395 Amines
Hence, the lone pair of electrons on nitrogen is less available for
donation to benzene ring by resonance. Therefore, activating
effect of –NHCOCH
3
group is less than that of amino group.
(b) Nitration: Direct nitration of aniline yields tarry oxidation
products in addition to the nitro derivatives. Moreover, in the
strongly acidic medium, aniline is protonated to form the
anilinium ion which is meta directing. That is why besides the
ortho and para derivatives, significant amount of meta derivative
is also formed.
However, by protecting the –NH
2
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.
(c) Sulphonation: Aniline reacts with concentrated sulphuric acid
to form anilinium hydrogensulphate which on heating with
sulphuric acid at 453-473K produces p-aminobenzene sulphonic
acid, commonly known as sulphanilic acid, as the major product.
Aniline does not undergo Friedel-Crafts reaction (alkylation and
acetylation) due to salt formation with aluminium chloride, the
Lewis acid, which is used as a catalyst. Due to this, nitrogen of
aniline acquires positive charge and hence acts as a strong
deactivating group for further reaction.
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396Chemistry
Intext QuestionsIntext Questions
Intext QuestionsIntext Questions
Intext Questions
13.4 Arrange the following in increasing order of their basic strength:
(i) C
2
H
5
NH
2
, C
6
H
5
NH
2
, NH
3
, C
6
H
5
CH
2
NH
2
and (C
2
H
5
)
2
NH
(ii) C
2
H
5
NH
2
, (C
2
H
5
)
2
NH, (C
2
H
5
)
3
N, C
6
H
5
NH
2
(iii) CH
3
NH
2
, (CH
3
)
2
NH, (CH
3
)
3
N, C
6
H
5
NH
2
, C
6
H
5
CH
2
NH
2
.
13.5 Complete the following acid-base reactions and name the products:
(i) CH
3
CH
2
CH
2
NH
2
+ HCl → (ii) (C
2
H
5
)
3
N + HCl →
13.6 Write reactions of the final alkylation product of aniline with excess of methyl
iodide in the presence of sodium carbonate solution.
13.7 Write chemical reaction of aniline with benzoyl chloride and write the name of
the product obtained.
13.8 Write structures of different isomers corresponding to the molecular formula,
C
3
H
9
N. Write IUPAC names of the isomers which will liberate nitrogen gas on
treatment with nitrous acid.
II. DIAZONIUM SALTS
The diazonium salts have the general formula
–
2
R N X
+
where R stands
for an aryl group and
–
X
ion may be Cl
–
Br,
–
4
HSO
−
,
BF
−
, etc. They are
named by suffixing diazonium to the name of the parent hydrocarbon
from which they are formed, followed by the name of anion such as
chloride, hydrogensulphate, etc. The
2
N
+
group is called diazonium
group. For example,
–
2
6 5
C H N C l
+
is named as benzenediazonium
chloride and C
6
H
5
N
2
+
HSO
4
–
is known as benzenediazonium
hydrogensulphate.
Primary aliphatic amines form highly unstable alkyldiazonium salts
(refer to Section 13.6). Primary aromatic amines form arenediazonium
salts which are stable for a short time in solution at low temperatures
(273-278 K). The stability of arenediazonium ion is explained on the
basis of resonance.
Benzenediazonium chloride is prepared by the reaction of aniline with
nitrous acid at 273-278K. Nitrous acid is produced in the reaction
mixture by the reaction of sodium nitrite with hydrochloric acid. The
conversion of primary aromatic amines into diazonium salts is known
as diazotisation. Due to its instability, the diazonium salt is not
generally stored and is used immediately after its preparation.
−
+
+ + → + +
2
273 2 78K
6 5 2
–
6 5 22
C H NaNO 2H C H NaCl Cl ClH N 2HN
O
13.713.7
13.713.7
13.7
Method ofMethod of
Method ofMethod of
Method of
PreparationPreparation
PreparationPreparation
Preparation
of Diazoniunof Diazoniun
of Diazoniunof Diazoniun
of Diazoniun
SaltsSalts
SaltsSalts
Salts
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397 Amines
Benzenediazonium chloride is a colourless crystalline solid. It is readily
soluble in water and is stable in cold but reacts with water when
warmed. It decomposes easily in the dry state. Benzenediazonium
fluoroborate is water insoluble and stable at room temperature.
The reactions of diazonium salts can be broadly divided into two
categories, namely (A) reactions involving displacement of nitrogen and
(B) reactions involving retention of diazo group.
A. Reactions involving displacement of nitrogen
Diazonium group being a very good leaving group, is substituted
by other groups such as Cl
–
, Br
–
,
I
–
,
CN
–
and OH
–
which displace
nitrogen from the aromatic ring. The nitrogen formed escapes from
the reaction mixture as a gas.
1. Replacement by halide or cyanide ion: The Cl
–
, Br
–
and CN
–
nucleophiles can easily be introduced in the benzene ring in the
presence of Cu(I) ion. This reaction is called Sandmeyer reaction.
Alternatively, chlorine or bromine can also be introduced in the
benzene ring by treating the diazonium salt solution with corresponding
halogen acid in the presence of copper powder. This is referred as
Gatterman reaction.
The yield in Sandmeyer reaction is found to be better than
Gattermann reaction.
2. Replacement by iodide ion: Iodine is not easily introduced into
the benzene ring directly, but, when the diazonium salt solution
is treated with potassium iodide, iodobenzene is formed.
3. Replacement by fluoride ion: When arenediazonium chloride is
treated with fluoroboric acid, arene diazonium fluoroborate is
precipitated which on heating decomposes to yield aryl fluoride.
4. Replacement by H: Certain mild reducing agents like
hypophosphorous acid (phosphinic acid) or ethanol reduce
diazonium salts to arenes and themselves get oxidised to
phosphorous acid and ethanal, respectively.
13.813.8
13.813.8
13.8
PhysicalPhysical
PhysicalPhysical
Physical
PropertiesProperties
PropertiesProperties
Properties
13.913.9
13.913.9
13.9
ChemicalChemical
ChemicalChemical
Chemical
ReactionsReactions
ReactionsReactions
Reactions
2 2
22
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