Química Orgânica 2 · Revisão de substituição aromática para a prova
Friedel–Crafts alkylation with 1-chloro-2-methylpropane: a hydride shift
Condições: Benzene (large excess), 1-chloro-2-methylpropane, AlCl3 (cat.), 0–25 °C
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Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
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The chloride binds aluminium chloride
A chlorine lone pair of 1-chloro-2-methylpropane coordinates to aluminium. The C–Cl bond is now weak and the primary carbon strongly electron-poor.
As setas curvas
- The chlorine lone pair forms a dative bond to aluminium.
Ionisation with a hydride shift
As the C–Cl bond breaks, the hydrogen on the neighbouring tertiary carbon moves with its bonding pair to the primary carbon. The positive charge ends on the tertiary carbon: the tert-butyl cation forms directly, and AlCl₄⁻ is released. The charges drawn on chlorine and aluminium in the complex, and on aluminium in AlCl₄⁻, are Lewis formal charges; aluminium stays Al(III) throughout.
As setas curvas
- The C–H pair carries the hydrogen to the primary carbon (a 1,2-hydride shift).
- The C–Cl pair leaves with chlorine as AlCl₄⁻.
Why the carbon skeleton changes
A tertiary carbocation is far more stable than a primary one, and a hydride on the neighbouring carbon can reach the electron-poor primary carbon as the C–Cl bond breaks.
Benzene captures the tert-butyl cation
A ring π bond forms the C–C bond to the tertiary carbon, giving the arenium ion.
As setas curvas
- The ring π electrons bond to the cationic tertiary carbon.
AlCl₄⁻ restores the ring
A chlorine of AlCl₄⁻ removes the proton on the sp³ ring carbon, leaving aluminium as HCl; the C–H electrons restore the aromatic ring and AlCl₃ is regenerated.
As setas curvas
- A chlorine lone pair takes the ring proton.
- The Al–Cl pair returns to chlorine, so HCl leaves and AlCl₃ is regenerated.
- The C–H pair forms the ring π bond.
tert-Butylbenzene
tert-Butylbenzene forms; the hydrogen that migrated now sits on what was the CH₂Cl carbon, which has become one of the three methyl groups. HCl and AlCl₃ are regenerated.
O que você vai praticar
Draw how the 1-chloro-2-methylpropane–AlCl₃ complex ionises with a hydride shift to the tert-butyl cation, how benzene captures that cation, and how AlCl₄⁻ restores the ring to give tert-butylbenzene.
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