Química Orgánica 2 · Adición y condensación aldólica
Diethyl 2-benzylidenemalonate by Knoevenagel condensation
Condiciones: piperidine (cat.), benzene, Δ (−H2O)
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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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Piperidine forms the malonate enolate
Piperidine (pKaH about 11) is basic enough to deprotonate diethyl malonate (pKa about 13) to a useful extent, because the anion is delocalized over two ester carbonyls. A weak amine base cannot enolize simple ketones or esters, so self-condensation does not compete.
Las flechas curvas
- The piperidine nitrogen lone pair takes a CH2 hydrogen.
- The C–H pair forms the enolate C=C bond.
- The ester C=O π pair moves onto oxygen.
Why is a weak base enough?
The acidity of the donor decides which base is needed.
The enolate adds to benzaldehyde
The stabilized enolate carbon attacks the benzaldehyde carbonyl carbon, forming the C–C bond and an alkoxide.
Las flechas curvas
- The oxygen lone pair re-forms the ester C=O.
- The enolate π pair forms the new C–C bond.
- The aldehyde π pair moves onto oxygen.
Piperidinium protonates the alkoxide
The piperidinium ion gives its proton to the alkoxide, giving the neutral aldol and regenerating piperidine.
Las flechas curvas
- The alkoxide takes the N–H proton.
- The N–H pair stays on nitrogen.
Second deprotonation
Piperidine removes the remaining acidic hydrogen between the two esters, giving the enolate that will expel hydroxide.
Las flechas curvas
- Piperidine takes the α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The ester C=O π pair moves onto oxygen.
E1cB loss of hydroxide
The enolate re-forms the ester C=O and expels hydroxide from the benzylic carbon. The new C=C is conjugated with the ring and both esters.
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 hydroxide.
Hydroxide is neutralized
The released hydroxide takes a proton from piperidinium, giving water and regenerating the piperidine catalyst. Removing water drives the condensation to completion.
Las flechas curvas
- Hydroxide takes the N–H proton.
- The N–H pair stays on nitrogen.
Diethyl 2-benzylidenemalonate
Diethyl 2-benzylidenemalonate forms with water; piperidine is regenerated as a catalyst.
Qué vas a practicar
Draw the piperidine-catalysed Knoevenagel condensation of diethyl malonate with benzaldehyde through the enolate addition and E1cB dehydration to diethyl 2-benzylidenemalonate.
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