Química Orgánica 2 · Condensación de Claisen y adición conjugada
Benzoylacetone from acetophenone and ethyl acetate
Condiciones: NaH; then H3O⁺
Dibuja este problema tú mismo
Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 7 pasos escritos
Form the enolate
Sodium hydride removes an α-hydrogen from acetophenone. The ketone α-hydrogens (pKa ≈ 19) are far more acidic than those of ethyl acetate (pKa ≈ 25), so the ketone is the donor; hydride leaves as H2 gas, so this deprotonation is irreversible.
Las flechas curvas
- A lone pair on the hydride takes the α-hydrogen.
- The C–H bonding pair stays on the α-carbon, forming the enolate.
Enolate attacks the acceptor carbonyl
The nucleophilic α-carbon adds to the carbonyl carbon of ethyl acetate, and the C=O π electrons move onto oxygen to give a tetrahedral alkoxide.
Las flechas curvas
- The enolate carbon's lone pair forms the new C–C bond.
- The C=O π pair moves onto oxygen, keeping the carbonyl carbon at an octet.
Which partner is the donor?
Acidity decides the donor.
Collapse and expel ethoxide
The alkoxide re-forms the C=O bond and pushes out ethoxide, completing the acyl substitution and giving the β-dicarbonyl compound.
Las flechas curvas
- An oxygen lone pair re-forms the C=O π bond.
- The C–O bond breaks and ethoxide leaves with the bonding pair.
Deprotonate the β-dicarbonyl product
The hydrogen on the carbon between the two carbonyls (pKa ≈ 9–11) is far more acidic than ethanol, so ethoxide removes it. This favorable step consumes the base and drives the reversible condensation to completion.
Las flechas curvas
- The alkoxide lone pair takes the doubly activated hydrogen.
- The C–H pair stays on carbon, delocalized over both carbonyls.
Acidic workup
Hydronium, added in a separate workup, protonates the stabilized anion on carbon to give the neutral β-dicarbonyl product.
Las flechas curvas
- The carbanion lone pair bonds to a proton of hydronium.
- The O–H pair returns to oxygen, releasing water.
Benzoylacetone
The product is benzoylacetone (1-phenylbutane-1,3-dione), a 1,3-diketone formed by acetylating the methyl group of acetophenone. Ethanol, hydrogen and water are by-products.
Qué vas a practicar
Draw the mixed Claisen in which the acetophenone enolate (from NaH) is acetylated by ethyl acetate, through product trapping and acidic workup.
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