Química Orgânica 2 · Substituição eletrofílica aromática
Friedel–Crafts alkylation — rearranging alcohol donor
Condições: Benzene in excess, 3-methylbutan-2-ol and strong acid. Follow only the monoalkylation branch in which the explicit neighboring H migrates before ring capture, yielding tert-amylbenzene. Elimination and unrearranged capture compete but are not the requested branches. Hydronium models the acid.
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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
Activate the alcohol partner
The neutral hydroxyl group is not treated as a leaving hydroxide.
As setas curvas
- Alcohol oxygen accepts a proton.
- The acid O–H pair returns to solvent oxygen.
Continue from the activated alcohol
The water molecule remains present throughout the following steps.
As setas curvas
- The C–O pair departs with oxygen as neutral water, leaving a secondary carbocation.
Follow the specified rearrangement branch
This is a hydride shift, not a proton transfer.
Nota: Use the drawn C–H bond on the carbon next to the carbocation.
As setas curvas
- The neighboring C–H bond electrons form a bond between hydrogen and the cationic carbon. The tertiary carbon becomes positively charged.
Capture the rearranged electrophile
Form the bond to the tertiary carbocation, not the original alcohol carbon.
Nota: Use the ring π bond touching the carbon with the drawn H to form the new C–C bond at that carbon.
As setas curvas
- The aromatic pair bonds to the rearranged tertiary carbocation.
Finish aromatic substitution
Return the catalyst charge to hydronium.
Nota: Water accepts the proton from the carbon that gained the alkyl group.
As setas curvas
- Water takes the hydrogen drawn on the carbon that gained the new group.
- The C–H electrons form the missing ring π bond and remove the positive charge.
Rearranged alkylbenzene, water and acid
The original alcohol carbon has gained the migrating H and is now CH₂ in an ethyl arm. Its neighbor bears the aromatic bond and two methyl groups. The alcohol oxygen leaves as water, hydronium is regenerated, and the aromatic ring is neutral.
O que você vai praticar
Link alcohol activation, water loss and hydride migration to a rearranged aromatic C–C bond.
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