The document discusses the physical and chemical properties of amines. It describes how amines have higher boiling points than compounds of similar molar mass due to their ability to form hydrogen bonds between nitrogen atoms and hydrogen atoms of other molecules. However, their boiling points are lower than alcohols and carboxylic acids which form stronger hydrogen bonds. The basicity of amines increases with increasing alkyl substituents due to an inductive effect. However, the order of basicity in aqueous solution is altered by hydration and steric effects. Aromatic amines are less basic than ammonia due to resonance delocalization of the nitrogen lone pair.
2. Physical properties of amines
1. Physical state
The lower aliphatic amines are gases with fishy odour. Primary amines
with three or more carbon atoms are liquid and still higher ones are
solid.
3. Physical properties of amines
2. Boiling Points :
Primary and secondary amines are capable of forming
intermolecular hydrogen bonds between N atom of one molecule and H-
atom of the other due to the presence of polar N—H bonds.
Amines have higher boiling points than the non-polar compounds of
comparable molar masses. However, amines have lower boiling points
than those of corresponding alcohols or carboxylic acids. This is due to
the reason that O—H bond is more polar than N—H bond and therefore,
intermolecular hydrogen bonds in alcohols and carboxylic acids are
relatively more stronger than those in amines.
4. Physical properties of amines
2. Boiling Points :
Primary and secondary amines are capable of forming
intermolecular hydrogen bonds between N atom of one molecule and H-
atom of the other due to the presence of polar N—H bonds.
Amines have higher boiling points than the non-polar compounds of
comparable molar masses. However, amines have lower boiling points
than those of corresponding alcohols or carboxylic acids. This is due to
the reason that O—H bond is more polar than N—H bond and therefore,
intermolecular hydrogen bonds in alcohols and carboxylic acids are
relatively more stronger than those in amines.
5. (CH3)3N
CH3CH2CH2NH2
propylamine
Trimethylamine
Physical properties of amines
CH3
I
CH2CH2NH
Ethyl methylamine
3. Boiling point in isomeric amines
Among isomeric amines, the primary amines have higher boiling points
than corresponding seconding amines.
Tertiary amines do not have any N-H bond in their molecules.
Therefore, they are unable to form H-bonds and consequently, have
lower boiling point
6. 4. Solubility :
Aliphatic amines of lower molar mass are soluble in
water. With increase in molar mass, solubility in water decreases
Physical properties of amines
7. Chemical reactions
Amines acts as lewis bases due to the presence of lone pair of
electrons on nitrogen. They are stronger bases than water and are,
therefore, protonated by water in their aqueous solutions.
RNH2 + H2O
RNH3 + OH
+ -
⇌
Basic character of amines
8. Kb = 3
[RNH + ] [OH]-
[RNH2]
The constant Kb is called basicity constant.
Larger the value of Kb or smaller the value of pKb, more is
the basic strength of amine,
Basic character of amines
9. 1. Alkyl amines versus ammonia
a. Alkyl amines are stronger bases than ammonia :
Explained in terms of electron releasing inductive effect (+I
effect) of alkyl group.
Structure basicity relationship of amines
10. b. Basic strength in primary secondary and tertiary alkyl amines
Structure basicity relationship of amines
R
H
H
.. ..
N N
R
R
H
N
..
R
R
R
+I increases
11. The order of basicity in aqueous solution does not follow the expected
trend and gets altered
The actual order of basic strength among 1o, 2o, 3o methyl amines and
ethyl amines as follows
Order of basicity in aqueous solution
12. Structure basicity relationship of amines
(CH3)2NH
20
CH3NH2
1o
(CH3) N
3p
(C2H5)2NH
2o
(C2H5)3N
3o
C2H5 NH2
1o
Basic character decreases
15. Structure basicity relationship of amines
Why order of basic strength gets altered in aqueous medium ?
1. Hydration effect
Protonated cation of 1o amine
(maximum hydration)
Protonated cation of 3o amine
(Least hydration)
Protonated cation of 2o amine
(Less hydration)
Hydration due to
H-bonding
-decreases
16. Structure basicity relationship of amines
2. Steric factors
Refers to the crowding of alkyl groups around N atom which causes
hindrance and cause obstruction to H-bonding of protonated amine.
19. Aromatic amines such as aniline are less basic than ammonia
The less basic character of aniline can be explained as follows
Delocalisation of lone pair of nitrogen on the ring
Structure basicity relationship of amines
20. Aromatic amines such as aniline are less basic than ammonia
The less basic character of aniline can be explained as follows
Delocalisation of lone pair of nitrogen on the ring
Structure basicity relationship of amines
21. In general, the electron-repelling groups (ERG) such as -OCH3, -
CH3, -NH2, etc. increase the basic strength. The electron
withdrawing groups (EWG) such as -NO2, -CN, halo (-X), etc,
decrease the
basic strength.
Effect of substituents on the basic character of aromatic bases
22. 1. Arrange the following sets in order of their basic strengths
a. ETNH2, NH3, triethylamine
b. Aniline, p-nitroaniline, p-toluidine
23. 1. Arrange the following sets in order of their basic strengths
a. ETNH2, NH3, triethylamine
b. Aniline, p-nitroaniline, p-toluidine
26. The products obtained by acylation reaction are known as amides. The
reaction is carried out in the presence of a base stronger than the
amine, like pyridine, which removes HCl so formed and shifts the
equilibrium to the right hand side
C2H5-N-C-CH3 + H - Cl
I II
H O
Base
N-ethylethanamide
Reaction with acid chlorides and acid anhydrides
CH3
.. I
C2H5-N-H C-Cl
II
O
27. ..
C2H5-N- C-CH3 + H - Cl
I II
C2H5 O
II
O
Base
H
I
C2H5-N + CH3-C -Cl
I
C2H5
N-Ethylethanamine
N,N- Diethylethanamide
CH3 - C - O - C - CH3
..
C6H5-N - H
I
H
Benzenamine
Ethanoic anhydride
..
C6H5-N- C-CH3 + CH3COOH
I II
C2H5 O
N-phenylethanamide or
acetanilide
Reaction with acid chlorides and acid anhydrides
II
O
31. Main problem with aromatic amines is that of their very high
reactivity. Substitution tends to occur at ortho- and para- positions
32. To get a monobromo compound, the amino group is acetylated before
bromination
2
3
NH2
O
II
H-N-C-CH3
NH2
1
O
II
H-N-C-CH3
Aniline N-phenylethanamide
(acetanilide)
Major 4-bromoaniline
33. To get a monobromo compound, the amino group is acetylated before
bromination
2
3
NH2
O
II
H-N-C-CH3
NH2
1
O
II
H-N-C-CH3
Aniline N-phenylethanamide
(acetanilide)
Major 4-bromoaniline
34. Purpose of acylation
In acyl derivative of aniline, the electron pair of nitrogen is withdrawn
towards carbonyl group as is shown by the following
Resonating structures
35. Purpose of acylation
As a result, the activation power of -NH2 group is reduced. This
method is called protection of -NH2 group by acylation. It is often
used to control the rate of substitution and to prevent the formation
of di and tri substituted products.
37. However, by protecting the -NH2 group by acetylation reaction with
acetic anhydride, the nitration reaction can be controlled and the p-
nitro derivative can be obtained as the major product.
NH2
NH NH2
(CH3CO)2O
Pyridine
NH2
HNO3, H2SO4, 288K
OH- or H+
NO2
p-nitroacetanilidine
NO2
p-nitroaniline
acetanilide
38. Sulphonation:
Aniline does not undergo friedel-Crafts reaction, due to salt formation
with aluminium chloride, the lewis acid which is used as a catalyst
..
NH2
+ -
NH3HSO4 NH2
H2SO4
+
NH3
453-473 K
SO H
3
Sulphanilic acid
SO3
Zwitter ion
Anilinium
Hydrogensulphate
-
Reaction with acid chlorides and acid anhydrides
