Substitution, electrophilic encounter complex

Electrophilic aromatic substitution is generally considered to proceed via a 71-complex (or an encounter complex) between the electrophile E+ and the... 

One interesting proposal86 is that the encounter pair is a radical pair N02 ArH formed by an electron transfer (SET), which would explain why the electrophile, once in the encounter complex, can acquire the selectivity that the free N02+ lacked (it is not proposed that a radical pair is present in all aromatic substitutions only in those that do not obey the selectivity relationship). The radical pair subsequently collapses to the arenium ion. There is evidence87 both for and against this proposal.88... 

Electrophilic aromatic substitution represents one of the most important applications of the transformation of an arene via a CT (precursor) complex. The process is considered to proceed via an (encounter) -complex between the electrophile (E+) and the aromatic substrate (ArH), which collapses in a single rate-limiting step to the Wheland intermediate or a-... 

A similar problem of complex formation may be encountered if either amino or phenol groups are present in the substrate, and the reaction may fail. Under such circumstances, these groups need to be blocked (protected) by making a suitable derivative. Nevertheless, Friedel-Crafts acylations tend to work very well and with good yields, uncomplicated by multiple acylations, since the acyl group introduced deactivates the ring towards further electrophilic substitution. This contrasts with Friedel-Crafts alkylations, where the alkyl substituents introduced activate the ring towards further substitution (see Section 8.4.3). 

Solvent for kinetic studies. Brown and Wirkala found trifluoroacetic acid a superior medium for kinetic studies of aromatic electrophilic substitutions (bromina-tion, nitration, mercuration). In contrast to the complex kinetics encountered in acetic acid, simple second-order kinetics apply. In addition, the reactions are faster... 

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