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Chapter 24. Amines and Heterocycles

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Chapter 24. Amines and Heterocycles Based on McMurry s Organic Chemistry, 7th edition * 13C NMR Carbons next to amine N are slightly deshielded - about 20 ppm ... – PowerPoint PPT presentation

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Title: Chapter 24. Amines and Heterocycles


1
Chapter 24. Amines and Heterocycles
Based on McMurrys Organic Chemistry, 7th edition
2
Amines Organic Nitrogen Compounds
  • Organic derivatives of ammonia, NH3,
  • Nitrogen atom with a lone pair of electrons,
    making amines both basic and nucleophilic
  • Occur in plants and animals

3
Why this Chapter?
  • Amines and carbonyl compounds are the most
    abundant and have rich chemistry
  • In addition to proteins and nucleic acids, a
    majority of pharmaceutical agents contain amine
    functional groups

4
Common Names of Heterocyclic Amines
  • If the nitrogen atom occurs as part of a ring,
    the compound is designated as being heterocyclic
  • Each ring system has its own parent name

5
24.2 Properties of Amines
  • Bonding to N is similar to that in ammonia
  • N is sp3-hybridized
  • CNC bond angles are close to 109 tetrahedral
    value

6
Chirality Is Possible (But Not Observed)
  • An amine with three different substituents on
    nitrogen is chiral (in principle but not in
    practice) the lone pair of electrons is the
    fourth substituent
  • Most amines that have 3 different substituents on
    N are not resolved because the molecules
    interconvert by pyramidal inversion

7
24.3 Basicity of Amines
  • The lone pair of electrons on nitrogen makes
    amines basic and nucleophilic
  • They react with acids to form acidbase salts and
    they react with electrophiles

8
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9
24.5 Biological Amines and the Henderson-Hasselbal
ch Equation
  • What form do amines exist at physiological pH
    inside cells

10
Selective Preparation of Primary Amines the
Azide Synthesis
  • Azide ion, N3? displaces a halide ion from a
    primary or secondary alkyl halide to give an
    alkyl azide, RN3
  • Alkyl azides are not nucleophilic (but they are
    explosive)
  • Reduction gives the primary amine

11
Gabriel Synthesis of Primary Amines
  • A phthalimide alkylation for preparing a primary
    amine from an alkyl halide
  • The N-H in imides (?CONHCO?) can be removed by
    KOH followed by alkylation and hydrolysis

12
Reductive Amination of Aldehydes and Ketones
  • Treatment of an aldehyde or ketone with ammonia
    or an amine in the presence of a reducing agent

13
Mechanism of Reductive Amination
14
Reducing Step
  • Sodium cyanoborohydride, NaBH3CN, reduces CN but
    not CO
  • Stable in water

15
Hofmann and Curtius Rearrangements
  • Carboxylic acid derivatives can be converted into
    primary amines with loss of one carbon atom by
    both the Hofmann rearrangement and the Curtius
    rearrangement

16
24.7 Reactions of Amines
  • Alkylation and acylation have already been
    presented

17
Hofmann Elimination
  • Converts amines into alkenes
  • NH2? is very a poor leaving group so it converted
    to an alkylammonium ion, which is a good leaving
    group

18
Orientation in Hofmann Elimination
  • We would expect that the more highly substituted
    alkene product predominates in the E2 reaction of
    an alkyl halide (Zaitsev's rule)
  • However, the less highly substituted alkene
    predominates in the Hofmann elimination due to
    the large size of the trialkylamine leaving group
  • The base must abstract a hydrogen from the most
    sterically accessible, least hindered position

19
24.8 Reactions of Arylamines
  • Amino substituents are strongly activating,
    ortho- and para-directing groups in electrophilic
    aromatic substitution reactions
  • Reactions are controlled by conversion to amide

20
Arylamines Are Not Useful for Friedel-Crafts
Reactions
  • The amino group forms a Lewis acidbase complex
    with the AlCl3 catalyst, preventing further
    reaction
  • Therefore we use the corresponding amide

21
Diazonium Salts The Sandmeyer Reaction
  • Primary arylamines react with HNO2, yielding
    stable arenediazonium salts

22
Uses of Arenediazonium Salts
  • The N2 group can be replaced by a nucleophile

23
Reduction to a Hydrocarbon
  • By treatment of a diazonium salt with
    hypophosphorous acid, H3PO2

24
Mechanism of Diazonium Replacement
  • Through radical (rather than polar or ionic)
    pathways

25
Diazonium Coupling Reactions
  • Arenediazonium salts undergo a coupling reaction
    with activated aromatic rings, such as phenols
    and arylamines, to yield brightly colored azo
    compounds, Ar?NN?Ar?

26
How Diazonium Coupling Occurs
  • The electrophilic diazonium ion reacts with the
    electron-rich ring of a phenol or arylamine
  • Usually occurs at the para position but goes
    ortho if para is blocked

27
Azo Dyes
  • Azo-coupled products have extended ? conjugation
    that lead to low energy electronic transitions
    that occur in visible light (dyes)

28
24.9 Heterocycles
  • A heterocycle is a cyclic compound that contains
    atoms of two or more elements in its ring,
    usually C along with N, O, or S

29
Pyrole and Imidazole
  • Pyrole is an amine and a conjugated diene,
  • however its chemical properties are not
    consistent with either of structural features

30
Chemistry of Pyrole
  • Electrophilic substitution reactions occur at C2
    b/c it is position next to the N
  • A more stable intermediate cation having 3
    resonance forms
  • At C3, only 2 resonance forms

31
Polycyclic Heterocycles
32
24.10 Spectroscopy of Amines -Infrared
  • Characteristic NH stretching absorptions 3300 to
    3500 cm?1
  • Amine absorption bands are sharper and less
    intense than hydroxyl bands
  • Protonated amines show an ammonium band in the
    range 2200 to 3000 cm?1

33
Nuclear Magnetic Resonance Spectroscopy
  • NH hydrogens appear as broad signals without
    clear-cut coupling to neighboring CH hydrogens
  • In D2O exchange of ND for NH occurs, and the
    NH signal disappears

34
13C NMR
  • Carbons next to amine N are slightly deshielded -
    about 20 ppm downfield from where they would
    absorb in an alkane
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