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Chapter 18

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  1. Chapter 18 Reactions at the a Carbon of Carbonyl Compounds Enols and Enolates

  2. Keto, enol and enolate structures: • What is a keto form ? A structure that contains a carbonyl. E.g. aldehyde, ketone, ester, acid, etc. • What is an enol ? A compound that has an alkene and an OH attached to the same carbon atom. • What is an enolate ? An enol with the proton removed. May be formed by removing a proton from the a carbon atom of a keto form.

  3. The a carbon and a hydrogens: • What is an a carbon ? A carbon atom adjacent to a carbonyl. • What is an a hydrogen ? A hydrogen attached to an a carbon atom. aHydrogens are weakly acidic (pKa = 19 – 20) due to the e-withdrawing C=O.

  4. Reactions at the a Carbon of Carbonyl Compounds:Enols and Enolates This shows a reaction at the carbonyl carbon atom. Tetrahedral intermediate

  5. The Acidity of the a Hydrogens of Carbonyl Compounds: Enolate Anions Comparison of pKas.

  6. Deprotonation: Removal of a H forms a resonance stabilized enolate. Resonance structures for the delocalized enolates

  7. Protonation: Protonation of a carbon. Protonation of oxygen.

  8. Keto and Enol Tautomers • Interconvertible keto and enol forms are called tautomers, and their interconversion is called tautomerization. • The keto and enol forms are in equilibrium (not resonance structures) because a proton transfer occurs.

  9. E.g.

  10. E.g. Resonance stabilization of the enol form

  11. Reactions via Enols & Enolates 3A. Racemization Racemization at an a carbon takes place in the presence of acids or bases

  12. Base-Catalyzed Enolization:

  13. Acid-Catalyzed Enolization:

  14. 3B. Halogenation at the a Carbon

  15. Base-Promoted Halogenation:

  16. Acid-Promoted Halogenation:

  17. 3C. The Haloform Reaction

  18. The Iodoform reaction:

  19. Mechanism:

  20. Acyl Substitution Step:

  21. 3D. a-Halo Carboxylic Acids: The Hell–Volhard–Zelinski Reaction

  22. Example:

  23. Mechanism:

  24. Further reaction of an a halo acid:

  25. Lithium Enolates

  26. Prep. of lithium diisopropylamide: (LDA)

  27. 4A. Regioselective Formation of Enolates • Formation of a Kinetic Enolate: (Dimethoxyethane) This enolate is formed faster because the hindered strong base removes the less hindered proton faster.

  28. Formation of a Thermodynamic Enolate: This enolate is more stable because the double bond is more highly substituted. It is the predominant enolate at equilibrium.

  29. 4B. Direct Alkylation of Ketones via Lithium Enolates Lithium diisopropylamide = LDA or LiN(iPr)2.

  30. 4C. Direct Alkylation of Esters

  31. Examples:

  32. Enolates of b-Dicarbonyl Compounds

  33. Recall: • a-hydrogens of b-dicarbonyl compounds are more acidic:

  34. Contributing resonance structures Resonance hybrid

  35. Synthesis of Methyl Ketones: The Acetoacetic Ester Synthesis

  36. Synthesis of monosubstituted methyl ketones:

  37. Synthesis of disubstituted methyl ketones:

  38. Synthesis of g-keto acids and g-diketones:

  39. 6A. Acylation • Synthesis b-diketones:

  40. Synthesis of Substituted Acetic Acids: The Malonic Ester Synthesis

  41. Synthesis of substituted acetic acid:

  42. Synthesis of monoalkylacetic acid:

  43. Synthesis of dialkylacetic acid:

  44. Example 1:

  45. Example 2:

  46. Further Reactions of Active Hydrogen Compounds

  47. Example:

  48. Synthesis of Enamines: Stork Enamine Reactions

  49. 2° amines most commonly used to prepare enamines: • e.g.