Friedel-Crafts Reaction: Theoretical Study of the Mechanism of Benzene Alkylation with Isopropyl Chloride Catalyzed by AlCl3 and Al2Cl6

The classical alkylation reaction of benzene with isopropyl chloride catalyzed by the species AlCl3 and Al2Cl6 was studied using reliable calculations at M06-2X/def2-TZVPP//X3LYP/def2/SVP level of theory and SMD model for solvent effect. We evaluated the formation of dimers, trimers, tetramers, and pentamers and showed that Al2Cl6 dimers exist in greater proportion, in agreement with experimental observations. The experimental solubility of Al2Cl6 in benzene was also included in the theoretical kinetics analysis. The reaction catalyzed by AlCl3 species presents the highest barrier, in part due to unfavorable dissociation of the Al2Cl6 species. The mechanism via Al2Cl6 catalysis is more effective and even considering its low solubility, the calculated observed DG‡ is only 20.6 kcal mol-1, indicating a fast reaction rate. The mechanism involves the formation of the CH3CHCH3+…Al2Cl7- ion pair, which reacts with benzene to form a Wheland intermediate and this carbon-carbon bond formation step corresponds to the rate-determining one.

本研究针对三氯化铝（AlCl₃）与二氯化二铝（Al₂Cl₆）物种催化的苯与异丙基氯经典烷基化反应展开了系统探究，采用M06-2X/def2-TZVPP//X3LYP/def2/SVP理论级别的可靠量子化学计算，并结合SMD溶剂化模型（SMD model）考量溶剂效应。我们对二聚体、三聚体、四聚体及五聚体的生成过程进行了评估，结果表明Al₂Cl₆二聚体的占比更高，这与实验观测结果一致。本研究还将Al₂Cl₆在苯中的实验溶解度纳入理论动力学分析范畴。由AlCl₃物种催化的反应具有最高的反应能垒，这在一定程度上源于Al₂Cl₆物种的解离过程较为不利。通过Al₂Cl₆催化的反应机制更为高效；即便考虑到其溶解度较低，计算得到的表观吉布斯自由能活化能（DG‡）仅为20.6 kcal mol⁻¹，表明反应速率较快。该反应机制涉及异丙基阳离子（CH₃CHCH₃⁺）与二氯化铝七阴离子（Al₂Cl₇⁻）形成离子对，该离子对可与苯反应生成维兰德中间体（Wheland intermediate），而这一碳-碳键形成步骤即为反应的决速步。