Electrophilic aromatic substitution

Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic nitration, aromatic halogenation, aromatic sulfonation, and acylation and alkylating Friedel–Crafts reaction.[1]

Illustrative reactions

The most widely practiced example of this reaction is the ethylation of benzene.

Benzene ethylation.svg

Approximately 24,700,000 tons were produced in 1999.[2] (After dehydrogenation and polymerization, the commodity plastic polystyrene is produced.) In this process, solid acids are used as catalyst to generate the incipient carbocation. Many other electrophilic reactions of benzene are conducted, although on much smaller scale, they are valuable routes to key intermediates. The nitration of benzene is achieved via the action of the nitronium ion as the electrophile. The sulfonation with fuming sulfuric acid gives benzenesulfonic acid. Aromatic halogenation with bromine, chlorine, or iodine gives the corresponding aryl halides. This reaction is typically catalyzed by the corresponding iron or aluminum trihalide.

Benzene halogenation en.svg

The Friedel–Crafts reaction can be performed either as an acylation or as an alkylation. Often, aluminium trichloride is used, but almost any strong Lewis acid can be applied. For the acylation reaction a stoichiometric amount of aluminum trichloride is required.

Benzene Friedel-Crafts acylation.svg
Benzene Friedel-Crafts alkylation.svg
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