Química Orgánica 2 · Adición y condensación aldólica
(E)-Cinnamic acid by the Perkin reaction
Condiciones: Ac2O, NaOAc, 180 °C
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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.
Leer los 9 pasos escritos
Acetate forms the anhydride enolate
Acetate, a weak base, removes an α-hydrogen of acetic anhydride. Only a small amount of enolate forms, which is why the reaction needs high temperature.
Las flechas curvas
- Acetate takes an α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The C=O π pair moves onto oxygen.
Aldol addition to benzaldehyde
The enolate carbon attacks benzaldehyde, forming the C–C bond and an alkoxide.
Las flechas curvas
- The oxygen lone pair re-forms the C=O.
- The enolate π pair forms the new C–C bond.
- The aldehyde π pair moves onto oxygen.
Intramolecular acetyl transfer
The alkoxide attacks the nearby acetyl carbonyl of the anhydride through a six-membered ring, giving a cyclic tetrahedral intermediate.
Las flechas curvas
- The alkoxide lone pair bonds to the acetyl carbon.
- The C=O π pair moves onto oxygen.
Ring opening releases a carboxylate
The tetrahedral intermediate collapses, breaking the anhydride C–O bond: the acetyl group stays on the benzylic oxygen as an acetate ester and a carboxylate is released.
Las flechas curvas
- The oxygen lone pair re-forms the C=O.
- The C–O pair leaves on the carboxylate oxygen.
Proton transfer to the carboxylate
Acetic acid protonates the carboxylate, giving the 3-acetoxy carboxylic acid.
Las flechas curvas
- The carboxylate takes the acetic acid proton.
- The O–H pair stays on oxygen as acetate.
Acetate removes the α-hydrogen
At high temperature acetate removes the α-hydrogen of the acid, forming an enolate next to the acetoxy-bearing carbon.
Las flechas curvas
- Acetate takes the α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The C=O π pair moves onto oxygen.
Which group leaves?
The β-carbon now carries an acetate ester instead of an OH.
E1cB loss of acetate
The enolate re-forms C=O and expels acetate from the benzylic carbon, giving the conjugated (E)-alkene.
Las flechas curvas
- The oxygen lone pair re-forms the C=O.
- The enolate π pair forms the C=C bond.
- The C–O pair leaves as acetate.
(E)-Cinnamic acid
(E)-Cinnamic acid forms (as its salt until acidic workup), with acetate and acetic acid; the trans geometry keeps the phenyl and carboxyl apart.
Qué vas a practicar
Draw the Perkin reaction of benzaldehyde with acetic anhydride and sodium acetate: anhydride enolate formation, aldol addition, intramolecular acetyl transfer, and E1cB loss of acetate to (E)-cinnamic acid.
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