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Reactions of a- Hydrogens: Condensation Reactions

Reactions of a- Hydrogens: Condensation Reactions

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Reactions of a- Hydrogens: Condensation Reactions

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  1. Reactions of a-Hydrogens:Condensation Reactions Chapter 21 These images should be printed in color, if possible, otherwise the images are very faint! It would be better to look at them on a computer monitor. WWU -- Chemistry

  2. Assignments for Chapter 21 and 22 Chapter 20, hardly any, attend class! Skip these sections from Chap. 21 21.4, 21.6, 21.8, 21.11, 21.17 and 21.21 Skip these sections from Chap. 22 22.3, 22.4, 22.13 through 22.15 Keep 22.16 and 22.17!!! WWU -- Chemistry

  3. Problems for Chapter 21 • In-Text Problems: • 2, 3, 4, 8, 13, 14, 18, 19, 26, 29, 30, 32, 34, 35, 36, 37, 40, 46 and 50 • End-of-Chapter Problems: • 2, 4, 5, 9, 13, 16, 17 and 21 WWU -- Chemistry

  4. Sect. 21.1: Keto-Enol Tautomerism WWU -- Chemistry

  5. Keto-Enol Tautomerism in 1,3-Dicarbonyl Compounds WWU -- Chemistry

  6. Keto-Enol Tautomerism in 1,3-Dicarbonyl Compounds • The equilibrium lies substantially to the right with 1,3-dicarbonyl compounds. • In simple ketones, such a hydrogen-bonded structure cannot form, and the percentage of enol found in an equilibrium mixture is very small (equilibrium lies to left). WWU -- Chemistry

  7. Some Representative Enol Percents WWU -- Chemistry

  8. Sect. 21.2 Acidity of a-Hydrogens • The acidity of a hydrogen attached to the a-carbon of a carbonyl compound is much higher than the acidity of a typical C-H hydrogen. • pKa values range from about 19 to 20 (compared with 48 to 50) for alkanes. WWU -- Chemistry

  9. Acidity of a-Hydrogens: The Reason WWU -- Chemistry

  10. Acidity of a-Hydrogens • Resonance stabilization of the enolate ion shifts the equilibrium to the right, thereby making the C-H bond more acidic. • Once formed, the enolate ion is capable of reacting as a nucleophile. The a-carbon of the enolate ion bears substantial negative charge. WWU -- Chemistry

  11. Sect. 21.3 Halogenation of Ketones WWU -- Chemistry

  12. Mechanism WWU -- Chemistry

  13. Example WWU -- Chemistry

  14. But... • The halogenation is difficult to stop at the mono-substitution stage. • Often, poly-halogenated products are formed in this reaction. WWU -- Chemistry

  15. With an excess of bromine: WWU -- Chemistry

  16. In base, bromoform is formed WWU -- Chemistry

  17. Section 21.5: Iodoform reaction WWU -- Chemistry

  18. Sect. 21.7: Alkylation Reactions WWU -- Chemistry

  19. Example WWU -- Chemistry

  20. Sect. 21.9: Alkylation of Active Methylene Compounds WWU -- Chemistry

  21. Sect: 21.10: Hydrolysis and decarboxylation WWU -- Chemistry

  22. Mechanism of decarboxylation WWU -- Chemistry

  23. Alkylation of ethyl acetoacetate WWU -- Chemistry

  24. Sect. 21.10: Decarboxylation WWU -- Chemistry

  25. Sect. 21.12: The Aldol Condensation WWU -- Chemistry

  26. Aldol Condensation -- Mechanism WWU -- Chemistry

  27. An example WWU -- Chemistry

  28. One More: WWU -- Chemistry

  29. Synthesis Problem WWU -- Chemistry

  30. Sect. 21.13: Dehydration of Aldol products • Aldol products easily dehydrate in acid and sometimes in base. WWU -- Chemistry

  31. Dehydration of Aldol Products WWU -- Chemistry

  32. Dehydration of Aldol Products Note here that iodine is a sufficiently strong Lewis acid to bring about dehydration. WWU -- Chemistry

  33. Also: WWU -- Chemistry

  34. Synthesis of: WWU -- Chemistry

  35. Synthesis of a Compound used in Perfumery WWU -- Chemistry

  36. Preparation via Aldol Condensation and reduction WWU -- Chemistry

  37. Preparation via Aldol Condensation and reduction! WWU -- Chemistry

  38. Synthesis of an Insect Repellent WWU -- Chemistry

  39. Sect. 21.14: Crossed Aldol Condensations • Reaction of two different aldehydes: • One with an "-hydrogen (donor) • Other with no "-hydrogen (acceptor) WWU -- Chemistry

  40. Crossed Aldol Condensations acceptor donor WWU -- Chemistry

  41. Chalcone formation: part 1 WWU -- Chemistry

  42. Chalcone Formation: part 2 (formed from an aldol condensation) WWU -- Chemistry

  43. Do the synthesis of: WWU -- Chemistry

  44. Sect. 21.15: The Claisen Ester Condensation WWU -- Chemistry

  45. Claisen Ester Condensation -- Mechanism (Part One) WWU -- Chemistry

  46. Claisen Ester Condensation -- Mechanism (Part Two) WWU -- Chemistry

  47. Example WWU -- Chemistry

  48. Another Example WWU -- Chemistry

  49. Example of a Ring Formation This reaction is known as the Dieckmann cyclization. WWU -- Chemistry

  50. Sect. 21.16: Crossed Claisen Condensation • In the crossed Claisen condensation, we choose one of the esters to be the acceptor. The acceptor does not have a-hydrogens. • The other ester, the donor, does have a-hydrogens. It can react with base to form a nucleophilic enolate ion. • With such an experimental design, the crossed Claisen condensation can be used successfully. WWU -- Chemistry