Química Orgânica 2 · Substituição eletrofílica aromática
Brominate an arene with a Lewis-acid catalyst
Condições: 1,4-Dimethylbenzene (p-xylene) with Br₂ and a catalytic amount of FeBr₃ in the dark. All four ring C–H positions are equivalent; substitute at the carbon bearing the drawn hydrogen. FeBr₃ is drawn as one Lewis-acid unit; the electron count at iron is not assessed. Further bromination is not requested.
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Activate bromine with the Lewis acid
Benzene rings are poor nucleophiles. Coordinating bromine to FeBr₃ weakens the Br–Br bond and makes the other bromine far more electrophilic.
As setas curvas
- A bromine lone pair bonds to iron, the Lewis acid.
The ring attacks bromine
Aromaticity is lost in this slow step. The positive charge appears on the neighboring methyl-bearing carbon. That extra stabilization is one reason p-xylene reacts faster than benzene.
As setas curvas
- The ring π pair forms a bond between the hydrogen-bearing carbon and the bromine not bonded to iron.
- The Br–Br pair stays on the bromine bonded to iron, giving FeBr₄⁻.
Move the charge para to the new C–Br bond
The σ complex (arenium ion) is one delocalized cation; its contributors differ only in π-electron placement.
As setas curvas
- The adjacent ring π pair forms a π bond to the cationic methyl-bearing carbon. The carbon para to the new C–Br bond becomes electron-poor.
Move the charge to the other ortho carbon
Positive charge is shared by the three carbons ortho and para to the new C–Br bond.
As setas curvas
- The next ring π pair forms a π bond to the cationic para carbon. The other carbon ortho to the new C–Br bond becomes electron-poor.
Restore the aromatic ring
The arenium ion is a very strong acid, so even the weakly basic bromide carried by FeBr₄⁻ removes the proton. Regaining aromaticity makes this step fast. HBr (pKa approximately −9) is released.
Nota: Remove the proton from the ring carbon that now bears bromine.
As setas curvas
- The Fe–Br bond pair forms a new Br–H bond, so FeBr₄⁻ releases bromide as it takes the proton.
- The ring C–H bond electrons form a π bond to the neighboring cationic carbon, restoring aromaticity.
Aryl bromide, HBr and recovered catalyst
Bromine replaced a ring hydrogen: 2-bromo-1,4-dimethylbenzene. The ring has three alternating π bonds again. The other bromine formed HBr, and FeBr₃ is regenerated unchanged, so it is a catalyst.
O que você vai praticar
Follow Br₂ activation by FeBr₃, σ-complex formation and its contributors, and catalyst regeneration during aromatic bromination.
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