Química Orgânica 2 · Adição a aldeídos e cetonas
2-Hydroxy-2-methylpropanoic acid from acetone cyanohydrin
Condições: H3O⁺, H2O, Δ
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Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
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Protonate the nitrile nitrogen
The nitrile nitrogen lone pair takes a proton from hydronium. The nitrilium ion has a much more electrophilic carbon, so weak nucleophilic water can add.
As setas curvas
- The nitrogen lone pair takes the proton from hydronium.
- The O–H bond pair stays on water.
Water adds to the nitrilium carbon
A water lone pair forms a C–O bond to the nitrilium carbon; one C≡N π pair moves onto nitrogen, which becomes neutral.
As setas curvas
- The water oxygen lone pair attacks the electrophilic nitrilium carbon.
- One π pair of the C≡N moves onto nitrogen.
Deprotonate the added oxygen
Water removes a proton from the positive oxygen, giving the neutral imidic acid, the tautomer of an amide.
As setas curvas
- A water lone pair takes the proton from the oxonium oxygen.
- The O–H bond pair stays on oxygen.
Protonate the imidic acid nitrogen
Tautomerisation to the amide is relayed in two steps: hydronium first protonates the nitrogen.
As setas curvas
- The nitrogen lone pair takes a proton from hydronium.
- The O–H bond pair stays on water.
Lose the O–H proton to form the amide
Water removes the O–H proton; that oxygen lone pair forms the C=O π bond while the C=N π pair moves onto nitrogen, giving the neutral amide.
As setas curvas
- A water lone pair takes the O–H proton.
- The O–H bond pair becomes the C=O π bond.
- The C=N π pair moves onto nitrogen, which becomes neutral.
The amide intermediate
Nitrile hydrolysis passes through an amide.
Protonate the amide oxygen
The amide carbonyl oxygen is its most basic site, because the resulting cation is stabilised by the nitrogen lone pair. Hydronium protonates it.
As setas curvas
- The carbonyl oxygen lone pair takes the proton from hydronium.
- The O–H bond pair stays on water.
Water adds to the carbonyl carbon
Water attacks the activated carbonyl carbon and the C=O π pair moves onto the protonated oxygen, giving a tetrahedral intermediate.
As setas curvas
- A water lone pair forms the new C–O bond.
- The C=O π pair moves onto the positive oxygen.
Deprotonate the new oxonium
Water removes a proton from the added oxygen, giving a neutral tetrahedral intermediate.
As setas curvas
- A water lone pair takes the proton.
- The O–H bond pair stays on oxygen.
Protonate the nitrogen
Nitrogen is the most basic atom of the tetrahedral intermediate. Protonating it turns NH2 into an NH3⁺ group that can leave as neutral ammonia.
As setas curvas
- The nitrogen lone pair takes the proton from hydronium.
- The O–H bond pair stays on water.
Expel ammonia
An oxygen lone pair re-forms the C=O bond and pushes out ammonia, a far better leaving group than amide ion.
As setas curvas
- An oxygen lone pair forms the C=O π bond.
- The C–N bond pair leaves with nitrogen as ammonia.
Deprotonate the carbonyl oxygen
Water removes the proton from the protonated carboxylic acid, giving the neutral acid and regenerating hydronium.
As setas curvas
- A water lone pair takes the proton.
- The O–H bond pair stays on oxygen.
Ammonia is protonated
In the acidic solution, ammonia is immediately protonated to ammonium. This removes the nucleophile and makes the overall hydrolysis irreversible.
As setas curvas
- The ammonia lone pair takes the proton from hydronium.
- The O–H bond pair stays on water.
2-Hydroxy-2-methylpropanoic acid
The nitrile carbon is now a carboxylic acid carbon; nitrogen ends up as ammonium. The tertiary OH from the cyanohydrin is untouched.
O que você vai praticar
Draw acid-catalysed hydrolysis of the nitrile of acetone cyanohydrin through the amide to 2-hydroxy-2-methylpropanoic acid and ammonium.
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