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Electrophilic Aromatic Substitution

Electrophilic attack on the ring is the most important type of aromatic reaction, as we might expect. After all the aromatic ring has electron rich 'clouds' above and below the ring. Many different substituents can be introduced onto the ring using proper reagents: -X, -NO₂, -SO₃H, -R (alkyl grps).

Bromination of Benzene: However, the reactions are not always what one would expect based on an alkene model. For example, bromination gives a single, mono-substituted product:

So why don't we see the di-substituted product? What mechanism accounts for the mono-substitution? A look at the possible carbocation intermediates helps in understanding this process:

A reaction coordinate diagram for this reaction is shown below (note that it has the same shape and general characteristics as the halogenation of ethylene we saw earlier).

So looking at the mechanism and the diagram, why don't we see the formation of the dibromo-product? It turns out that due to the loss of the delocalized electron system represented by the resonance forms, the dibromo-product, 5,6-dibromo-1,3-cyclohexadiene, has an energy close to that of the intermediate. Thus any 5,6-dibromo-1,3-cyclohexadiene formed in the reaction would be in rapid equilibrium with this intermediate because of the relatively low activation energy, and it would all go to the final product, bromobenzene.

Chlorination of Benzene: This reaction is like the bromination reaction, using ferric chloride as catalyst. It is used in a variety of synthess reactions including pharmaceuticals and (more commonly in the past) for herbacide (e.g. 2,4,5-T, 2,4-D etc.) and insecticide (e.g. DDT, alare, etc.) synthesis:

Nitration of Benzene: In this reaction the nitronium ion, NO₂⁺, is the electrophilic species, as seen for Br⁺ above. Concentrated nitric acid asts as a source for this ion, while concentrated sulfuric acid acts as a catalyst. Nitration is important for the production of explosives (TNT = TriNitroToluene), dyes, pharmaceuticals, etc.:

Sulfonation of Benzene: In this reaction the bisulfite ion, HSO₃⁺, is the electrophilic species. Fuming sulfuric acid (sulfuric acid saturated with sulfur trioxide) serves as the catalyst and source of HSO₃:

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Electrophilic Aromatic Substitution