How would you explain the electrophilic aromatic substitution with a diagram of a reaction mechanism?

1 Answer

Electrophilic aromatic substitution requires of an aromatic reactant, a pre-electrophile and an electrophile maker.

Explanation:

The reaction begins with a 'pre-step' which involves the electrophile maker turning the pre-electrophile into a very reactive electrophile.

Only reactive electrophiles can cause a reaction because aromatic rings are very stable.

The aromatic rings acts as a nucleophile.

I will take the bromination of benzene as an example:

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1. The reaction starts with the production of a good electrophile by a catalyst.

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.

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2. The electrophile attacks the π electron system of the benzene ring to form a non-aromatic carbocation intermediate.

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3. The positive charge on the carbocation is delocalized throughout the molecule.

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4. The aromaticity is restored by the loss of a proton from the atom to which the bromine atom (the electrophile) has bonded.

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5. Finally, the proton reacts with the #"FeBr"_4^−# to regenerate the #"FeBr"_3# catalyst and form the product HBr.

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Here's an energy diagram that corresponds for the bromination of benzene.

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