The charge transfer mechanism of aromatic substitution presented by the present author and Tanaka in the previous paper is further developed from the viewpoint of the resonance interaction between no-bond and charge-transfer structures. According to the present mechanism which includes symmetrical charge transfer complexes of a-type as stable intermediates, the reaction process can be divided into two steps, first of which is the formation of the a-type complex and the second, the decomposition of the complex into final products: moreover, these two steps are shown to be interpreted as transfer processes between the non-bond and charge-transfer structures.
Approximate formulae for the activation energy are derived. They show that the orientation of substitution is determined by localization energies of aromatic cations and anions for electrophilic and nucleophilic substitution, respectively. These quantities are actually evaluated with some aromatic hydrocarbons by a simple LCAO approximation. The calculated localization energies can explain satisfactorily the observed orientation rule. Furthermore, the necessary condition for the occurrence of aromatic substitution is deduced with regard to ionization potentials and electron affinities of aromatic substrate molecules and reagents, and also with regard to binding energies between two components in intermediate e-complexes.
I. INTRODUCTION
FOR many years the mechanism of aromatic substitution has been a subject of experimental and theoretical investigations. In 1942 Wheland 1 presented the localization model of the reaction intermediate in which a reagent is connected with one of the ring carbon atoms of an aromatic substrate molecule by a bond of a-type and he succeeded in explaining the observed orientation rule, namely, relative rates of substitution reactions on various positions of a certain aromatic molecule. In addition, several other theoretical studies have been carried out, and consequently some quantum mechanical concepts useful for interpreting chemical reactivity of aromatic substrate molecules, such as ~r-electron density, 2 free valency, 3 frontier electron density, 4 etc., s t Most parts of the present paper were presented at the Symposium of Electronic Structures of Molecules held at Tokyo in October 1957.