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4284279 
Journal Article 
Thoughts about the electrophilic aromatic substitution mechanism: the Friedel-crafts alkylation and acylation of benzene with acetyl and t-butyl cations in the gas phase 
Oliveira, FG; Rodrigues, FL; de Oliveira, AVB; Marcal, DVLM; Esteves, PM 
2017 
1 
Structural Chemistry
ISSN: 1040-0400
EISSN: 1572-9001 
28 
2 
545-553 
WOS:000394341700030 
The mechanism of electrophilic aromatic substitution (SEAr) is still matter of debate and interest in the literature. In this work, the Friedel-Crafts alkylation and the acylation in the gas phase were investigated in the context of the unified mechanism for SEAr. In this unified proposal three kinds of intermediates can potentially be formed: oriented and unoriented pi-complexes, intimate single electron transfer (SET) intermediates and sigma-complexes. Quantum chemical calculations at M06-2X/6-311++G(d,p) level were carried out for the investigation of the reaction of benzene with acetyl and tert-butyl ions as model non-oxidant electrophiles for acylation and alkylation, respectively, in the gas phase. It was found that both the tert-butyl and the acetyl cations prefer to form oriented pi-complexes. Both electrophiles do not react through a SET pathway with benzene. The pi-complex between tert-butyl cation and benzene can evolve to a sigma-complex, while in the case of the acetyl cation and benzene the sigma-complex was not found as a minimum on the potential energy surface. Instead, it corresponds to a transient species or a very shallow minimum. The outcome of this is that the pi-complex would only react with the aromatic ring evolving to the product with nucleophilic assistance by a species of the reaction medium, in either through a concerted mechanism or a specific interaction. This is also observed for aromatics with low ionization energies/nucleophilicities. However, very electron rich aromatic systems afford sigma-complexes, and as their ionization energies increases (i.e., less nucleophilic), the more the resulting complex resembles a pi-complex, more or less continuously. This suggests out that electrophilic aromatic substitution reactions cannot be rationalized within a single mechanistic framework. Instead, a continuum of mechanistic possibilities may be involved. 
Density functional calculations; Electrophilic aromatic substitution; Friedel-crafts; Alkylation; Acylation