Amines constitute an important class of organic
compounds derived by replacing one or more hydrogen
atoms of ammonia molecule by alkyl/aryl group(s). In
nature, they occur among proteins, vitamins, alkaloids
and hormones. Synthetic examples include polymers,
dye stuffs and drugs. Two biologically active
compounds, namely adrenaline and ephedrine, both
containing secondary amino group, are used to increase
blood pressure. Novocain, a synthetic amino compound,
is used as an anaesthetic in dentistry. Benadryl, a well
known antihistaminic drug also contains tertiary amino
group. Quaternary ammonium salts are used as
surfactants. Diazonium salts are intermediates in the
preparation of a variety of aromatic compounds
including dyes. In this Unit, you will learn about amines
and diazonium salts.
I. AMINES
Amines can be considered as derivatives of ammonia,
obtained by replacement of one, two or all the three
hydrogen atoms by alkyl and/or aryl groups.
For example:
Like ammonia, nitrogen atom of amines is trivalent and
carries an unshared pair of electrons. Nitrogen orbitals
in amines are therefore, sp
3
hybridised and the geometry
of amines is pyramidal. Each of the three sp
3
hybridised
orbitals of nitrogen overlap with orbitals of hydrogen or
carbon depending upon the composition of the amines.
The fourth orbital of nitrogen in all amines contains an
unshared pair of electrons. Due to the presence of
unshared pair of electrons, the angle C–N–E, (where E is
After studying this Unit, you will be
able to
describe amines as derivatives of
ammonia having a pyramidal
structure;
classify amines as primary,
secondary and tertiary;
name amines by common names
and IUPAC system;
describe some of the important
methods of preparation of amines;
explain the properties of amines;
distinguish between primary,
secondary and tertiary amines;
describe the method of prepara-
tion of diazonium salts and their
importance in the synthesis of a
series of aromatic compounds
including azo dyes.
Objectives
“The chief commercial use of amines is as intermediates in the
synthesis of medicines and fibres” .
Unit
Unit
Unit
Unit
Unit
13
A
A
A
A
A
minesmines
minesmines
mines
13
AA
AA
A
minesmines
minesmines
mines
13.1 Structure of Amines13.1 Structure of Amines
13.1 Structure of Amines13.1 Structure of Amines
13.1 Structure of Amines
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C or H) is less than 109.5°; for instance, it is 108
o
in case of
trimethylamine as shown in Fig. 13.1.
Amines are classified as primary (1
o
), secondary (2
o
) and tertiary (3
o
)
depending upon the number of hydrogen atoms replaced by alkyl or
aryl groups in ammonia molecule. If one hydrogen atom of ammonia
is replaced by R or Ar , we get RNH
2
or ArNH
2
, a primary amine (1
o
).
If two hydrogen atoms of ammonia or one hydrogen atom of R-NH
2
are
replaced by another alkyl/aryl(R) group, what would you get? You
get R-NHR’, secondary amine. The second alkyl/aryl group may be
same or different. Replacement of another hydrogen atom by alkyl/aryl
group leads to the formation of tertiary amine. Amines are said to be
‘simple’ when all the alkyl or aryl groups are the same, and ‘mixed’
when they are different.
In common system, an aliphatic amine is named by prefixing alkyl
group to amine, i.e., alkylamine as one word (e.g., methylamine). In
secondary and tertiary amines, when two or more groups are the same,
the prefix di or tri is appended before the name of alkyl group. In
IUPAC system, primary amines are named as alkanamines. The name
is derived by replacement of ‘e’ of alkane by the word amine. For
example, CH
3
NH
2
is named as methanamine. In case, more than one
amino group is present at different positions in the parent chain, their
positions are specified by giving numbers to the carbon atoms bearing
–NH
2
groups and suitable prefix such as di, tri, etc. is attached to the
amine. The letter ‘e’ of the suffix of the hydrocarbon part is retained. For
example, H
2
N–CH
2
–CH
2
–NH
2
is named as ethane-1, 2-diamine.
To name secondary and tertiary amines, we use locant N to designate
substituent attached to a nitrogen atom. For example, CH
3
NHCH
2
CH
3
is
13.213.2
13.213.2
13.2
ClassificationClassification
ClassificationClassification
Classification
Fig. 13.1 Pyramidal shape of trimethylamine
13.313.3
13.313.3
13.3
NomenclatureNomenclature
NomenclatureNomenclature
Nomenclature
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Amines
named as N-methylethanamine and (CH
3
CH
2
)
3
N is named as N, N-
diethylethanamine. More examples are given in Table 13.1.
In arylamines, –NH
2
group is directly attached to the benzene ring.
C
6
H
5
NH
2
is the simplest example of arylamine. In common system, it
is known as aniline. It is also an accepted IUPAC name. While naming
arylamines according to IUPAC system, suffix ‘e’ of arene is replaced by
‘amine’. Thus in IUPAC system, C
6
H
5
–NH
2
is named as benzenamine.
Common and IUPAC names of some alkylamines and arylamines are
given in Table 13.1.
Amine Common name IUPAC name
CH
3-
–CH
2
NH
2
Ethylamine Ethanamine
CH
3
–CH
2
–CH
2
NH
2
n-Propylamine Propan-1-amine
Isopropylamine Propan-2-amine
Ethylmethylamine N-Methylethanamine
Trimethylamine N,N-Dimethylmethanamine
N,N-Diethylbutylamine N,N-Diethylbutan-1-amine
Allylamine Prop-2-en-1-amine
Hexamethylenediamine Hexane-1,6-diamine
Aniline Aniline or Benzenamine
o-Toluidine 2-Methylaniline
p-Bromoaniline 4-Bromobenzenamine
or
4-Bromoaniline
N,N-Dimethylaniline N,N-Dimethylbenzenamine
Table 13.1: Nomenclature of Some Alkylamines and Arylamines
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392Chemistry
Amines are prepared by the following methods:
1. Reduction of nitro compounds
Nitro compounds are reduced to amines by passing hydrogen gas
in the presence of finely divided nickel, palladium or platinum and
also by reduction with metals in acidic medium. Nitroalkanes can
also be similarly reduced to the corresponding alkanamines.
Reduction with iron scrap and hydrochloric acid is preferred because
FeCl
2
formed gets hydrolysed to release hydrochloric acid during the
reaction. Thus, only a small amount of hydrochloric acid is required
to initiate the reaction.
2. Ammonolysis of alkyl halides
You have read (Unit 10, Class XII) that the carbon - halogen bond
in alkyl or benzyl halides can be easily cleaved by a nucleophile.
Hence, an alkyl or benzyl halide on reaction with an ethanolic
solution of ammonia undergoes nucleophilic substitution reaction
in which the halogen atom is replaced by an amino (–NH
2
) group.
This process of cleavage of the C–X bond by ammonia molecule is
known as ammonolysis. The reaction is carried out in a sealed
tube at 373 K. The primary amine thus obtained behaves as a
nucleophile and can further react with alkyl halide to form secondary
and tertiary amines, and finally quaternary ammonium salt.
13.413.4
13.413.4
13.4
PreparationPreparation
PreparationPreparation
Preparation
of Amines
of Amines
of Aminesof Amines
of Amines
13.1 Classify the following amines as primary, secondary or tertiary:
13.2 (i) Write structures of different isomeric amines corresponding to the molecular
formula, C
4
H
11
N.
(ii) Write IUPAC names of all the isomers.
(iii) What type of isomerism is exhibited by different pairs of amines?
Intext QuestionsIntext Questions
Intext QuestionsIntext Questions
Intext Questions
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Amines
The free amine can be obtained from the ammonium salt by treatment
with a strong base:
Ammonolysis has the disadvantage of yielding a mixture of primary,
secondary and tertiary amines and also a quaternary ammonium salt.
However, primary amine is obtained as a major product by taking
large excess of ammonia.
The order of reactivity of halides with amines is RI > RBr >RCl.
3. Reduction of nitriles
Nitriles on reduction with lithium aluminium hydride (LiAlH
4
) or
catalytic hydrogenation produce primary amines. This reaction is
used for ascent of amine series, i.e., for preparation of amines
containing one carbon atom more than the starting amine.
4. Reduction of amides
The amides on reduction with lithium aluminium hydride yield
amines.
Example 13.1Example 13.1
Example 13.1Example 13.1
Example 13.1
SolutionSolution
SolutionSolution
Solution
Write chemical equations for the following reactions:
(i) Reaction of ethanolic NH
3
with C
2
H
5
Cl.
(ii) Ammonolysis of benzyl chloride and reaction of amine so formed
with two moles of CH
3
Cl.
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