Effect of Hydroxyl Group of Catalyst on Formation of 2-Phenyl-Benzimidazole: A Theoretical Elucidation of Mechanism

Abstract

In this work, the capability of step1 and step2 of the synthesis reaction mechanism of the 2-phenyl-1H-benzo[d]imidazole as a rate-determining step (RDS) has been energetically and thermodynamically evaluated using theoretical methods. The results were profoundly compared to the experimental approaches. The role of population explosion resulting from the increased hydroxyl groups within the catalyst structure was considered using some models with different hydroxyl groups on the reaction. Herein, the theoretical rate constants were obtained using the potential energy surfaces (PESs) and the transition state theory (TST) at the M06-2X/6-31 + G(d,p) level of theory. The calculated results showed that the second step (step2) is an RDS. In step2, H2O elimination can be proceeded by effective participation of catalyst for making a four-membered ring structure of TS that is generated harder than a six-membered ring mechanism (Ea = 56.3 and Ea = 29.7 kcal mol−1, respectively). Moreover, the reaction rate (for step2 as the RDS) in the presence of a modified catalyst (similar to sucrose containing five hydroxyl groups) speeds up 100,000 times more than the simple catalyst such as methanol (involving one hydroxyl group) and ethylene glycol (with the two hydroxyl groups). Theoretical calculations exhibited that the catalyst enables to facilitate a proton transfer process in step4 to make a great facility for doing oxidation process easily in step5 as the fastest step. In addition, when the reaction was followed without assisted-catalyst, herein, activation energy for step4 increased up to 34.8 kcal mol−1 while it was 10.8 kcal mol−1 in the presence of a catalyst.