Química Orgánica 2 · Enoles, enolatos y reacciones en alfa
Trap a designated kinetic enolate
Condiciones: 2-Butanone with LDA in dry THF at −78 °C; the methyl-side kinetic enolate is specified. Methyl iodide is introduced afterward for C-alkylation. No aqueous quench precedes trapping. A final check compares this enolate with the thermodynamic one. Counterions are not drawn; no stereochemistry is assigned without bond markers.
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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 5 pasos escritos
Move the electrons
Bulky lithium diisopropylamide removes the designated less-hindered methyl-side proton in the supplied kinetic branch. Diisopropylamine (pKa ≈ 36) is a far weaker acid than the ketone α C–H (pKa ≈ 20), so K ≈ 10¹⁶: deprotonation is complete and does not reverse at −78 °C.
Las flechas curvas
- The base removes an alpha hydrogen next to the carbonyl.
- The alpha C–H electrons form a π bond to the carbonyl carbon.
- The carbonyl π electrons move onto oxygen.
Move the electrons
These are two resonance forms of the same enolate, not separate molecules. No proton moves between them.
Las flechas curvas
- An oxygen lone pair forms the carbonyl π bond.
- The π electrons move onto the alpha carbon as a lone pair.
Move the electrons
Methyl iodide is introduced after the dry kinetic-enolate preparation. Carbon alkylation gives a neutral ketone and iodide.
Las flechas curvas
- The enolate carbon uses its lone pair to form the new C–C bond.
- The leaving group takes the bond electrons as the new bond forms.
Compare the kinetic and thermodynamic enolates
Kinetic control: a strong, bulky base at low temperature removes the more accessible CH₃ proton faster and irreversibly (diisopropylamine pKa ≈ 36 against α C–H pKa ≈ 20). Thermodynamic control: when proton exchange is possible (ethanol pKa ≈ 16, or excess ketone), the enolates interconvert and the more substituted one predominates. The pKa values are approximate.
Nota: Compare the two structures.
Products
The selected methyl side gains one carbon to give the 3-pentanone skeleton. Diisopropylamine retains the removed alpha hydrogen, and iodide retains the negative charge initially carried by the base.
Qué vas a practicar
Combine regioselective strong-base deprotonation with complete C-alkylation rather than starting with an isolated carbanion.
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