Química Orgânica 1 · Adição a alcenos e regioquímica
Add HBr and let a hydride move
Condições: 3-Methylpent-1-ene with anhydrous HBr in a nonpolar solvent, in the dark and without peroxides, so the addition is ionic rather than radical. Follow the rearranged branch to 3-bromo-3-methylpentane; the unrearranged 2-bromo-3-methylpentane also forms and no product ratio is claimed. The acid supplies one hydrogen; the hydrogen on the branched carbon next to the alkene undergoes the hydride shift.
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 4 passos escritos
Protonate the alkene
HBr (pKa approximately −9) is acidic enough to protonate an alkene. Add the proton to the terminal carbon so the positive charge is on the more substituted alkene carbon.
As setas curvas
- The alkene π pair forms a bond between the terminal carbon and the acid proton, leaving an empty orbital on the internal carbon.
- The H–Br pair stays on bromine as bromide.
Shift a hydrogen with its pair
The cation is secondary, but the neighboring branched carbon is attached to three carbons. A hydride shift from that neighbor converts the secondary cation into a more stable tertiary one.
Nota: Use the C–H bond on the branched carbon next to the carbocation. That carbon becomes positively charged; no proton is released.
As setas curvas
- The C–H bond electrons form a new bond between that hydrogen and the neighboring cationic carbon. The carbon that loses the C–H bond is left with an empty orbital.
Capture the tertiary cation
The bromide made in the first step is still next to the cation. It bonds to the rearranged, tertiary center.
As setas curvas
- Bromide donates a lone pair to the empty orbital on the tertiary carbon.
Rearranged alkyl bromide
The terminal alkene carbon gained the acid proton, and the internal alkene carbon gained a hydrogen through the hydride shift. Both are now saturated. Bromine is bonded to the branched carbon, which also carries the methyl and ethyl groups: 3-bromo-3-methylpentane. Net: one HBr added, with the hydrogen and bromine ending on carbons that were not neighbors of each other in the alkene.
O que você vai praticar
Recognize that the secondary cation from HBr addition can become tertiary through a 1,2-hydride shift before bromide bonds.
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