The halogenation of monosubstituted benzenes in aqueous solvent was studied using density functional theory at the PCM-M06-2X/6-311G(d,p) level. The reaction with Cl₂ begins with the formation of C atom coordinated π-complex and is followed by the formation of the σ-complex, which is rate-determining. The final part proceeds via the abstraction of the proton by a water molecule or a weak base. We evaluated the use of the σ-complex as a model for the rate-determining transition state (TS) and found that this model is more accurate the later the TS comes along the reaction coordinate. This explains the higher accuracy of the model for halogenations (late TS) compared to nitrations (early TS); that is, the more deactivated the substrate the later the TS. The halogenation with Br₂ proceeds with a similar mechanism as the corresponding chlorination, but the bromination has a very late rate-determining TS that is similar to the σ-complex in energy. The iodination with ICl follows a different mechanism than chlorination and bromination. After the formation of the π-complex, the reaction proceeds in a concerted manner without a σ-complex. This reaction has a large primary hydrogen kinetic isotope effect in agreement with experimental observations.

中文翻译：

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亲电子芳香卤代反应速率决定因素的理论研究

使用密度泛函理论在PCM-M06-2 X / 6-311G（d，p）浓度下研究了单取代苯在水性溶剂中的卤化作用。与Cl ₂的反应开始于C原子配位的π络合物的形成，然后是σ络合物的形成，这是决定速率的。最后一部分通过水分子或弱碱对质子的提取来进行。我们评估了使用σ-络合物作为确定速率的过渡态（TS）的模型，发现该模型在TS沿反应坐标出现的时间越晚越准确。这解释了与硝化反应（早期TS）相比，卤化反应模型（TS后期）的准确性更高；也就是说，基板越失活，TS越晚。溴卤化_{图2的过程}与相应的氯化反应的机理相似，但是溴化反应的速率决定性TS非常晚，类似于能量中的σ-复合物。与氯化和溴化作用不同，用ICl进行碘化作用的机理不同。在形成π-络合物之后，反应以一致的方式进行，而没有σ-络合物。与实验观察结果一致，该反应具有较大的初级氢动力学同位素效应。