAL-CONTROLLED REACTIONS

Electrophilic aromatic substitution reactions have been divided into three classes based on data obtained for substrate selectivity in the first stage of reactions in the series benzene to hexamethylbenzene: weakly or nonselective reactions, selective reactions controlled by arene basicity, and selective reactions controlled by ionization potential. Based on the results of kinetic isotope effects and product composition we have proposed a generalized reaction scheme for these types of reactions, involving as key intermediates either a sigma-complex between the arene and electrophilic reagent or a charge transfer complex. The criteria underlying mechanism selection are discussed.