Chapter 10: Amines
Sections 10.1-10.8
Chemistry 2060, Spring 2060, LSU
10-1
Sections
Chapter 10: Amines
1.
2.
3.
4.
5.
6.
7.
8.
Introduction
Structure and classification
Nomenclature
Physical Properties
Basicity of amines
Reaction with acids
Synthesis of arylamines: Reduction of –NO2 group
Reactions of primary amines with nitrous acid
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10-2
Structure & Classification
Classification
• 1°, 2°, or , 3° amines: amines in which 1, 2, or 3
hydrogens of NH3 are replaced by alkyl or aryl groups
• 4° amines: ions in which nitrogen is bonded to four
carbons and bears a positive charge
CH 3
CH 3 - NH 2
CH 3 - N
CH 3
Methylamine
(a 1° amine)
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Trimethylamine
(a 3° amine)
CH 3
Br
+
CH 3 - N- CH 3
CH 3
Tetramethylammonium
bromide
(a 4° ammonium salt)
10-3
Structure & Classification
Amines are further divided into aliphatic, aromatic,
and heterocyclic amines
• aliphatic amine: an amine in which nitrogen is bonded
only to alkyl groups
• aromatic amine: an amine in which nitrogen is bonded
to one or more aryl groups
CH3
NH2
N-H
CH3
CH2 -N-CH3
A niline
N-Methylaniline Benzyldimethylamine
(a 1° aromatic (a 2° aromatic
(a 3° aliphatic amine)
amine)
amin e)
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10-4
Structure & Classification
• heterocyclic amine: an amine in which nitrogen is one
of the atoms of a ring
N
N
H
H
Piperidine Pyrrolidine
(heterocyclic
aliphatic amines)
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N
N
H
Pyrrole Pyridine
(heterocyclic
aromatic amines )
10-5
Structure & Classification
Example: classify each amino group by type
H
(a)
H
N
H
(S)-(+)-Coniine
H3 C
N
N
(b)
N
CH3
(S)-(-)-N icotin e
O
( c)
OCH3
O
Cocain e
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O
10-6
Nomenclature
Aliphatic amines: replace the suffix -e of the parent
alkane by -amine
NH2
NH2
2-Propan amin e
C6 H5
(S)-1-Ph enyleth anamine
NH2
H2 N
1,6-Hexanediamine
The IUPAC system retains the common name
aniline
NH2
NH2
NH2
NH2
CH3
OCH3
NO2
An iline 4-N itroanilin e
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2-Meth ylan iline 3-Methoxyan iline
(o-Toluidin e)
(m-Anisid ine) 10-7
Nomenclature
Among the various functional groups discussed in
the text, -NH2 has one of the lowest priorities
COOH
H2 N
OH
NH2
2-Aminoethan ol
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2-Amin ob enzoic acid
(A nthranilic acid )
10-8
Nomenclature
Common names for most aliphatic amines are
derived by listing the alkyl groups bonded to
nitrogen in one word ending with the suffix amine
H
CH3 N H2
Methylamine
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N H2
tert- Butylamine
N
Dicyclopentylamine
N
Triethylamine
10-9
Physical Properties
Amines are polar compounds, and both 1° and 2°
amines form intermolecular hydrogen bonds
• N-H- - - -N hydrogen bonds are weaker than O-H- - - -O
hydrogen bonds because the difference in
electronegativity between N and H (3.0 - 2.1 = 0.9) is
less than that between O and H (3.5 - 2.1 = 1.4)
CH3 CH3 CH3 NH2
molecu lar w eight (g/mol)
30.1
boiling point (°C) -88.6
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31.1
-6.3
CH3 OH
32.0
65.0
10-10
Basicity
All amines are weak bases, and aqueous solutions
of amines are basic
H
CH3 -N:
H
+ H-O-H
Meth ylamine
H
+
CH3 -N-H
:O-H
H
Methylammonium
hyd roxide
+
Kb = Keq [H 2 O] =
Chemistry 2060, Spring 2060, LSU
-
[CH 3 NH3 ][OH ]
= 4.37 x 10-4
[CH 3 NH2 ]
10-11
Basicity
it is also common to discuss the basicity of amines
by reference to the acid ionization constant of the
corresponding conjugate acid
CH3 NH3
+
+
+ H2 O
CH3 NH2 + H3 O
+
Ka =
[ CH3 NH2 ][ H3 O ]
+
[CH3 NH3 ]
= 2.29 x 10-1 1
pK a = 10.64
• for any acid-conjugate base pair
pKa + pKb = 14.00
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10-12
Basicity
Using values of pKa, we can predict the position of
equilibrium in acid-base reactions
CH3 NH2 + CH3 COOH
p Ka 4.76
(stron ger
base)
(stron ger
acid)
+
-
CH3 NH3
+ CH3 COO
pKa 10.64
(w eak er
acid)
(w eaker
bas e)
• acetic acid is the stronger acid and, therefore, the
position of this equilibrium lies to the right
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10-13
Basicity
• aliphatic amines have about the same base strength,
pKb 3.0 - 4.0, and are slightly stronger bases than
ammonia
Amine
Ammon ia
Primary Amines
methylamine
ethylamin e
cyclohexylamin e
Secon dary Amines
dimethylamin e
dieth ylamine
Tertiary Amines
trimethylamine
triethylamine
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Structu re
pK b
pK a
NH3
4.74
CH3 NH2
CH3 CH2 NH2
3.36 10.64
3.19 10.81
C6 H1 1 NH2
3.34 10.66
( CH3 ) 2 NH
( CH3 CH2 ) 2 NH
3.27 10.73
3.02 10.98
( CH3 ) 3 N
( CH3 CH2 ) 3 N
4.19 9.81
3.25 10.75
9.26
10-14
Basicity
• aromatic amines are considerably weaker bases than
aliphatic amines
pKb 3.34
NH2 + H2 O
A niline
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-
OH
Cyclohexylammon ium
hydroxid e
Cyclohexylamine
NH2 + H2 O
NH3
+
pKb 9.37
NH3
+
-
OH
An ilinium hydroxide
10-15
Basicity
• aromatic amines are weaker bases than aliphatic
amines because the resonance stabilization of the free
base is lost on protonation
H
N H
H + H
N
H + H
N
H + H
N
Interaction of th e electron pair on
nitrogen w ith the p i system of the
aromatic rin g
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10-16
Basicity
Electron-withdrawing groups, such as halogen,
nitro, and carbonyl, decrease the basicity of
aromatic amines by decreasing the availability of
the electron pair on nitrogen
NH2
A niline
(pK b 9.37)
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O2 N
NH2
4-N itroanilin e
(pK b 15.0)
10-17
Basicity
Example: select the stronger base in each pair of
amines
O
or
(a)
N
H
(B)
N
(A)
NH2
CH3 or
(b)
( C)
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CH2 NH2
(D)
10-18
Guanidine
Guanidine, pKb 0.4, is the strongest base among
neutral organic compounds
NH
NH2
pKb 0.4
H2 N-C-NH2 + H2 O
Guanidine
+
H2 N-C-NH2
+ OH
Guanidinium ion
-
• its basicity is due to the delocalization of the positive
charge over the three nitrogen atoms
+
NH2
H2 N
C
NH2
+
H2 N
NH2
C
NH2
H2 N
NH2
+
C NH2
Three equivalent con trib uting structures
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10-19
Reaction with Acids
All amines, whether soluble or insoluble in water,
react quantitatively with strong acids to form
water-soluble salts
OH
OH
HO
HO
NH2
HO
(R)-N orep ineph rin e
(only slightly
soluble in w ater)
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+ HCl
H2 O
+
-
NH3 Cl
HO
(R)-N orepin ephrine
h yd roch loride
(a w ater-soluble s alt)
10-20
Synthesis of 1° Arylamines
Most are synthesized by nitration of an aromatic
ring followed by catalytic reduction of the nitro
group to a primary amine
COOH
COOH
+ 3 H2
NO2
3-N itroben zoic
acid
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Ni
(3 atm)
+ 2 H2 O
NH2
3-Aminobenzoic
acid
10-21
Reaction with Nitrous Acid
Nitrous acid is a weak acid, most commonly
prepared by treating an aqueous solution of
NaNO2 with H2SO4 or HCl
HNO2 + H2 O
H3 O+ + NO2
-
Ka = 4.26 x 10-4
pKa = 3.37
• nitrous acid reacts with amines of different types,
depending on whether the amine is 1°, 2°, 3° and
whether it is aliphatic or aromatic
• we concentrate on the reaction of nitrous acid with 1°
aromatic amines because this reaction is useful in
organic synthesis
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10-22
Reaction with Nitrous Acid
Treating a 1° aromatic amine with HNO2 gives a
diazonium salt
NH2 + NaNO2 + HCl
Anilin e
(a 1° aromatic
amin e)
Sodium
n itrite
+
N N Cl + H2 O
H2 O
0°C
Ben zenediazoniu m
chloride
• warming the aqueous solution of the diazonium salt
gives a phenol and N2 gas
+
N N Cl + H2 O
Ben zenediazoniu m
chloride
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OH + N2
Ph enol
10-23
Reaction with Nitrous Acid
Example: show how to bring about each step in this
conversion
CH3
CH3
(1)
(2)
NO2
Toluene
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COOH
COOH
(3)
NO2
COOH
(4)
NH2
OH
4-Hydroxyben zoic acid
10-24