Molecular Mechanistic Study on the Electrophilic Substitution

芳烃的亲电取代反应是最基本的有机反应之一。芳香族亲电取代反应的反应机理分为两个阶段。第一步是π型络合物的形成，第二步是sigma-络合物形成后的质子消去。这一机理已被广泛接受，但详细的分子过程尚未了解，特别是与溶剂和Lowis酸催化剂的结合。本研究从理论上研究了芳香亲电取代反应的分子机理，以阐明反应配合物的分子结构及其势能面。选择苯和Cl_2分子作为第一种模型。已知苯-cl_2的pi型配合物是电荷转移配合物，但这种分子相互作用非常罕见，可能是范德华相互作用。由于pi型配合物反应的势能面表明取代反应和加成反应的能垒都很高。这意味着必须考虑生成氯离子的活性物质，我们研究了水合氢离子作为催化剂的苯氯化反应的势能面。当我们将水合氢离子加入到苯-氯配合物中时，发现了两个具有极低能垒的过渡态。这些过渡态对应于许多实验工作所支持的势能面。第一个是由-型络合物形成-型络合物。这个sigma配合物必须稳定，才能通过第二过渡态进行质子的消除。

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.