Química Orgânica 1 · Estereoquímica das reações de alcenos
Prilezhaev epoxidation — followed by acidic ring opening
Condições: Cyclohexene is first epoxidized with peracetic acid. In a second stage, aqueous hydronium and water enter; counterion omitted. Water opens the protonated epoxide, then another water removes a proton from the positive oxygen. Wedge/hash refer to the original ring plane.
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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 5 passos escritos
Transfer oxygen in one step
Draw the four arrows as one cycle: oxygen transfer happens in one step, without a free carbocation. Swapping the carbon ends of the two C–O arrows gives the same epoxide.
Nota: Use the terminal peroxide oxygen, the one bonded to hydrogen, to make the epoxide. Swapping the two carbon ends of the arrows gives the same ring.
As setas curvas
- The alkene π pair forms one C–O bond of the epoxide.
- The terminal O–H pair forms the other C–O bond. Swapping the carbon ends of these two arrows gives the same epoxide.
- The O–O pair forms the new C=O π bond in the acid.
- The original C=O π pair forms an O–H bond to the peracid proton.
Begin the aqueous stage
Aqueous acid and water now enter; the carboxylic-acid coproduct is retained.
Nota: Any of hydronium’s three hydrogens can be transferred.
As setas curvas
- The epoxide oxygen accepts the hydronium proton.
- The acid bond pair returns to water.
Continue the activated ring pathway
Backside opening sets the trans relationship.
Nota: Water attacks the right-hand ring carbon from below.
As setas curvas
- Water attacks opposite the protonated epoxide bridge.
- The breaking C–O bond pair returns to oxygen, releasing ring strain and neutralizing the original oxygen.
Complete the solvent step
Remove a proton from the incoming oxygen, not from carbon.
Nota: Water removes a proton from the positive oxygen.
As setas curvas
- Water uses a lone pair to take a proton from the positive oxygen.
- The O–H pair stays on oxygen, making it neutral.
Trans vicinal diol
The original epoxide oxygen remains above the ring and the solvent oxygen lies below it on the neighboring carbon. Each alcohol oxygen is neutral with two lone pairs; hydronium is regenerated and peracetic-acid atoms remain in the carboxylic acid coproduct.
O que você vai praticar
Combine syn epoxide formation with acid-catalyzed backside opening to obtain a trans diol without omitting proton transfers.
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