Química Orgânica 2 · Éteres e abertura de epóxidos
Epoxide opening — acid-catalysed, with methanol
Condições: 2-Methyl-1,2-Epoxypropane in methanol with a catalytic, non-nucleophilic acid, shown as hydronium and water. Draw attack at the tertiary carbon; you do not need a product ratio or an R/S assignment. The acid counterion is not drawn.
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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
Protonate the oxygen
Draw both arrows for the proton transfer. The water formed here can take back a proton later to regenerate hydronium. No halide is added.
Nota: Hydronium transfers a proton and becomes water. The acid counterion is not drawn.
As setas curvas
- An oxygen lone pair bonds to the hydronium proton, giving this oxygen a positive charge.
- The O–H bond electrons stay on the water oxygen. Hydronium becomes water.
Methanol opens the ring
Backside attack and ring opening happen together, without a free carbocation. Here, attack favors the tertiary carbon because it better supports positive charge. Primary and secondary epoxides can behave differently and give mixtures.
As setas curvas
- Methanol attacks the tertiary carbon from the side opposite its bond to the ring oxygen. This carbon better supports the developing positive charge.
- The attacked C–O bond electrons return to the ring oxygen as the new bond forms. Carbon keeps eight outer electrons.
Water takes back a proton
Water removes a proton from the product. The product becomes neutral and water becomes hydronium.
As setas curvas
- The water oxygen takes a proton.
- The O–H bond electrons return to the positively charged oxygen, making it neutral.
Products
The product is HOCH₂C(OCH₃)(CH₃)₂. The ring oxygen stays bonded to CH₂ as OH, and the methanol oxygen bonds to the tertiary carbon as an ether. Water takes the methanol proton to regenerate hydronium. Both methyl groups and all oxygens are still present; the total charge stays +1.
O que você vai praticar
Protonate the epoxide, then add methanol at the tertiary carbon while the ring oxygen stays on CH₂.
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