Química Orgánica 2 · Adición y condensación aldólica
(E)-But-2-enal from acetaldehyde in acid
Condiciones: HCl (cat.), H2O, Δ
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El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
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HCl protonates acetaldehyde
Hydrogen chloride protonates the carbonyl oxygen of one acetaldehyde, which makes its α-hydrogens acidic enough to be removed by water.
Las flechas curvas
- The carbonyl lone pair takes the HCl proton.
- The H–Cl pair stays on chlorine as chloride.
Water removes an α-hydrogen: the enol
Water removes an α-hydrogen from the protonated aldehyde, giving ethenol, the enol tautomer. In acid there is no enolate; the enol is the nucleophile.
Las flechas curvas
- A water lone pair takes an α-hydrogen.
- The C–H pair forms the enol C=C bond.
- The C=O+ π pair returns to oxygen.
Protonation of a second acetaldehyde
Hydronium protonates the carbonyl of a second acetaldehyde, activating it toward the weakly nucleophilic enol.
Las flechas curvas
- The carbonyl lone pair takes the hydronium proton.
- The O–H pair stays on oxygen as water.
The enol attacks
The enol oxygen lone pair pushes the C=C electrons onto the protonated carbonyl carbon, forming the new C–C bond.
Las flechas curvas
- The enol oxygen lone pair forms the C=O+ bond.
- The C=C π pair forms the new C–C bond.
- The C=O+ π pair moves onto oxygen.
Deprotonation gives 3-hydroxybutanal
Water removes the proton from the protonated aldehyde, giving the aldol, 3-hydroxybutanal.
Las flechas curvas
- Water takes the O–H proton.
- The O–H pair stays on oxygen.
Protonation of the aldehyde again
Hydronium protonates the aldehyde of the aldol, starting the acid-catalysed enolization needed for dehydration.
Las flechas curvas
- The carbonyl lone pair takes the hydronium proton.
- The O–H pair stays on oxygen as water.
Enol of the aldol
Water removes the α-hydrogen between the protonated aldehyde and the carbinol carbon, giving the enol.
Las flechas curvas
- Water takes an α-hydrogen.
- The C–H pair forms the enol C=C bond.
- The C=O+ π pair returns to oxygen.
Protonation of the hydroxyl group
Hydronium protonates the β-OH, turning it into water, a good leaving group. In acid, hydroxide is never the leaving group.
Las flechas curvas
- The hydroxyl lone pair takes the hydronium proton.
- The O–H pair stays on oxygen as water.
What pushes water out?
Loss of water next to an enol is assisted by the enol oxygen.
Loss of water
The enol oxygen lone pair pushes the C=C electrons toward the β-carbon, which expels water and gives the protonated, conjugated enal; the E geometry places the methyl trans to the aldehyde.
Las flechas curvas
- The enol oxygen lone pair forms the C=O+ bond.
- The C=C π pair forms the new C=C bond.
- The C–O pair leaves as water.
Deprotonation gives crotonaldehyde
Water removes the O–H proton, giving neutral (E)-but-2-enal and regenerating hydronium.
Las flechas curvas
- Water takes the O–H proton.
- The O–H pair stays on oxygen.
(E)-But-2-enal
(E)-But-2-enal (crotonaldehyde) forms with water; the acid catalyst is regenerated as hydronium chloride.
Qué vas a practicar
Draw the HCl-catalysed aldol condensation of acetaldehyde: enol formation, attack on a protonated acetaldehyde, deprotonation to 3-hydroxybutanal, and acid-catalysed dehydration through the enol to (E)-but-2-enal.
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