Important to B.Sc. students

1. 1

3. Unit-IV
A) AROMATIC NITRO
COMPOUNDS:
By
Dr. P. R. Padole
Department of Chemistry
Shri Shivaji Science College, Amravati.

4. Contents
A) Aromatic nitro compounds:
Nitrobenzene:1
Synthesis from benzene2
Reduction of nitrobenzene3
in acidic, neutral and
alkaline (Basic) medium
Electrolytic Reduction
of Nitrobenzene:
4

5. Introduction:
 The nitro group (-NO2) is the most highly oxygenated
form of the nitrogen atom.
 The nitro group can be introduced in the benzene ring
very easily by electrophilic substitution but it is rather
difficult to replace a hydrogen atom in an aliphatic
compound by a nitro group.
 Hence the aromatic nitro compounds are much more
readily available than their aliphatic counterparts.
5
Ar N
O
O
Ar N
O
O
N
O
O
N
O
O
I II
N
O
O
Classical Polar
Resonance Form Resonance Hybrid
1
2
Resonating structure of -NO2
I II
1
2

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7. 7

8. 8
Azinitro form
Acidic Nature

9. Do you know?
Aromatic Nitro Compounds:

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11. Aromatic Nitro Compounds:
Nitro Arenes:
Aromatic nitro compounds are the substitution
products of aromatic hydrocarbons in which one
or more ring hydrogen atoms have been replaced
by –NO2 group.
 {Note that: The nuclear nitro compounds are by far more important
because they can be conveniently prepared from the aromatic
hydrocarbons by direct nitration. They have been used as dyes,
pharmaceuticals and use as explosives is too well-known.}
NO2
- H
+ NO2
Benezene Nitrobenzene
(Nuclear nitro compound)
H

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Aromatic Nitro Compounds:
 In modern view, aromatic nitro compounds are molecules
in which –NO2 group is directly bonded to an aromatic
ring.
 They have general formula Ar-NO2. The -NO2 group may
be represented as…..
Ar N
O
O
Ar N
O
O
N
O
O
N
O
O
I II
N
O
O
Classical Polar
Resonance Form Resonance Hybrid
1
2
Resonating structure of -NO2
I II
1
2
It has been shown by bond length and dipole measurements that both N–O bonds are equivalent.
Therefore, the structure of nitro group is as one in which resonance gives it symmetry.

13. Laboratory preparation of Nitrobenzene:
Preparation or Synthesis of
Nitrobenzene from Benzene
(Nitration):
Q.1) How will you prepare Nitrobenzene from Benzene? (W-16, 2 Mark)
Q.2) How will you obtain Nitrobenzene from Benzene?
(S-17 & W-19, 2 Mark)
Q.3) What is nitration?
Q.4) What is nitrating mixture? How benzene is nitrated with nitrating
mixture? (S-18, 4 Mark)
Q.5) Complete the following reaction: (S-19, 2 Mark)
 Conc. HNO3
Conc. H2SO4
?

14. Nitration of Benzene:
 Defination: Replacement of hydrogen atom of benzene ring by
nitro (-NO2) group is called nitration.
 When benzene is reacted with concentrated
nitric acid (con. HNO3) in presence of
concentrated sulphuric acid (nitrating
mixture) at 60o C (333 K); to form
nitrobenzene.
14
con.HNO3 + Con.H2SO4)Mixture of ( is known as nitrating mixture.
H
HO-NO2
con. HNO3
con. H2SO4
NO2
H2 O
Benzene
+
or
Heat at 60o
C
or at 333K
+
Nitrobenzene
HO---NO2

15. Formation of nitronium ion (NO2)+ :
 Formation of electrophile, i.e., NO2
+ ion:
 The active electrophile, nitronium ion, NO2
+ is
formed from nitrating mixture as follows.
15
Q.1) Explain the formation of nitronium ion (NO2)+ in the nitrating mixture. (W-12, 3 Mark)
Q.2) The compound or mixture of compounds which is used as a source of electrophilic ,
NO2
+ is referred as nitrating agents
The electrophile in the reaction is NO2
+, why not H3O+?
Though Oxygen atom in H3O+ has a positive charge it is not
an electrophilic centre, because the atom has a complete
octet in its outermost shell hence cannot accommodate any
more electrons.

16. Formation of Nitrobenzene:
16

17. Do you know?
Reduction of Nitrobenzene :

18. Addition of hydrogen or removal of oxygen is termed
as reduction.
18
Conversion of nitro (-NO2) group to amino group (-NH2)
is termed as reduction.

19. Reduction of Nitrobenzene under
Different Conditions:
Electrolytic
Electrolytic reduction of nitrobenzene:
(a) Reduction by dil. H2SO4 (Weakly acidic medium)
Preparation of Aniline from Nitrobenzene:
(b) Reduction by con. H2SO4 (Strongly acidic medium)
Preparation of p-Amino Phenol from Nitrobenzene:
Hydrazobenzene
Reduction of nitrobenzene in Alkaline Medium (Basic Medium):
Or Preparation of Hydrazobenzene from Nitrobenzene:
Reducing Agents: Zn dust & NaOH
Phenyl hydroxyl amine
Reduction of nitrobenzene in Neutral Medium:
Or
Preparation of Phenyl hydroxyl amine from Nitrobenzene:
Reducing Agents: Zn dust and NH4Cl
Aniline
Reduction of nitrobenzene in Acidic Medium:
Or
Preparation of Aniline from Nitrobenzene:
Reducing Agents: Zn/HCl, Fe/HCl, ZnCl2/HCl, Sn/HCl
Aniline
p-Amino Phenol

20. (i) Reduction of nitrobenzene in
Acidic Medium:
Or Preparation of Aniline from Nitrobenzene:
 Reduction of nitrobenzene in Acidic Medium:
Or Metal-Acid Reduction:
 Reducing Agents: Zn/HCl, Fe/HCl, ZnCl2/HCl, Sn/HCl
Ans: When nitrobenzene is reduced by metal like Zn, Fe, Sn in
presence of HCl acid (i.e., in acidic medium); to form aniline.
20
NO2
Nitrobenzene
+ 6 [H] Zn or Fe or Sn in HCl
or Zn / HCl
NH2
Aniline
Acidic medium
+ 2 H2O
or Fe / HCl

21. (i) Reduction of nitrobenzene in
Acidic Medium:
21
NO2
Nitrobenzene
+ 6 [H] Zn or Fe or Sn in HCl
or Zn / HCl
NH2
Aniline
Acidic medium
+ 2 H2O
or Fe / HCl

22. LOGO(ii) Reduction of nitrobenzene in Neutral Medium:
Preparation of Phenyl hydroxyl amine
from Nitrobenzene:
Reduction of nitrobenzene in Neutral Medium:
 Reducing Agents: Zn dust and boiling aq. ammonium chloride solution (NH4Cl).
Ans: When nitrobenzene is reduced by Zn dust and ammonium chloride solution
(i.e., in neutral medium); to form N-phenyl hydroxyl-amine or
Phenyl hydroxyl-amine.
Q.1) How will you convert Nitrobenzene to phenylhydroxyl amine? (W-14, 2 Mark)
Q.2) Complete the following reaction: (S-18, 2 Mark)
Q.3) Discuss the reduction of nitrobenzene in neutral medium. (W-18 & S-19, 2 Mark)
NO2
Nitrobenzene
+ 4 [H]
Zn-dust & NH4Cl solution NH-OH
Phenyl hydroxyl amineNeutral medium
+ H2Owarm
NO2
Zn/NH4Cl
2 [H] ?

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Q.1) Discuss the reduction of nitrobenzene to hydrazobenzene in alkaline medium. (S-12 & W-14, 2 Mark)
Q.2) Complete the following reaction. (S-14, 2 Mark)
Q.3) Complete the following reaction. (S-14, W-15 & W-16, 2 Mark)
Q.4) What are products of reduction of nitrobenzene in alkaline medium? (W-15, 2 Mark)
Q.5) Complete the following reaction. (W-17, 2 Mark)
Q.6) What is the product of reduction of nitrobenzene in basic medium? (Ans: hydrazobenzene)
(iii) Reduction of nitrobenzene in Alkaline
Medium (Basic Medium):
Or Preparation of Hydrazobenzene from
Nitrobenzene:
NO2
Zn / NaOH / CH3OH2 ?
NO2
Zn / NaOH
2 ?
NO2
Zn dust
?+ 6 [H]
NaOH/ H2O

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Reduction of nitrobenzene in Alkaline Medium:
i.e., Basic medium
Reducing Agents:
Zn dust & NaOH, SnCl2 & NaOH, aq. Sodium Arsenite. (Na3AsO3)
Ans:
When nitrobenzene is reduced straight way by
Zn dust and NaOH solution (i.e., in alkaline medium /
Basic medium); to form Hydrazobenzene.
NO2
Nitrobenzene
+ 6 [H] Zn-dust & NaOH solution
HN
Hydrazobenzene
Alkaline medium + 4 H2O+
O2N
Nitrobenzene
NH
NO2
6 [H]+
Zn dust
& NaOH / H2O
2 NH
hydrazobenzene
NH
C2H5OH
or
OR

25. Do you know?
Electrolytic reduction:

26. Electrolytic reduction of nitrobenzene:
a) Reduction
by dil. H2SO4
(Weakly acidic
medium)
Preparation of
Aniline from
Nitrobenzene:
Electrolytic
reduction
(b) Reduction by
con. H2SO4
(Strongly acidic
medium)
Preparation of
Phenyl hydroxyl
amine &
p-Amino Phenol :

27. (a) Reduction by dil. H2SO4 (Weakly acidic medium)
Preparation of Aniline from Nitrobenzene:
 Reduction of nitrobenzene in weakly Acidic Medium (dil. H2SO4):
 Reducing Agents: dil. H2SO4
 When nitrobenzene is reduced by dil. H2SO4
(i.e., in weakly acidic medium); to form aniline.
27
Q.1) Explain: Electrolytic reduction of nitrobenzene. (S-12, 2 Mark)
Q.2) How will you convert Nitrobenzene to Aniline? (S-13, 2 Mark)
Q.3) Main product in the electrolytic reduction of nitrobenzene in weakly acidic medium is:(W-13, ½ Mark)
(a) Hydrazobenzene (b) Aniline (c) Azobenzene (d) p-amino phenol
NO2
Nitrobenzene
+ 6 [H]
NH2
Aniline
Weakly Acidic medium
+ 2 H2O
dil. H2SO4
Electolytic Reduction

28. (b) Reduction by con. H2SO4 (Strongly acidic medium)
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Preparation of Phenyl hydroxyl amine & p-Amino Phenol
from Nitrobenzene:
NO2
?
Conc acid
H2SO4
Rearrangement
?
Reduction of nitrobenzene in strongly Acidic Medium (con. H2SO4):
Reducing Agents: con. H2SO4
When nitrobenzene is reduced by con. H2SO4 (i.e., in strongly acidic
medium); to form p-Amino-phenol.
Nitrobenzene
NHOHNO2
NH2
OH
Strong Acid
conc. H2SO4
Phenyl hydroxyl amine p-Amino phenol
Rearrengment
Electrolytic
Reduction
H

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30. THE END!