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Chapter 20

Chapter 20. Amines. Nomenclature. 1 o Amines. 2 o Amines. 3 o Amines. 1A. Arylamines. 1B. Heterocyclic Amines. Physical Properties and Structure of Amines. 2A. Physical Properties. 2B. Structure of Amines. N: sp 3 hybridized Trigonal pyramidal Bond angles close to 109.5 o.

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Chapter 20

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  1. Chapter 20 Amines

  2. Nomenclature • 1o Amines • 2o Amines

  3. 3o Amines

  4. 1A. Arylamines

  5. 1B. Heterocyclic Amines

  6. Physical Properties and Structure of Amines 2A. Physical Properties

  7. 2B. Structure of Amines • N: sp3 hybridized • Trigonal pyramidal • Bond angles close to 109.5o

  8. 3o Amines with three different groups • The two enantiomeric forms interconvert rapidly: • Impossible to resolve enantiomers. • Pyramidal or nitrogen inversion: • Barrier ~ 25 kJ/mol. • Enough to occur rapidly at room temperature.

  9. Ammonium salts with four different groups. enantiomers can be resolved

  10. Basicity of Amines:Amine Salts

  11. The aminium ion of a more basic amine will have a larger pKa than the aminium ion of a less basic amine

  12. > > By releasing electrons, R— stabilizes the alkylaminium ion through dispersal of charge >

  13. Basicity of amines:

  14. 3A. Basicity of Arylamines Compare cyclohexylamine with arylamines.

  15. Aniline • Five resonance structures

  16. Only two resonance structures

  17. 3B. Basicity of Heterocyclic Amines

  18. 3C. Amines versus Amides • Amides are far less basic than amines (even less basic than arylamines). The pKa of the conjugate acid of a typical amide is about zero Larger resonance stabilization Smaller resonance stabilization

  19. < >

  20. 3D. Aminium Salts and QuaternaryAmmonium Salts Cannot act as bases However, R4N⊕⊖OH are strong bases (as strong as NaOH)

  21. 3E. Solubility of Amines in AqueousAcids

  22. 3F. Amines as Resolving Agents • Enantiomerically pure amines are often used to resolve racemic forms of acidic compounds by the formation of diastereomeric salts.

  23. Preparation of Amines 4A. Through Nucleophilic Substitution Reactions • Alkylation of ammonia

  24. Alkylation of azide ion and reduction

  25. The Gabriel synthesis

  26. Alkylation of 3o amines

  27. 4B. Preparation of Aromatic Amines through Reduction of Nitro Compounds

  28. 4C. Preparation of Primary, Secondary, and Tertiary Amines through Reductive Amination

  29. Mechanism Sodium cyanoborohydride, NABH3CN, is often used to reduce the imine.

  30. Examples

  31. 4D. Preparation of Primary, Secondary, or Tertiary Amines through Reduction of Nitriles, Oximes, and Amides

  32. Examples

  33. Examples

  34. 4D. Preparation of Primary Amines through the Hofmann and Curtius Rearrangements • Hofmann rearrangement:

  35. Examples

  36. Mechanism

  37. Curtius rearrangement:

  38. Reactions of Amines • Acid-base reactions • Alkylation

  39. Acylation

  40. Electrophilic aromatic substitution NH2: powerful activating group, ortho-para director

  41. 5A. Oxidation of 3o Amines

  42. Reactions of Amines withNitrous Acid • Nitrous acid (HONO) is a weak, unstable acid

  43. 6A. Reactions of Primary Aliphatic Amines with Nitrous Acid 1o aliphatic amine (aliphatic diazonium salt) (highly unstable)

  44. 6B. Reactions of Primary Arylamines with Nitrous Acid (arenediazonium salt) (stable at <5oC)

  45. Mechanism

  46. Mechanism (Cont'd) diazonium ion

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