Although aromatic compounds have multiple double bonds, these compounds do not undergo addition reactions. Their lack of reactivity toward addition reactions is due to the great stability of the ring systems that result from complete π electron delocalization (resonance). Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved. For example, benzene reacts with bromine to form bromobenzene.
Many functional groups can be added to aromatic compounds via electrophilic aromatic substitution reactions. A functional group is a substituent that brings with it certain chemical reactions that the aromatic compound itself doesn't display.
All electrophilic aromatic substitution reactions share a common mechanism. This mechanism consists of a series of steps.
1. An electrophile — an electron‐seeking reagent — is generated. For the bromination of benzene reaction, the electrophile is the Br+ ion generated by the reaction of the bromine molecule with ferric bromide, a Lewis acid.
2. The electrophile attacks the π electron system of the benzene ring to form a nonaromatic carbocation.
3. The positive charge on the carbocation that is formed is delocalized throughout the molecule.
4. The aromaticity is restored by the loss of a proton from the atom to which the bromine atom (the electrophile) has bonded.
5. Finally, the proton reacts with the FeBr 4 − to regenerate the FeBr 3 catalyst and form the product HBr.
You can summarize this particular electrophilic aromatic substitution mechanism like this:
In another example of an electrophilic aromatic substitution reaction, benzene reacts with a mixture of concentrated nitric and sulfuric acids to create nitrobenzene.
The mechanism for the nitrobenzene reaction occurs in six steps.
1. Sulfuric acid ionizes to produce a proton.
2. Nitric acid accepts the proton in an acid‐base reaction.
3. The protonated nitric acid dissociates to form a nitronium ion ( +NO 2).
4. The nitronium ion acts as an electrophile and is attracted to the π electron system of the benzene ring.
5. The nonaromatic carbocation that forms has its charge delocalized around the ring.
6. The aromaticity of the ring is reestablished by the loss of a proton from the carbon to which the nitro group is attached.
The sulfonation of benzene
The reaction of benzene with concentrated sulfuric acid at room temperature produces benzenesulfonic acid.]
The mechanism for the reaction that produces benzenesulfonic acid occurs in the following steps:
1. The sulfuric acid reacts with itself to form sulfur trioxide, the electrophile.
This reaction takes place via a three‐step process:
2. The sulfur trioxide is attracted to the π electron system of the benzene molecule.
The remaining steps in the mechanism are identical with those in the bromination and nitration mechanisms: The charge around the ring is delocalized, and then the loss of a proton reestablishes the aromaticity of the ring.