Computational Study of Aluminum Chloride Activation for Toluene Chlorination

Abstract

Aluminum chloride, in the absence of solvent and moisture, was observed to exert a negligible catalytic effect upon the chlorination of toluene, but in the presence of atmospheric moisture, a very large catalytic effect was evident. DFT calculations were performed on a number of possible intermediates involved in the chlorination process. The calculations suggest that dimeric aluminum chloride predominates in the absence of solvents or moisture, and it is not active as a catalyst. It is suggested that HCl is produced by reaction of water with the aluminum chloride leading to the formation of monomeric complexes with molecular chlorine. These complexes are believed to serve as the electrophile during catalytic toluene chlorination. The results further suggest that very strong complexes can form between water and monomeric aluminum chloride. Complexes between aluminum chloride and toluene were also identified in the calculations, but their catalytic significance remains unclear.

Prereactive Complexes in Chlorination of Benzene, Triazine, and Tetrazine: A Quantum Chemical Study

“…72 We have found that BF 4 − and AlCl 4 − anions are suitable counterions, which can effectively stabilize σ-complex C 6 H 5 Cl + (Figure 2). It is in agreement with analogous results from molecular orbital studies of chlorination of toluene 5 and benzene 6 catalyzed by AlCl 3 . These computational results indicate that not only is the reaction rate increased with Lewis acid catalyst but also the general mechanism of the aromatic chlorination is changed.…”

“…We have found that BF 4 – and AlCl 4 – anions are suitable counterions, which can effectively stabilize σ-complex C 6 H 5 Cl + (Figure ). It is in agreement with analogous results from molecular orbital studies of chlorination of toluene and benzene catalyzed by AlCl 3 .…”

“…Despite the huge body of data that has been accumulated, the mechanism of halogenation of benzene continues to be the subject of active research and some controversy. For example, several authors have claimed that the σ-complex, commonly accepted as transient Wheland intermediate in electrophilic aromatic substitutions (EAS), can successfully be located at the energy surface by using quantum-chemical approach in gas phase. − In contrast, others have claimed that no σ-complex is existent until Lewis acid catalyst or high-polarity model solvents are included in calculations. − Similar debates have been ongoing over the existence of benzenium/ethene complex − and the exact nature of chlorine atom/benzene complex, , to name but a few examples that are closely related to prereactive complexes in EAS. Recently, Schleyer et al have challenged the reaction mechanism paradigm for the halogenation of benzene, demonstrating that alternative pathways, such as the addition–elimination route, may compete successfully with those depicted in textbooks for EAS reactions .…”

Pathways of electrophilic aromatic substitution reactions catalyzed by group 13 trihalides: An ab initio study

“…riedel-Crafts reactions have been the topic of the numerous experimental [1] and theoretical [2][3][4][5] studies. In our recent report [6] we tested several computational approaches to describe the thermodynamics of such reactions and found that molecules containing dimeric form of the metal halides may be important intermediates.…”

Theoretical Study on σ-Complexes in Ring-Chlorination of Toluene

“…In recent decades, experimental reports on chlolo-substitution mechanism of aromatic compounds [5] have put forward that positive-charged Cl δ+ is the electrophilic reagent in the reaction. Meanwhile, theoretical studies since the 1960s were only related to solvent effect on the chloro-substitution reaction rate and the catalysis of AlCl 3 in toluene ring chlorination [6][7][8][9][10]. According to the co-effect of conjugation and electron donating inductivity of methyl, its para-and ortho-ring hydrogens are preferred ones for chloro-substitution, which results in the predominance of OCT and PCT over MCT.…”