1. Chapter 13 Reactions at the α-Carbon of Carbonyl Compounds
Paula Yurkanis Bruice
University of California,
Santa Barbara

2. Contents of Chapter 13 Acidity of Alpha Hydrogens Keto-enol Tautomers
Alkylation of Enolate Ions
Aldol Condensation
No Claisen, Malonic/Acetoacetic Ester, or Decarboxylation
No Bioloogical Reactions
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3. Acidity of a Hydrogens
In the presence of a strong base, a proton a to a carbonyl group can be removed to form an enolate ion stabilized by electron delocalization:
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4. Alkylation of Enolate Ions
Enolate ions are alkylated (bonds made to carbon) at the alpha carbon:
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5. Product Analysis
Polarize the alpha carbon negative and an alpha hydrogen positive and swap partners with the positive and negative fragments of the alkylating reagent
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6. Acidity of a Hydrogens This is Base-Catalyzed Enolization
Reprotonation of the enolate ion can give an enol:
This is Base-Catalyzed Enolization
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7. Keto–Enol Tautomerism
Under ordinary conditions, all aldehydes and ketones are in equilibrium with the corresponding enol forms.
Interconversion of these isomers is catalyzed by acids or bases.
For most simple aldehydes and ketones, the equilibrium lies on the side of the keto form because the carbon–oxygen double bond is stronger than a carbon–carbon double bond.
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8. Acid-Catalyzed Enolization
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9. Sample Problem Draw two enol forms for the following compound:
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10. The Aldol Reaction
The most important reaction of an enolate anion derived from an aldehyde or ketone is nucleophilic addition to the carbonyl group of another carbonyl-containing compound.
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11. Mechanism of the Aldol Addition
Step 1 – Deprotonation:
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12. Mechanism of the Aldol Addition
Step 2 – Nucleophilic Addition of Enolate Ion:
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13. Mechanism of the Aldol Addition
Step 3 – Reaction of alkoxide anion with solvent (protonation):
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14. Dehydration of Aldol Products
b-Hydroxyaldehydes and b-hydroxyketones are very easily dehydrated to give a,b-unsaturated aldehydes or ketones.
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15. Aldol Reaction Product Analysis
To figure out dehydration product of aldol reaction place carbonyl O of one molecule under 2 alpha H’s of the other molecule
Erase the H’s and the O
Extend the carbonyl double bond from the carbonyl C of the lower molecule to the alpha carbon of the upper molecule
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16. Aldol Reaction Product Analysis
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17. Retrosynthetic Analysis
Draw structural formulas for the two carbonyl-containing compounds that react to give cinnamaldehyde:
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