Spectroscopy of Amines - IR

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Spectroscopy of Amines - IR

• Characteristic NH stretching absorptions 3300 to
  3500 cm-1. NH2 group shows an irregular doublet,
  NH - weak multiple bands. Ammonium ions show N-H
  at 2600 cm-1. Amine absorption bands are sharper
  and less intense than hydroxyl bands. 1o amines
  show NH2 deformation band at 1650-1590 cm-1.
• C-N stretching vibrations are found at 1090-1068
  cm-1 in 1o amines with a 1o a carbon, 1140-1080
  cm-1 with a 2o a carbon, and at 1240-1170 cm-1
  with a 3o a carbon. 3o amines show no C-N
  vibrations. In aromatic amines, this band is at
  1330-1260 cm-1.
• N-H wagging bands are found at 850-750 cm-1 as
  strong, broad, multiple bands. These are weak in
  aromatic amines.

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Mass Spectrometry

• Since N is a compound with an odd number of
  nitrogen atoms has an odd-numbered molecular
  weight and a corresponding parent ion
• Alkylamines cleave at the CC bond nearest the
  nitrogen to yield an alkyl radical and a
  nitrogen-containing cation

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Mass Spectrum of N-Ethylpropylamine

• The two modes of a cleavage give fragment ions at
  m/z 58 and m/z 72.

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1H - NMR Spectroscopy

• NH hydrogens appear as broad signals, either
  fully coupled to neighboring CH hydrogens, or
  more frequently with no coupling (just like the
  H-bonded OH signals)
• Hydrogens on C next to N and absorb at lower
  field than alkane hydrogens
• N-CH3 gives a sharp three-H singlet at d 2.2 to
  2.6

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C3H9NO
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C4H11NO2
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C8H11NO
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C9H13N
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C15H17N
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C6H15N - IR
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C6H15N - 1H-NMR
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Structure, Properties and Reactivity of Amines

• Organic derivatives of ammonia, NH3
• Nitrogen atom with a lone pair of electrons,
  making amines both basic and nucleophilic
• Occur in plants and animals

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IUPAC nomenclature of simple amines

• For simple amines, the suffix -amine is added to
  the name of the alkyl substituent

• The suffix -amine can also be used in place of
  the final -e in the name of the parent compound

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IUPAC nomenclature of simple amines

• Alkyl-substituted (alkylamines) or
  aryl-substituted (arylamines)
• Classified 1 (RNH2), methyl (CH3NH2), 2
  (R2NH), 3 (R3N)

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IUPAC nomenclature of complex amines.Amines with
more than one functional group.

• Consider the NH2 as an amino substituent on the
  parent molecule

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IUPAC nomenclature of complex amines.Amines with
multiple alkyl groups.

• Symmetrical secondary and tertiary amines are
  named by adding the prefix di- or tri- to the
  alkyl group

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IUPAC nomenclature of complex amines.Amines with
multiple different alkyl groups.

• Named as N-substituted primary amines
• Largest alkyl group is the parent name, and other
  alkyl groups are considered N-substituents

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Common Names

• Alkylamines do not have common names unless they
  are biological molecules, such as putrycine
  (1,6-hexanediamine)
• or cadaverine (1,7-heptanediamine)
• Simple arylamines have common names

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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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Quaternary Ammonium Ions

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

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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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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
• 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

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Amines as Acids

• Loss of the NH proton requires a very strong base

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Synthesis of Amines
SN2 Reactions of Alkyl Halides

• Ammonia and other amines are good nucleophiles

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Uncontrolled Multiple Alkylation are Unavoidable
when Sterically Un-hindered Amines React
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• Reduction of nitriles and amides

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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

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Selective Preparation of Primary Aminesthe
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

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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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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 (Mechanism) Part I

• RCONH2 reacts with Br2 and base to give electron
• deficient nitrogen

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Hofmann Rearrangement (Mechanism) Part II
Alkyl group (-R) migrates to the neighboring
electron-deficient nitrogen. Hydration of the
resultant isocyanate gives carboxamic acid.
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Hofmann Rearrangement (Mechanism) Part III
Deprotonation-reprotonation produces a
protonated ammonium zwitterion (a good leaving
group). Its elimination produces the amine and
carbon dioxide by-product.
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Curtius Rearrangement

• Heating an acyl azide prepared from substitution
  an acid chloride
• This rearrangement also involves migration of R
  from CO to the neighboring electron-deficient
  nitrogen with simultaneous loss of a leaving group

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Reactions of Amines

• Acylation leads to amides (1o, 2o, or 3o.)

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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

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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