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Chapter 20 Amines.

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Presentation on theme: "Chapter 20 Amines."— Presentation transcript:

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 Five resonance structures
Aniline Five resonance structures

16 Only two resonance structures


18 3B. Basicity of Heterocyclic Amines


20 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

21 < >

22 3D. Aminium Salts and Quaternary Ammonium Salts
Cannot act as bases However, R4N⊕ ⊖OH are strong bases (as strong as NaOH)

23 3E. Solubility of Amines in Aqueous Acids

24 3F. Amines as Resolving Agents
Enantiomerically pure amines are often used to resolve racemic forms of acidic compounds by the formation of diastereomeric salts.


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


28 Alkylation of azide ion and reduction

29 The Gabriel synthesis

30 Alkylation of 3o amines

31 4B. Preparation of Aromatic Amines through Reduction of Nitro Compounds

32 4C. Preparation of Primary, Secondary,
and Tertiary Amines through Reductive Amination

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

34 Examples

35 4D. Preparation of Primary, Secondary,
or Tertiary Amines through Reduction of Nitriles, Oximes, and Amides

36 Examples

37 Examples

38 4D. Preparation of Primary Amines
through the Hofmann and Curtius Rearrangements Hofmann rearrangement:

39 Examples

40 Mechanism

41 Curtius rearrangement:

42 Reactions of Amines Acid-base reactions Alkylation

43 Acylation

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

45 5A. Oxidation of 3o Amines

46 Reactions of Amines with Nitrous Acid
Nitrous acid (HONO) is a weak, unstable acid

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

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

49 Mechanism

50 Mechanism (Cont'd) diazonium ion

51 6C. Reactions of Secondary Amines with Nitrous Acid
N-Nitroso- amines

52 6D. Reactions of Tertiary Amines with Nitrous Acid

53 Replacement Reactions of Arenediazonium Salts

54 7A. Syntheses Using Diazonium Salts

55 7B. The Sandmeyer Reaction: Replacement of the Diazonium Group by -Cl, -Br, or -CN
The Sandmeyer reactions use a copper containing reagent.

56 A Sandmeyer reaction.

57 A Sandmeyer reaction.

58 7C. Replacement by —I

59 7D. Replacement by —F

60 7E. Replacement by —OH

61 7F. Replacement by Hydrogen: Deamination by Diazotization

62 Coupling Reactions of Arenediazonium Salts

63 Examples

64 Examples

65 Examples

66 Reactions of Amines with Sulfonyl Chlorides
A sulfonamide

67 9A. The Hinsberg Test Sulfonamide formation is the basis for a chemical test, called the Hinsberg test, that can be used to demonstrate whether an amine is primary, secondary, or tertiary.

68 Hinsberg with a 1o Amine

69 Hinsberg with a 2o Amine

70 Hinsberg with a 3o Amine

71 Synthesis of Sulfa Drugs

72 Analysis saltsof Amines
11A. Chemical Analysis They dissolve in dilute aqueous acid Moist pH paper is  basic Hinsberg test for 1o, 2o and 3o. 1o aromatic amines form diazo salts.  azo dye formation with 2-naphthol

73 11B. Spectroscopic Analysis
IR 1o amines 3300 – 3555 cm-1 (N–H)  two bands. 2o amines  one band only. 3o amines No bands at 3300 – 3555 cm-1 region.

74 Aliphatic amines 1020 – 1220 cm-1 (C–N) Aromatic amines 1250 – 1360 cm-1 (C–N)

75 1H NMR spectra 1o and 2o amines
N–H d (0.5 – 5 ppm), usually broad, exact position depends on the solvent, concentration, purity and temperature. N–H protons are not usually coupled to protons on adjacent carbons. Protons on the a carbon of an aliphatic amine are deshielded by the electron-withdrawing effect of the nitrogen and absorb typically in the d 2.2–2.9 region.

76 Protons on the b carbon are not deshielded as much and absorb in the range d 1.0–1.7

77 13C NMR chemical shifts (d)
13C NMR spectra 23.0 34.0 14.3 29.7 42.5 13C NMR chemical shifts (d)

78 Mass spectra The molecular ion in the mass spectrum of an amine has an odd number mass (unless there is an even number of nitrogen atoms in the molecule). The peak for the molecular ion is usually strong for aromatic and cyclic aliphatic amines but weak for acyclic aliphatic amines. Cleavage between the a and b carbons of aliphatic amines is a common mode of fragmentation.

79 Eliminations Involving Ammonium Compounds
12A. The Hofmann Elimination

80 Preparation of the 4o ammonium hydroxide

81 Although most eliminations involving neutral substrates tend to follow the Zaitsev rule, eliminations with charged substrates tend to follow what is called the Hofmann rule and yield mainly the least substituted alkene

82 An anti mechanism

83 12B. The Cope Elimination Prep of amine oxide, see slide 45.
A syn mechanism

84 Summary of Preparations and Reactions of Amines
Preparation of amines


86 Reactions of amines



89  END OF CHAPTER 20 

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