Química Orgânica 1 · Rearranjos de carbocátions e expansões de anel
Friedel–Crafts alkylation of benzene with 1-chloropropane
Condições: benzene (excess), 1-chloropropane (1 equiv), AlCl3 (0.1 equiv), 25 °C; then H2O
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Chlorine coordinates to AlCl3
A chlorine lone pair of 1-chloropropane donates into the empty orbital of aluminium. The C–Cl bond is now weakened, because chlorine carries a positive formal charge.
As setas curvas
- A chlorine lone pair forms the Cl–Al bond.
Ionisation with a 1,2-hydride shift
As the C–Cl bond breaks, a hydrogen on the middle carbon moves with its bonding pair to the CH2 carbon that is losing chlorine. No primary cation is ever formed: the positive charge appears directly on the middle carbon, giving the secondary isopropyl cation and AlCl4−.
As setas curvas
- The C–H bonding pair carries the hydrogen to the CH2 carbon.
- The C–Cl bonding pair stays on chlorine, which remains bonded to aluminium as AlCl4−.
Why the ring gains an isopropyl group
A primary cation is too unstable to be an intermediate. The hydride shift during ionisation delivers the secondary isopropyl cation, so the ring becomes bonded to the middle carbon of the propyl chain.
Benzene attacks the isopropyl cation
A π bond of benzene attacks the cationic carbon, forming a C–C bond and the resonance-stabilised arenium ion; aromaticity is temporarily lost.
As setas curvas
- The ring π electrons form the new bond to the isopropyl cation, leaving the positive charge in the ring.
Deprotonation restores aromaticity
A chloride of AlCl4− removes the proton from the ring carbon that bears the isopropyl group. The C–H pair re-forms the ring π bond, restoring aromaticity and regenerating AlCl3 along with HCl.
As setas curvas
- A chlorine lone pair forms the new Cl–H bond.
- The C–H bonding pair re-forms the ring π bond.
- The Al–Cl bond breaks, releasing neutral AlCl3.
Isopropylbenzene
Isopropylbenzene (cumene) forms with HCl, and AlCl3 is regenerated, so it is catalytic. The ring is attached to the middle carbon of the original propyl chain because of the hydride shift.
O que você vai praticar
Draw activation of 1-chloropropane by AlCl3 with the hydride shift that gives the isopropyl cation, attack of benzene, and deprotonation of the arenium ion giving isopropylbenzene.
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