2-氯吡啶气相光氯化反应机理的量子化学研究

Quantum Chemistry Studies on Mechanism of Gas Phase Photochemical Chlorination of 2-Chloropyridine

用量子化学DFT方法在B3LYP/3-21G*水平下研究了2-氯吡啶气相光氯化取代反应生成2,3-二氯吡啶、2,4-二氯吡啶、2,5-二氯吡啶和2,6-二氯吡啶不同产物的过渡态,并计算了活化能.结果表明,生成2,6-二氯吡啶过渡态的能量最低,所需的活化能也最低,反应优先生成2,6-二氯吡啶.生成2,6-二氯吡啶的IRC结果显示反应过程中C—H键的断裂和C—C l键的生成协同但不同步.过渡态的构型接近于产物,是一个晚期过渡态.C l原子在反应进程中是给电子的,因此,氯自由基与2-氯吡啶反应是亲核取代的SN2机理.

The gas phase reaction mechanism of the photochemical chlorination of 2-chloropyridine was theoretically studied by using DFT B3LYP/3-21G^＊ method. The activation energy leading to 2,3-dichloropyridine, 2,4-dichloropyridine, 2,5-dichloropyridine and 2,6-dichloropyridine were also calculated respectively. The calculated results consistently show that the energy of transition state and the activation energy for the formation of 2,6-dichloropyridine were the lowest among the four reaction pathways. Therefore, the main product should be 2,6-dichloropyridine. The IRC result showed that the breaking of C-H and the forming of C-Cl were concerted but not synchronous. The structure of the transition state was near to 2,6-dichloropyridine, which showed that it was a late transition state. During the whole reaction processing, the Cl^＊ acted as a nucleophilic specie, donating the electron. Therefore, SN2 mechanism was speculated for the reaction of Cl^＊ with 2-chloropyridine.