Química Orgânica 1 · Raciocínio ácido–base
A strong base turns a ketone into its enolate
Condições: Acetone with lithium diisopropylamide (LDA) in dry THF at −78 °C. The separately drawn H is one of the six equivalent alpha hydrogens. First draw deprotonation to the carbon-centered contributor, then the resonance form with the charge on oxygen. Li⁺ is shown as a spectator counterion. Approximate pKa values: acetone alpha C–H 19–20, diisopropylamine 36.
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LDA removes an alpha proton
Only the hydrogens on carbons next to the C=O are acidic, because the anion they leave behind is delocalized onto the carbonyl oxygen. LDA is a very strong base, and its two bulky isopropyl groups keep nitrogen from adding to the carbonyl carbon.
As setas curvas
- A lone pair on the negatively charged nitrogen of LDA forms a bond to the alpha hydrogen.
- The C–H pair stays on the alpha carbon as a lone pair.
Move the charge onto oxygen
Resonance, not a reaction: no atom moves, only electrons. The contributor with the negative charge on the more electronegative oxygen is the major one.
As setas curvas
- The alpha carbon lone pair forms a C=C π bond.
- The C=O π pair moves onto oxygen so the carbonyl carbon keeps four bonds.
Why LDA gives complete conversion
Diisopropylamine (pKa about 36) is a far weaker acid than acetone (pKa about 19–20), so K ≈ 10^16 and essentially every acetone molecule becomes enolate. With hydroxide or ethoxide the conjugate acid (water or ethanol, pKa about 16) is a stronger acid than acetone, so only a small equilibrium amount of enolate forms.
Lithium enolate and diisopropylamine
The enolate is shown as its major contributor: C=C between the former carbonyl carbon and the alpha carbon, and oxygen with three lone pairs and charge −1, balanced by Li⁺. Nitrogen now has three bonds (two carbon groups and the new N–H) and one lone pair: neutral diisopropylamine. The net charge is conserved.
O que você vai praticar
Remove an alpha hydrogen from acetone with LDA, draw the resonance that moves the charge onto oxygen, and explain why conversion is essentially complete.
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