Química Orgânica 2 · Hidrólise e esterificação
Acid-catalysed hydrolysis of methyl acetate to acetic acid
Condições: H3O⁺ (cat.), H2O (excess), Δ
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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 8 passos escritos
Protonate the carbonyl oxygen
Water is a weak nucleophile, so the ester is first activated: hydronium protonates the carbonyl oxygen, making the carbonyl carbon much more electrophilic.
As setas curvas
- The carbonyl oxygen lone pair takes the proton.
- The O–H bond pair stays on the water oxygen.
Water adds to the activated carbonyl
A neutral water molecule attacks the protonated carbonyl carbon, giving a tetrahedral intermediate with an oxonium ion.
As setas curvas
- A water lone pair forms the new C–O bond.
- The C=O π electrons move onto the protonated oxygen.
Water removes a proton from the oxonium
A water molecule takes the extra proton from the added oxygen, giving the neutral tetrahedral intermediate.
As setas curvas
- A lone pair on the base takes the proton.
- The O–H bond pair stays on the atom that loses the proton.
Which oxygen gets protonated next?
The leaving group must leave as a neutral molecule.
Protonate the methoxy oxygen
Hydronium protonates the methoxy oxygen so that it can leave as neutral methanol.
As setas curvas
- The methoxy oxygen lone pair takes the proton.
- The O–H bond pair stays on the atom that loses the proton.
Methanol leaves
An OH lone pair pushes to re-form the C=O π bond and neutral methanol departs, leaving the protonated acid.
As setas curvas
- An oxygen lone pair forms the new C=O π bond.
- The C–O bond breaks onto the protonated methoxy oxygen.
Water regenerates the catalyst
Water removes the proton from the protonated carbonyl, giving acetic acid and regenerating hydronium.
As setas curvas
- A lone pair on the base takes the proton.
- The O–H bond pair stays on the atom that loses the proton.
Acetic acid and methanol
Acetic acid and methanol form and hydronium is regenerated. In excess water the equilibrium lies on this side; with excess methanol it would run backward (Fischer esterification).
O que você vai praticar
Draw the six proton-transfer, addition and elimination steps that convert methyl acetate and water into acetic acid and methanol with hydronium as catalyst (methyl acetate).
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