Transcript Chapter 24. Amines

Spectroscopy of Amines - IR
 Characteristic N–H 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 C–C bond nearest the
nitrogen to yield an alkyl radical and a nitrogencontaining 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
 N–H hydrogens appear as broad signals, either fully
coupled to neighboring C–H 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 δ 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
C–N–C 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 acid–base 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 N–H 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 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
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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
O
O
R
N
O
R
H
OHR
H
N
N
H
+ H2O
H
+ Br -
Br
+ H2O
O
H
Br2
R
N
Br
O
O
R
N
H
OHR
N
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Br
Hofmann Rearrangement (Mechanism) Part II
Alkyl group (-R) migrates to the neighboring electron-deficient
nitrogen. Hydration of the resultant isocyanate gives
carboxamic acid.
O
O
R
N
R
Br
O
R
R
N
N
H2O
R
N
+ Br -
N
C
O
C
(isocyanate)
O
R
N
H
O
(carboxamic acid)
C
OH
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Hofmann Rearrangement (Mechanism) Part III
Deprotonation-reprotonation produces a protonated
ammonium zwitterion (a good leaving group). It’s
elimination produces the amine and carbon dioxide by-product.
O
R
N
N
OH
R
O
O
O
R
H
H
N
H2O
C
O
O
R
N
C
H
O
O
R
H
O
H2O
+
C
H
H
C
H
O
R
C
H
N
OH-
H
N
+
C
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O
Curtius Rearrangement
 Heating an acyl azide prepared from substitution an acid
chloride
 This rearrangement also involves migration of R from C=O
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
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