Arrow Lab

Química Orgânica 2 · Acetais, iminas e equilíbrios relacionados

Mecanismo de formação de acetal

O mecanismo de formação de acetal em 8 etapas, a partir do exemplo resolvido do Arrow Lab: Acetal formation — ethanal and methanol.

O que você vai praticar

  • Add two methanol molecules to an aldehyde and regenerate the acid catalyst.

O exemplo e suas 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.

Mecanismo de formação de acetal, passo a passo

Cada etapa começa com o seu desenho, setas curvas incluídas; o texto abaixo dele descreve esse desenho.

  1. Protonate the carbonyl

    Etapa 1 de 8, Protonate the carbonyl: as estruturas com 2 setas curvas desenhadas

    Hydronium gives a proton to the carbonyl oxygen, making the carbonyl easier to attack. Hydronium becomes water.

    As setas curvas do desenho:

    • 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.

  2. Add the first methanol

    Etapa 2 de 8, Add the first methanol: as estruturas com 2 setas curvas desenhadas

    The first alcohol attacks the carbonyl carbon. Its oxygen gains a bond and a positive charge.

    As setas curvas do desenho:

    • The nucleophile uses a lone pair to form the new single bond.
    • The π electrons move off carbon, keeping eight electrons around it.

  3. Remove a proton

    Etapa 3 de 8, Remove a proton: as estruturas com 2 setas curvas desenhadas

    Water removes a proton from the incoming alcohol oxygen. This gives a neutral hemiacetal.

    As setas curvas do desenho:

    • 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.

  4. Turn OH into water

    Etapa 4 de 8, Turn OH into water: as estruturas com 2 setas curvas desenhadas

    Add a proton to the OH group on the former carbonyl carbon. It can now leave as water.

    As setas curvas do desenho:

    • 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.

  5. Let water leave

    Etapa 5 de 8, Let water leave: as estruturas com 2 setas curvas desenhadas

    The ether oxygen forms a π bond to carbon as the original carbonyl oxygen leaves in water.

    As setas curvas do desenho:

    • A lone pair forms the new π bond.
    • The leaving group takes the electrons from the single bond.

  6. Add the second methanol

    Etapa 6 de 8, Add the second methanol: as estruturas com 2 setas curvas desenhadas

    The second alcohol attacks carbon. Move the C=O π electrons onto oxygen so carbon keeps eight electrons.

    As setas curvas do desenho:

    • The nucleophile uses a lone pair to form the new single bond.
    • The π electrons move off carbon, keeping eight electrons around it.

  7. Return the acid catalyst

    Etapa 7 de 8, Return the acid catalyst: as estruturas com 2 setas curvas desenhadas

    Water removes the proton from the positively charged oxygen. The acetal becomes neutral and hydronium returns.

    As setas curvas do desenho:

    • 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.

  8. Products

    Etapa 8 de 8, Products: as estruturas, com os produtos finais

    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.

Desenhe você mesmo, com retorno sobre cada seta

Abra a demonstração interativa gratuita de formação de acetal: você desenha cada seta curva, do par isolado ou da ligação até o átomo que ela ataca, e recebe retorno sobre cada movimento. Não é preciso criar conta.

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