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Chapter 24. Amines

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Title: Chapter 24. Amines


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Chapter 24. Amines
  • Based on McMurrys Organic Chemistry, 6th 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
24.1 Naming Amines
  • Alkyl-substituted (alkylamines) or
    aryl-substituted (arylamines)
  • Classified 1 (RNH2), methyl (CH3NH2), 2
    (R2NH), 3 (R3N)

4
Quaternary Ammonium Iions
  • A nitrogen atom with four attached groups is
    positively charged
  • Compounds are quaternary ammonium salts

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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

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

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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

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24.3 Properties and Sources of Amines
  • Simple methylated amines from reaction of NH3
    with CH3OH and alumina catalyst
  • Yields a mixture of monomethylated, dimethylated,
    and trimethylated products that are easily
    separated by distillation

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Amines Form H-Bonds
  • Amines with fewer than five carbons are
    water-soluble
  • Primary and secondary amines form hydrogen bonds,
    increasing their boiling points

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24.4 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

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Relative Basicity
  • Amines are stronger bases than alcohols, ethers,
    or water
  • Amines establish an equilibrium with water in
    which the amine becomes protonated and hydroxide
    is produced
  • The most convenient way to measure the basicity
    of an amine (RNH2) is to look at the acidity of
    the corresponding ammonium ion (RNH3)
  • High pKa ? weaker acid and stronger conjugate
    base .

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General Patterns of Basicity
  • Table 24.1 pKa values of ammonium ions
  • Most simple alkylammmonium ions have pKa's of 10
    to 11
  • Arylamines and heterocyclic aromatic amines are
    considerably less basic than alkylamines
    (conjugate acid pKa 5 or less)

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Amides
  • Amides (RCONH2) in general are not proton
    acceptors except in very strong acid
  • The CO group is strongly electron-withdrawing,
    making the N a very weak base
  • Addition of a proton occurs on O but this
    destroys the double bond character of CO as a
    requirement of stabilization by N

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Amines as Acids
  • Loss of the NH proton requires a very strong base

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24.5 Basicity of Substituted Arylamines
  • The N lone-pair electrons in arylamines are
    delocalized by interaction with the aromatic ring
    ? electron system and are less able to accept H
    than are alkylamines

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Substituted Arylamines
  • Can be more basic or less basic than aniline
  • Electron-donating substituents (such as ?CH3,
    ?NH2, ?OCH3) increase the basicity of the
    corresponding arylamine
  • Electron-withdrawing substituents (such as ?Cl,
    ?NO2, ?CN) decrease arylamine basicity

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24.6 Synthesis of Amines
  • Reduction of nitriles and amides (review)

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Reduction Aryl Nitro Compounds
  • Arylamines are prepared from nitration of an
    aromatic compound and reduction of the nitro
    group
  • Reduction by catalytic hydrogenation over
    platinum is suitable if no other groups can be
    reduced
  • Iron, zinc, tin, and tin(II) chloride are
    effective in acidic solution

32
SN2 Reactions of Alkyl Halides
  • Ammonia and other amines are good nucleophiles

33
Uncontrolled Multiple Alkylation
  • Primary, secondary, and tertiary amines all have
    similar reactivity, the initially formed
    monoalkylated substance undergoes further
    reaction to yield a mixture of products

34
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

35
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

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

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Reductive Amination Is Versatile
  • Ammonia, primary amines, and secondary amines
    yield primary, secondary, and tertiary amines,
    respectively

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Mechanism of Reductive Amination
  • Imine is intermediate

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Reducing Step
  • Sodium cyanoborohydride, NaBH3CN, reduces CN but
    not CO
  • Stable in water

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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

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Hofmann Rearrangement
  • RCONH2 reacts with Br2 and base
  • Gives high yields of arylamines and alkylamines

49
Curtius Rearrangement
  • Heating an acyl azide prepared from substitution
    an acid chloride
  • Migration of ?R from CO to the neighboring
    nitrogen with simultaneous loss of a leaving group

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Reactions of Amines
  • Alkylation and acylation have already been
    presented

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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

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Silver Oxide Is Used for the Elimination Step
  • Exchanges hydroxide ion for iodide ion in the
    quaternary ammonium salt, thus providing the base
    necessary to cause elimination

59
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

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Steric Effects Control the Orientation
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Definition of Phase Transfer Catalysis
  • The transfer of an inorganic ion such as OH? from
    one phase to another is called phase transfer,
    and the tetraalkylammonium salt is a
    phase-transfer catalyst

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