Química Orgânica 2 · Acetais, iminas e equilíbrios relacionados
Acetal formation — ethanal and methanol
Condições: Ethanal in excess methanol with an acid catalyst; removing water favors acetal formation. Draw both methanol additions and use water to transfer protons. Counterions are not drawn; no stereochemistry is assigned without bond markers.
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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 8 passos escritos
Protonate the carbonyl
Hydronium gives a proton to the carbonyl oxygen, making the carbonyl easier to attack. Hydronium becomes water.
As setas curvas
- The base uses a lone pair to accept a proton from the O–H bond.
- The bond to hydrogen breaks, and its electrons stay on the atom losing the proton.
Add the first methanol
The first alcohol attacks the carbonyl carbon. Its oxygen gains a bond and a positive charge.
As setas curvas
- The nucleophile uses a lone pair to form the new single bond.
- The π electrons move off carbon, keeping eight electrons around it.
Remove a proton
Water removes a proton from the incoming alcohol oxygen. This gives a neutral hemiacetal.
As setas curvas
- The base uses a lone pair to accept a proton from the O–H bond.
- The bond to hydrogen breaks, and its electrons stay on the atom losing the proton.
Turn OH into water
Add a proton to the OH group on the former carbonyl carbon. It can now leave as water.
As setas curvas
- The base uses a lone pair to accept a proton from the O–H bond.
- The bond to hydrogen breaks, and its electrons stay on the atom losing the proton.
Let water leave
The ether oxygen forms a π bond to carbon as the original carbonyl oxygen leaves in water.
As setas curvas
- A lone pair forms the new π bond.
- The leaving group takes the electrons from the single bond.
Add the second methanol
The second alcohol attacks carbon. Move the C=O π electrons onto oxygen so carbon keeps eight electrons.
As setas curvas
- The nucleophile uses a lone pair to form the new single bond.
- The π electrons move off carbon, keeping eight electrons around it.
Return the acid catalyst
Water removes the proton from the positively charged oxygen. The acetal becomes neutral and hydronium returns.
As setas curvas
- The base uses a lone pair to accept a proton from the O–H bond.
- The bond to hydrogen breaks, and its electrons stay on the atom losing the proton.
Products
Both methanol oxygens are now neutral ether oxygens bonded to the former carbonyl carbon. The original carbonyl oxygen leaves in water with two hydrogens. Hydronium is regenerated.
O que você vai praticar
Add two methanol molecules to an aldehyde and regenerate the acid catalyst.
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