Química Orgânica 2 · Substituição eletrofílica aromática
Chlorobenzene from benzene with Cl₂ and FeCl₃
Condições: Cl2, FeCl3 (cat.)
Desenhe este problema você mesmo
Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 6 passos escritos
The ring attacks chlorine
FeCl₃ (not drawn) polarizes Cl₂, making one chlorine strongly electrophilic. In this slow step a ring π pair bonds to that chlorine and aromaticity is lost, giving the arenium ion (σ complex).
As setas curvas
- The ring π pair at the carbon bearing the drawn hydrogen forms the new C–Cl bond, leaving a positive charge on the neighboring carbon.
- The Cl–Cl bond pair moves onto the far chlorine, which leaves as chloride.
Delocalize the charge to the para carbon
The arenium ion is one delocalized cation: the positive charge is shared by the two carbons ortho to the new C–Cl bond and the carbon para to it. Each contributor differs only in where the π pairs sit.
As setas curvas
- The neighboring ring π pair shifts toward the cationic carbon, moving the positive charge to the carbon para to the new bond.
Return the charge beside the sp³ carbon
The arenium ion is one hybrid; redraw the contributor with the positive charge back on the carbon next to the sp³ C–H. That contributor shows which π bond the C–H electrons will re-form in the deprotonation.
As setas curvas
- The π pair shifts back toward the para carbon, leaving the positive charge on the ortho carbon next to the sp³ C–H.
Why is FeCl₃ needed?
Benzene is a weak nucleophile because attacking breaks its aromatic sextet; it needs a stronger electrophile than Cl₂ alone.
Lose the proton and restore aromaticity
The arenium ion is a very strong acid, so chloride (released as FeCl₄⁻, which regenerates FeCl₃) removes the proton from the sp³ carbon bearing chlorine. Regaining aromaticity makes this step fast, so substitution wins over addition.
As setas curvas
- A lone pair on the chloride ion takes the ring proton, forming HCl.
- The C–H bond electrons form the ring π bond to the neighboring cationic carbon, restoring aromaticity.
Chlorobenzene and HCl
Chlorine has replaced one ring hydrogen, giving chlorobenzene; the ring is aromatic again. The other chlorine leaves as HCl, and FeCl₃ is regenerated, so it is a catalyst.
O que você vai praticar
Draw chlorination of benzene: ring attack on Cl₂, the arenium-ion contributors, and loss of the ring proton to give chlorobenzene.
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