Molecular Mechanistic Study on the Electrophilic Substitution

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The electrophilic substitution reaction for aromatic hydrocarbon is one of most fundamental organic reactions. The reaction mechanism of aromatic electrophilic substitution reaction is thought to be via two stages. The first step is pi-type complex formation, and the second step is the proton elimination after the sigma-complex formation. This mechanism has been well accepted, but the detailed molecular processes have not been understood especially in conjunction with the solvent and Lowis acid catalyst. In this research, we have been studied the molecular mechanisms on the aromatic electrophilic substitution reactions theoretically in order to clarify the molecular structures of reaction complexes and theirpotential energy surfaces. Benzene and Cl_2 molecules are chosen as the first model. The pi-type complex of benzene-cl_2 is known to be charge-transfer complex, but this molecular interaction is very week and is concluded to be likely van der Waals interaction. Since the potential energy surfaces for the reaction from the pi-type complex indicate that the energy barriers are extremely high for both substitution pathway and addition reaction. This means that the active species to generate a chlorine cation must be considered, and we have examined the potential energy surface for the chlorination reaction of benzene with hydronium ion as a catalyst. When we include the hydronium ion to the benzene-Chlorine complex, two transition states have been found with very low energy barrier. These transition states correspond to the potential energy surface supported by the many experimental works. The first one is the formation of sigma-complex from pi-type complex. This sigma-complex must stabilize in order to proceed the elimination of proton via the second transition state.