AMINES
AMINES
Amines are derivatives of ammonia, in which one or more of the hydrogen atoms
have been replaced by alkyl groups or aryl groups. This gives three classes of amine:
RNH2
R2NH
R3N
primary
secondary
tertiary
Hydrogen bonding gives amines higher boiling points that alkanes of similar M r.
However, the hydrogen bonding is not as strong as that in alcohols, and therefore an
amine has a lower boiling point than the corresponding alcohol.
All classes of amines possess a lone pair of electrons on nitrogen and can therefore
act as electron pair donors. Consequently they can function as bases, nucleophiles
or ligands (see module CHM5).
Base Properties
The lone pair of electrons on nitrogen allows ammonia and amines to act as
Bronsted-Lowry bases, i.e. as proton acceptors. Ammonia and amines are weak
bases and dissolve in water to give alkaline solutions:
RNH3+ + OH-
RNH2 + H2O
The strength of a nitrogenous base depends on the ease with which it can accept a
proton and therefore on the electron density on the nitrogen atom: any structural
feature which increases the electron density will increase the base strength.
Primary aliphatic amines are stronger bases than ammonia because of the +I
inductive effect (electron-donating) of the alkyl group. This increases the electron
density on nitrogen.
H
CH3
N
H
pKb
NH3 4.75
CH3NH2 3.40
(N.B. The lower the pKb value, the stronger the base is.)
Primary aromatic amines are much weaker bases than ammonia, because the lone
pair on nitrogen interacts with the -orbital of the aromatic ring and is delocalised.
This greatly reduces the electron density on nitrogen.
-
..
NH2
+
NH2
+
NH2
pKb
TOPIC 13.8: AMINES 1
NH3 4.75
C6H5NH2 9.38
Nucleophilic Properties
The lone pair of electrons on nitrogen allows ammonia and amines to act as
nucleophiles, i.e. electron pair donors.
1. Reaction with haloalkanes
Haloalkanes react with ammonia or amines and undergo nucleophilic substitution
(see module CHM3). The reactants are heated together in a sealed tube. The
reaction proceeds in a series of mechanistically identical steps, forming finally a
quaternary ammonium salt.
NH3 + RBr
RNH3+ BrA proton is then removed by a second mole of ammonia:
RNH3+ Br- + NH3
RNH2 + NH4Br
RNH2 + RBr
R2NH2+ Br- + NH3
R2NH + RBr
R3NH+ Br- + NH3
R3N + RBr
primary amine
R2NH2+ BrR2NH + NH4Br
secondary
amine
R3NH+ BrR3N + NH4Br
tertiary amine
quaternary
ammonium
salt
R4N+ Br-
The product of the reaction is usually a mixture whose composition depends on the
initial ammonia:haloalkane mole ratio.
If a large excess of haloalkane is used, a high yield of the quaternary ammonium salt
can be obtained.
Quaternary ammonium salts containing two long-chain alkyl groups are used as
cationic surfactants in fabric conditioners.
For example:
[CH3(CH2)17]2N+(CH3)2 ClIf a large excess of ammonia is used, a reasonable yield of the primary amine can be
obtained.
This reaction is not a generally useful synthetic method because of the need to
separate products. These products could, however, be separated using
chromatography (see section on analytical techniques).
Mechanism:
H
H
C
H3N:
Br
+
H3N
-
C
:Br
H
H
H
The other three steps are mechanistically identical.
TOPIC 13.8: AMINES 2
H
Preparation of Amines
Primary Aromatic Amines
Primary aromatic amines are usually made by the reduction of nitro compounds.
A common method of reduction in the laboratory is to heat the nitro compound with a
mixture of granulated tin and concentrated hydrochloric acid. In industry, iron, which
is cheaper, is used instead of tin. Since the reaction mixture is acidic, the amine is
present in the protonated form, ArNH3+. The free amine is liberated by the addition of
an excess of sodium hydroxide and is then obtained by steam distillation.
ArNO2 + 6[H]
ArNH2 + 2H2O
Alternatively, the nitro compound can be reduced to the amine by catalytic
hydrogenation, using a nickel catalyst at about 150oC.
ArNO2 + 3H2
ArNH2 + 2H2O
Primary Aliphatic Amines
Primary aliphatic amines can be obtained by the reduction of a nitrile. The nitrile is
first obtained by the reaction of a haloalkane with potassium cyanide in ethanol. This
is a nucleophilic substitution reaction in which the nucleophile is the cyanide ion, CN -.
CH3CH2Br + KCN
CH3CH2CN + KBr
CH3CH2CN + 4[H]
CH3CH2CH2NH2
The nitrile can be reduced to the amine by catalytic hydrogenation, using a nickel
catalyst at about 150oC. Alternatively, lithium tetrahydridoaluminate(III), LiAlH4, can
be used in scrupulously dry ethoxyethane.
TOPIC 13.8: AMINES 3