Química Orgânica 1 · Rearranjos de carbocátions e expansões de anel
Pinacol rearrangement of 1-methylcyclohexane-1,2-diol
Condições: H2SO4 (cat.), H2O, heat
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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
Protonate the tertiary hydroxyl group
Hydronium protonates the OH on the methyl-bearing carbon. Loss of water from this oxygen gives a tertiary cation, so it is the productive site.
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.
Water leaves to give the tertiary cation
The C–O bond breaks and water departs, leaving a tertiary carbocation on the ring carbon that carries the methyl group.
As setas curvas
- The C–O bonding pair leaves with oxygen as water.
Hydride or ring bond?
The carbinol carbon offers a hydrogen and a ring C–C bond. Hydride migration gives the ketone in an unchanged six-membered ring; ring-bond migration would contract the ring to give an aldehyde.
Hydride shift gives the oxocarbenium ion
The hydrogen on the neighbouring carbinol carbon moves with its bonding pair to the tertiary cation. As it leaves, a lone pair on the hydroxyl oxygen forms a C=O π bond, so the new cation is a protonated ketone (oxocarbenium ion), far more stable than any carbocation.
As setas curvas
- The C–H bonding pair carries the hydrogen to the cationic carbon.
- An oxygen lone pair forms the C=O π bond, so the charge ends on oxygen.
Water removes the O–H proton
Water takes the proton from the protonated carbonyl oxygen, giving the neutral ketone and regenerating hydronium.
As setas curvas
- A water lone pair forms the new O–H bond.
- The O–H bonding pair stays on the carbonyl oxygen.
2-Methylcyclohexanone
The carbon that lost the hydrogen became the carbonyl carbon, and the carbon that lost water now carries the methyl and the migrated hydrogen: 2-methylcyclohexanone. The acid is regenerated.
O que você vai praticar
Draw the acid-catalysed pinacol rearrangement of 1-methylcyclohexane-1,2-diol: loss of the tertiary OH as water, hydride migration from the neighbouring carbinol carbon, and deprotonation giving 2-methylcyclohexanone.
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