Química Orgânica 1 · Eliminação e caminhos competitivos
E1 after a hydride shift onto a side chain
Condições: (1-Bromoethyl)cyclohexane heated in ethanol with no added base. Follow the rearranged E1 branch to 1-ethylcyclohexene. Substitution by ethanol, the unrearranged alkenes and the exocyclic isomer (ethylidenecyclohexane) also form; no ratio is claimed. The hydrogen on the ring carbon attached to the side chain undergoes the hydride shift. A β hydrogen on a neighboring ring carbon is removed at the end.
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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
Ionize the secondary bromide
With only weakly basic ethanol present and heat applied, the leaving group departs first.
As setas curvas
- The C–Br pair leaves with bromine, leaving a secondary cation on the side chain.
Draw the hydride shift from the ring
The ring carbon attached to the side chain is bonded to three carbons. A hydride shift from this carbon to the side-chain cation turns a secondary cation into a tertiary one.
As setas curvas
- The ring C–H bond electrons form a new bond between that hydrogen and the side-chain cationic carbon. The ring carbon becomes the cation.
Remove a ring β proton
Ethanol is a weak base (its conjugate acid has pKa approximately −2), but a C–H bond next to a carbocation is acidic enough for ethanol to remove its proton. Removing a ring hydrogen gives the trisubstituted endocyclic alkene.
Nota: Remove a β hydrogen from a ring carbon next to the carbocation. The two neighboring ring positions are equivalent.
As setas curvas
- An ethanol lone pair takes a β proton from a ring carbon next to the carbocation.
- The ring C–H bond electrons form a π bond to the neighboring cationic carbon and neutralize the cation.
Endocyclic alkene
The hydride shift formed a C–H bond on the side chain, which is now an ethyl group. The double bond lies within the ring at the carbon bearing that group: 1-ethylcyclohexene. Ethanol has become protonated ethanol, which with bromide accounts for the HBr lost.
O que você vai praticar
Follow an E1 elimination with a hydride shift from the ring to a side-chain cation before the solvent removes a β proton.
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