Química Orgânica 1 · Rearranjos de carbocátions e expansões de anel
E1 dehydration of 2,2-dimethylcyclohexanol with a methyl shift
Condições: H2SO4 (cat.), heat; alkene distilled out
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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
Protonation of the hydroxyl group
Hydronium protonates the alcohol oxygen, turning OH into an alkyloxonium ion that can leave as water.
As setas curvas
- An oxygen lone pair forms the new O–H bond.
- The O–H pair of hydronium stays on its oxygen, releasing water.
Loss of water
Water departs and leaves a secondary ring carbocation next to the carbon carrying two methyl groups.
As setas curvas
- The C–O bonding pair leaves with oxygen as water.
1,2-Methyl shift
The neighbouring ring carbon is quaternary, so a methyl group, not a hydrogen, migrates with its bonding pair onto the cationic carbon. The charge moves to the ring carbon that keeps one methyl: a tertiary cation.
As setas curvas
- The C–CH3 bonding pair carries the methyl to the cationic carbon.
Which proton is lost?
The tertiary cation can lose a proton from the methyl-bearing CH carbon, from the ring CH2 on its other side, or from its own methyl group. Only the first gives a tetrasubstituted alkene.
Water removes a β-hydrogen
A water lone pair takes the hydrogen from the ring carbon that now carries the migrated methyl; its C–H bonding pair becomes the new C=C π bond, giving the tetrasubstituted alkene.
As setas curvas
- A water lone pair forms the new O–H bond.
- The C–H bonding pair becomes the C=C π bond.
1,2-Dimethylcyclohexene
Each alkene carbon carries one methyl and one ring carbon: 1,2-dimethylcyclohexene. The two methyls, once both on the same carbon, now sit on adjacent carbons because of the shift. Hydronium is regenerated.
O que você vai praticar
Draw the E1 dehydration of 2,2-dimethylcyclohexan-1-ol: protonation, loss of water, the methyl shift to a tertiary cation, and loss of a proton giving 1,2-dimethylcyclohexene.
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