Química Orgânica 1 · Rearranjos de carbocátions e expansões de anel
E1 dehydration of 3-methyl-2-butanol with a hydride shift
Condições: H2SO4 (cat.), H2O, 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 transfers a proton to the alcohol oxygen, converting OH, a poor leaving group, into an alkyloxonium ion that can leave as neutral water.
As setas curvas
- An oxygen lone pair of the alcohol forms the new O–H bond.
- The O–H pair of hydronium stays on its oxygen, releasing water.
Loss of water
The C–O bond breaks heterolytically and water departs, leaving a secondary carbocation on the carbon that carried the hydroxyl group.
As setas curvas
- The C–O bonding pair leaves with oxygen as a water molecule.
1,2-Hydride shift
The hydrogen on the neighbouring isopropyl CH carbon moves with its bonding pair onto the cationic carbon. The charge moves to the carbon bearing two methyls, giving the tertiary 2-methylbutan-2-yl cation.
As setas curvas
- The C–H bonding pair migrates with the hydrogen to the cationic carbon.
Which β-hydrogen leaves?
The tertiary cation has β-hydrogens on the CH2 carbon and on two methyl groups. Removing one from the CH2 carbon gives the trisubstituted alkene, the most stable choice.
Water removes a β-hydrogen
A water lone pair takes a proton from the CH2 carbon next to the cation; the C–H bonding pair becomes the new C=C π bond, giving the trisubstituted alkene and hydronium.
As setas curvas
- A water lone pair forms the new O–H bond to a β-hydrogen on the CH2 carbon.
- The C–H bonding pair becomes the C=C π bond.
2-Methyl-2-butene
The major alkene is trisubstituted 2-methylbut-2-ene. The secondary cation could reach the same alkene by losing its own neighbouring hydrogen, but the fast hydride shift funnels it into the tertiary cation first, which is why little 3-methylbut-1-ene forms. Hydronium is regenerated, so the acid is catalytic.
O que você vai praticar
Draw the acid-catalysed E1 dehydration of 3-methylbutan-2-ol, including the 1,2-hydride shift before loss of a β-hydrogen, giving 2-methylbut-2-ene.
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