Wheland intermediate Friedel-Crafts reaction

This section will expand the theme of reactions between a carbocation and a nucleophilic species such as an alkene or an alcohol. In this section, however, the carbocation will be attacked by an aromatic ring to form a Wheland-type intermediate, which leads to a substituted aromatic system. These are the Friedel-Crafts reactions and they are among the most important reactions in organic chemistry. 

As described earlier, the S Ar reaction mechanism generally involves the formation of a a-complex intermediate. This species is formally a cyclohexadienyl cation, and it has also been called the Wheland intermediate, Pfeiffer-Wizinger complex, arenium ion, benzonium ion, and benzeniumion. Since it was first proposed as an intermediate in Friedel-Crafts reactions, there has been a considerable amount of evidence for its involvement in the reactions. As described previously, the involvement of the a-complex provides a good basis for the understanding of some activating and dbecting effects in S Ar reactions. 

The mechanism of the Friedel-Crafts alkylation with alkyl halides involves initial formation of the active alkylating agent, which then reacts with the aromatic ring. Depending on the catalyst, the solvent, the reaction conditions, and the alkyl halide, the formation of a polarized donor-acceptor complex 19) or real carbocations may take place (eq. 57). In a typical aromatic electrophilic substitution, the formed carbocation attacks the aromatic ring and forms the product through a a-complex or arenium ion (the Wheland intermediate) (20) (eq. 58). 

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