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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5574_Библиотеки_им_академика_М_И_Перельмана.pdf
HC
+
+
+
HC
HC
HC
CH
CH
Red hot tube
Benzene
1500–2000°C
CH
2
CH
2
CH
3
CH
2
HC
2
CH
3
Cyclohexane
331
aromatization:
/Al23 at high pressure and high
23
9.5 PROPERTIES OF BENZENE
9.5.1 Physical Properties of Benzene
odour. (aroma
Learning Plus
Prior to the 1920s, benzene was frequently used
as an industrial solvent, especially for degreasing
metal. As its toxicity became obvious, other solvents replaced benzene in application that directly
exposed the user to benzene.
magnitude of van der Waals forces of attraction.
sp2
9.5.2 Chemical Properties (Reactions of Benzene)
1. substitution reactions:
332
Benzene Nitrobenzene
+ HONO
2
+ H2O
H2SO
4
333 K
NO
2
Nitrobenzene m-Dinitrobenzene
+ HONO
2
+ H2O
H2SO
4
333 K
NO
2
NO
2
NO
2
Benzene Benzene sulphonic acid
+ HOSO
3
H + H2O
8 h
353 K
SO3H
Benzene
Oleum
Benzene disulphonic acid
+ H
2S2O7
+ H2O
SO3H
SO
3
H
a. Nitration: 3 in the presence
3 + conc..H2 is called as the
2 group at a higher temperature.
Sulphonation:
for 8 h.
m
Benzene
FeCl
3
Chlorobenzene
+ Cl
2
+ HCl
Cl
o-Dichlorobenzene p-DichlorobenzeneChlorobenzene
+ 2Cl
2
2
+ 2HCl+
Cl
Cl
Cl Cl
Cl
Benzene Toluene
AlCl
3
Chloromethane
+ CH
3
Cl + HCl
CH
3
Ethylbenzene
AlCl
3
Bromoethane
+ C
2H5
Br + HBr
C2H
5
c. Halogenation: 33 reacts
333
d. Friedel–Crafts reaction: 3), reacts
334
Acetophenone
AlCl
3
+ CH3COCl + HCl
COCH
3
C6H6 + 15 [O]
6CO2 + 3H2O; ∆H
− 650 kJ
C6H6 + 9/2 O
2
+ 2CO2 + H2O
Maleic anhydride
HC
HC CO
CO
O
V2O
5
773 K
Cyclohexane
Ni
200°C
+ 3H
2
3acylation of benzene.
e. Oxidation reactions:
H
2
25
Learning Plus
Benzopyrene (a polycyclic aromatic
compound) produced by incomplete
oxidation of organic compounds in
tobacco, is found in cigarette smoke.
2 and
f. addition reactions:
Learning Plus
In bacteria, dioxygenase enzyme can add an
oxygen molecule to the ring, and the unstable
product is immediately reduced (by NADH) to a
cyclic diol with two double bonds.
Sunlight
Benzene Hexachlorobenzene
+ 3Cl
2
Cl
Cl
Cl
Cl
Cl
Cl
O
O
O
O
O
O
O
O
O
HC
HC
HC
HC
CH
CH
2H
Glyoxal
Diphenyl
+ H
2
Red hot tube
2
2
335
g. Polymerization
9.5.3 Physiological Effects of Benzene
of the stomach, dizziness, sleepiness, convulsions and death.
Benzene is also carcinogenic in nature. Benzene damages
depress the immune system, increasing the chance of infection.
Learning Plus
The most widely produced derivatives of benzene
are styrene, which is used to make polymers
and plastics phenol for resins and adhesives (via
cumene), and cyclohexane, which is used in nylon
manufacturing. Smaller amounts of benzene are
used to make some types of rubbers, lubricants,
dyes, detergents, drugs, explosives and pesticides.
336
O
2
CO
2
+
H
2
O
Burn
CO + HCl C6H5CHO + HCl
AlCl
3
(Gattermann Koch
aldehyde synthesis)
O
2
H
2
C6H12 (Cyclohexane)
Ni or Pt
V2O5, 773 K
(Maleic anhydride)
Cl
2
Cl
2
Br
2
HNO
3
H2SO
4
CH3COCl
CH
3
Cl
C
6H6
C6H
6
or
FeCl
3
FeBr
3
Conc. H2SO
4
AlCl
3
AlCl
3
Sunlight
C6H5Cl + HCl
C
6H6
Br + HBr
C
6H5NO2
+ H2O
H2O + C6H5SO3H (Benzene sulphonic acid)
HCl + C6H5COCH3 (Acetophenone)
HCl + C
6H5CH3
(Toluene)
This reaction is called Friedel–Crafts acylation.
This reaction is called Friedel–Crafts alkylation.
C
6H6Cl6
(Benzene hexachloride or BHC)
HC CO
HC CO
O
MEMORY FOCUS
Kekule structure of benzene
C
H
H
C
Or,
HC
HC
HC HC
9.6 STRUCTURE OF BENZENE
9.6.1 Kekule Ring Structure of Benzene
6H6
it is an unsaturated compound. But it does not undergo the usual
Learning Plus
Kekule was the first to deduce the ring structure of
benzene; after years of studying carbon bonding,
benzene and related molecules, the solution to
the benzene structure came to him in a dream of
a snake eating its own tail. Upon waking, he was
inspired to deduce the ring structure of benzene.
attached to one hydrogen atom.
337
Limitations of the ring structure
o
compounds easily.
Å and for
338
Double bond between the
two substituents
Single bond between the
two substituents
X
X
X
X
CC CC
H H
+ Heat of hydrogenation
Catalyst
+ H
But only one o
9.6.2 Resonance Structure of Benzene
Facts in support of resonance structure of benzene
1. carbon–carbon bond length:
Å
Å Å as in
2. stability:
heat of hydrogenation.
-1
-1 of heat is evolved.
-1
+ H
2
; ∆H = −119.5 kJ
Cyclohexene
Catalyst
+ 3H
2
Cyclohexane
; ∆H = −358.5 kJ
Kekule structure
Catalyst
; ∆H = −208 kJ
Benzene Cyclohexane
+ 3H
2
Catalyst
H
HH
H
H
Orbital structure of benzene
H
1.397 A
1.09 A
120°
CC
C
C
C
C
339
9.6.3 Molecular Orbital Structure of Benzene
9.7 AROMATIC ELECTROPHILIC SUBSTITUTION (CHARACTERISTIC AROMATIC REACTION)
340
C6H6 + Y
+
C6H
5
Y
+ Z
–
.
.
⊕
H
(Slow; rate determining)
C6H
5
Y
⊕
H
(Fast)Z
C
6H5
Y + H
H
Step (i) Step (ii)
Intermediate
carbonium ion
Substituted
product
C6H6 + Y
+
C6H5Y + H
+
C6H
5
H
⊕
Y
9.7.1 Electronic Mechanism of Electrophilic Mono-Substitution in Benzene Ring
either as a electrophile (y
+
-) molecule.
same rate as protium H.
+, to the
form the products. Step (i) is thus the rate-determining step.