ROLE OF LEWIS-ACID (ALCL3) AROMATIC RING INTERACTIONS IN FRIEDEL-CRAFTS REACTION - AN AB-INITIO STUDY

Electrophilic aromatic substitution reactions, which are highly exempl ified by the widely used Friedel-Craft's reaction, have been extensive ly studied using theoretical and experimental techniques. A number of elegant mechanisms have been proposed for the Friedel-Craft's reaction till date. In all the proposed mechanisms, the role of the Lewis acid has been limited to the generation of the electrophile, which subsequ ently attacks the aromatic system to form either a pi or a sigma compl ex. A recent experimental report on the reaction of CO with benzene in zeolite catalysts intrigued us because the presence or absence of AlC l3 was found to govern the reaction product. These clearly indicated t hat AlCl3 has an additional role in the reaction. We probed this role theoretically by performing high-level ab initio calculations on two m odel systems C6H6-BCl3 and C6H6-AlCl3. Our results clearly indicate th at one of the benzene carbon tends to become highly nucleophilic, ther eby facilitating an attack by an incipient electrophile. There appear unusual molecular orbital interactions including the loss of the benze ne nodal plane and back-donation from Cl 3p orbital to the benzene HOM O. In what could be the first high-level theoretical study of Lewis ac id-aromatic reactions, we believe our results could help understand th e nature of the intermediates in electrophilic aromatic substitution r eactions.