Benzene
and
Electrophilic Aromatic
Substitution
1
1
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Aromatic Hydrocarbons (Arenes)
• Simplest member : Benzene, C6H6
• Characteristic aroma
• Alkylbenzene, Cn+6H2n+6
E.g.
CH3
C6H5CH3
2
2
C6H5C2H5
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Aromatic Hydrocarbons (Arenes)
• Generally less dense than water
• Insoluble in water but soluble in many
organic solvents
3
3
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Aromatic Hydrocarbons (Arenes)
• PAHs : Polycyclic aromatic hydrocarbons
• Fused aromatic rings
4
4
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Aromatic Hydrocarbons (Arenes)
• No heteroatoms
N
B
5
5
Not PAH
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Aromatic Hydrocarbons (Arenes)
• PAHs : Polycyclic aromatic hydrocarbons
Sources : - incomplete combustion of
wood, coal, diesel, fat, or tobacco
Q.38
6
6
benzocyclobutadiene
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PAHs : Polyaromatic hydrocarbons
Toxic and carcinogenic
Benzo(a)pyrene : - first carcinogen discovered
Found in : - tobacco smoke,
- char-grilled food
- burnt toast,
- edible oils
7
7
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Preparation of
Benzene
8
8
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1. Destructive Distillation of Coal
9
9
•
Gives coal gas, ammoniacal liquor,
coal tar and coke as products
•
The coal tar produced is a mixture
of many organic compounds (mainly
aromatic ones)
•
benzene and methylbenzene can be
obtained by fractional distillation
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1. Destructive Distillation of Coal
10
10
A laboratory
set-up of the
destructive
distillation of
coal
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Industrial Preparation
2. Catalytic trimerization of ethyne
organonickel cat alyst
3 HC
11
11
CH
70oC, under pressure
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Industrial Preparation
3. Catalytic Reforming of Petroleum
•
Converts alkanes and cycloalkanes
into aromatic hydrocarbons
Pt
C6H14  C6H6 + 4H2
500 oC, 10 – 20 atm
12
12
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Laboratory Synthesis
1. Decarboxylation of Sodium Salt of Benzoic Acid
•
When sodium benzoate is fused with
sodium hydroxide
 the carboxylate group is removed
13
13
volatile
Benzene is separated by fractional distillation
New Way Chemistry for Hong Kong A-Level Book 3A
2. Reduction of Phenol
•
Passing phenol vapour over heated zinc
dust (reducing agent)
 produce benzene and zinc(II) oxide
volatile
14
14
Benzene is separated by fractional distillation
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Reactions of
Benzene
15
15
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Reactivity of Benzene
Unreactive towards addition reactions due to
stabilization of the system by delocalization
of -electrons
16
16
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Reactivity of Benzene
Not oxidized by KMnO4
17
17
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Reactivity of Benzene
Br2 / HBr / H2O
Resistant to electrophilic
addition
18
18
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Reactivity of Benzene
Resistant to catalytic
hydrogenation
19
19
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Addition reactions occur only under drastic
conditions.
excess H2, high T &P
Ni or P t or P d
20
20
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Occur only
under drastic
conditions
No
reaction
21
21
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The -electron cloud is susceptible to
electrophilic attack.
Substitution is preferred to addition since
the former retains aromaticity.
Electrophilic aromatic substitution (SE)
22
22
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23
23
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Electrophilic Aromatic Substitution
Reactions
E+ : electrophile
24
24
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General Reaction Mechanism
Step 1:
•
25
25
Rate determining step
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General Reaction Mechanism
•
Stabilized by delocalization of 
electrons
+
26
26
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General Reaction Mechanism
Step 2:
•
The carbocation loses a hydrogen ion
 forms the substitution product
27
27
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1. Halogenation
No
apparent
reation
Catalysts : AlCl3, FeCl3 or FeBr3
28
28
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1. Halogenation - Mechanism
Step 1:
•
The catalyst (FeBr3) combines with
bromine to give a complex
Br
29
29
Br
FeBr3
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Br [FeBr4]
Step 2:
•
Formation of carbocation intermediate
•
Rate determining step
+
30
30
Br [FeBr]
Br
Br
Br
H
H
H
Stabilized by delocalization of -electrons
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Step 3:
•
The catalyst (FeBr3) is regenerated
Fumes of HBr(g) are produced, indicating
substitution rather than addition has occurred.
31
31
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2. Nitration
32
32
•
Benzene reacts readily with a mixture
of conc. HNO3 and conc. H2SO4
•
Conc. H2SO4 increases the rate of
reaction by increasing the concentration
of the electrophile, NO2+
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O
H2SO 4
acid
+
HO
H
N
base
O
O
O
N
H
O
N
O
O
+
H2O
electrophile
2HNO3
33
33
NO3 + H2O + NO2+
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NO2
NO2
conc. HNO3
conc. H2SO 4, 95oC
NO2
NO2
NO2
conc. HNO3
conc. H2SO 4, 110oC
NO2
Optional
34
34
O2N
NO2
meta-directing
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Q.39
H2SO4 + HNO3
NO2+ + H2O + HSO4
O
O
35
35
N
O
O
N
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O
O
N
36
O
O
N
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O
O
N
O
O
HSO4
NO2
N
H
+ H2SO4
37
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3. Sulphonation
•
(H2SO4 + SO3)
Benzene reacts with fuming sulphuric
acid at room temp
 form benzenesulphonic acid
+
38
38
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3. Sulphonation
•
Sulphonation is a reversible process
•
By heating an aqueous solution of
benzenesulphonic acid to above 100 oC
 benzene and sulphuric acid are formed
heat
39
39
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Q.40
H2SO4 + H2SO4
SO3 + H3O+ + HSO4
O
O
O
S
O
40
40
O
S
O
H
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O
O
41
S
O
O
O
H
O
S
O
H
O
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S
O
H
HSO4
O
O
S
O
H
SO3H
SO3
H+
+ H2SO4
42
New Way Chemistry for Hong Kong A-Level Book 3A
4. Friedel-Crafts Alkylation
•
When benzene is warmed with a
haloalkane in the presence of AlCl3 as a
catalyst
 alkylbenzene is formed
43
43
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4. Friedel-Crafts Alkylation
Important starting step in the manufacture
of styrene, phenol and detergents
The alkyl group introduced activates the ring
towards further alkylation by +ve I-effect.
 Friedel-Crafts alkylation is not a good way
to prepare alkylbenzene.
44
44
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deactivating
R
R
O
CH2
C
O
C
+
Cl
Hg/Zn
R
AlCl3
Friedel-Craft
acylation
45
45
conc. HCl
Clemensen
reduction
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Devise a reaction pathway of not more than
four steps for the following conversion.
CH3
COOH
6B 09/07/2011
46
46
New Way Chemistry for Hong Kong A-Level Book 3A
47
47
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Chlorination
Bromination or Friedel-Craft reaction
Soda lime to absorb
HBr or HCl fumes
Br2, RCl or RCOCl
or AlCl3
48
48
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Q.41
electrophile
CH3Cl + AlCl3  [CH3]+[AlCl4]
49
49
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AlCl4
H
CH3
H
C
H
H
t wo more structures
CH3
+ HCl + AlCl3
50
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Q.42(a)
2C6H6 + X
AlCl3
C6H5CH2C6H5 + 2HCl
X : CH2Cl2
51
51
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CH2Cl
CH2Cl2
CH2
AlCl3
AlCl3
(excess)
-CH2Cl deactivate the ring
Susceptibility to SE reaction :
52
52
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CH2Cl
>
Q.42(b)
3C6H6 + Y
AlCl3
(C6H5)3CH + 3HCl
Y : CHCl3
53
53
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CHCl2
CHCl3
CHCl
AlCl3
AlCl3
(excess)
AlCl3
C
54
54
H
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Q.42(c)
2C6H6 + Z
AlCl3
C6H5CH2CH2C6H5 + 2HCl
Z : CH2ClCH2Cl
55
55
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ClH2C
ClH2C
CH2Cl
AlCl3
AlCl3
(excess)
56
56
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Industrial preparation of toluene
CH3
1.
Fract ional
Coal t ar
dist illat ion
CH3
2.
57
57
P et roleum
Fract ional
dist illat ion
CH3
catalyst
high T & P
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+
3H2
Laboratory Preparation of Methylbenzene
1. Friedel-Crafts acylation of benzene
followed by Clemensen reduction.
2.Friedel-Crafts alkylation of benzene is not
recommended as further alkylation will
take place giving a mixture of
alkylbenzenes that are difficult to be
separated.
58
58
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Reactions of methylbenzene and Other
Alkylbenzenes
1. Electrophilic aromatic substitution rxs
more susceptible to electrophilic attacks
∵ alkyl groups activate the ring by positive
inductive effect.
59
59
New Way Chemistry for Hong Kong A-Level Book 3A
2. Reactions of the side chain
(a)
Free radical substitution vs
electrophilic substitution at the ring
Free radical
substitution
Electrophilic
substitution
60
60
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Benzylic hydrogens are much more susceptible
to free radical substitution than hydrogens at
other positions.
Major
products
61
61
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(b)
Oxidation at the benzylic carbon
Absence of
benzylic H
62
62
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If alkaline KMnO4 is used, the resulting
mixture must be acidified in order to obtain
the benzoic acid.
COO-
CH3
KMnO 4 / OH-
COOH
H+
heat
Or,
CH3
COOH
1. KMnO 4 / OH-, heat
2. H+
63
63
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Structural determination of isomeric alkylbenzene
H3C
CH3
CH3
CH2CH3
alkylbenzenes
with very close
boiling points
CH3
b.p. 144oC
b.p. 139oC
CH3
b.p. 138oC
b.p. 136oC
CH3
COOH
COOH
C8H10
COOH
COOH
acids with
different
melting points
COOH
COOH
o
m.p. 231C
o
m.p. 348C
m.p. 300oC
m.p. 122oC
COOH
64
64
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Structural determination of isomeric alkylbenzene
H3C
CH3
CH3
CH2CH3
alkylbenzenes
with very close
boiling points
CH3
b.p. 144oC
b.p. 139oC
CH3
b.p. 138oC
b.p. 136oC
CH3
COOH
COOH
COOH
COOH
COOH
COOH
o
m.p. 231C
o
m.p. 348C
acids with
different
melting points
m.p. 300oC
m.p. 122oC
COOH
65
65
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Q.43(a)
O
COOH
1. MnO4- / OH+
2. H+
66
66
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C
HO
CH3
Q.43(b)
Terminal alkene gives CO2
COOH
1. MnO4- / OH+
2. H+
COOH
COOH
1. MnO4- / OH2. H+
COOH
COOH
1. MnO4- / OH2. H+
67
67
HOOC
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+ H2O
CO2
Q.43(b)
1.
MnO4-
-
/ OH
2. H+
68
68
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COOH
Q.43(b)
COOH
1. MnO4- / OH2. H+
COOH
1. MnO4- / OH2. H+
69
69
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COOH
COOH
Q.43(b)
1.
MnO 4-
COOH
-
/ OH
2. H+
1.
MnO4-
-
/ OH
2. H+
70
70
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COOH
Q.43(c)
1. MnO4- / OH+
2. H
71
71
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O
+
CO2
+ H2O
The END
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29.2 Nomenclature of the Derivatives of Benzene (SB p.191)
Back
Draw the structural formula for each of the following
compounds:
(a) 1,3,5-Trichlorobenzene
(b) 2,5-Dibromophenol
(c) 2,4-Dinitrobenzoic acid
(a)
73
73
(b)
Answer
(c)
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29.2 Nomenclature of the Derivatives of Benzene (SB p.192)
Give the IUPAC name for each of the following
compounds:
(a)
(b)
74
74
(a) 1,2-Dimethylbenzene
(b) 1-Methyl-2-nitrobenzene or 2nitrotoluene
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Answer
29.2 Nomenclature of the Derivatives of Benzene (SB p.192)
Back
Give the IUPAC name for each of the following
compounds:
(c)
(c) 3-Bromo-5-chlorobenzoic acid
(d) 4-Bromo-2,6-dinitrophenol
(d)
75
75
Answer
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29.3 Structure of Benzene and Aromaticity (SB p.195)
The basic structural requirement for aromatic
compounds is that the molecule must be planar,
cyclic and with (4n + 2)  electrons delocalized in
the ring. n must be a natural number (i.e. n = 1, 2, 3, and
so on).
There are aromatic compounds without benzene ring.
An example is the 1,3-cyclopentadienyl anion. Can you
draw its structure and explain its aromaticity?
Answer
76
76
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29.3 Structure of Benzene and Aromaticity (SB p.195)
Back
Cyclopentadienyl anion is an aromatic anion. It has six π
electrons delocalized over a completely conjugated planar
monocyclic system of five sp2 hybridized carbon atoms.
Cyclopentadienyl anion
77
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29.4 Physical Properties of Aromatic Hydrocarbons (SB p.197)
PAHs are formed from partial combustion and pyrolysis
of aromatic compounds. They are in common occurrence
in our environment. List some important uses of
aromatic hydrocarbons and how they release PAHs to
our environment.
Answer
78
78
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29.4 Physical Properties of Aromatic Hydrocarbons (SB p.197)
Aromatic hydrocarbons are the raw materials for the manufacture of
monomers and plasticizers in polymers, commonly used as solvents
and important constituents of lead-free gasoline.
Incomplete combustion and pyrolysis process favour the production
of PAHs. These compounds are encountered abundantly in the
atmosphere, soil and elsewhere in the environment from sources
that include engine exhaust, wood stove smoke, cigarette smoke
and charbroiled food. Coal tar and petroleum residues such as road
and roofing asphalt have high levels of PAHs.
Back
79
79
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29.6 Reactions of Benzene (SB p.203)
Back
Complete each of the following by supplying the missing
reactant or product as indicated by the question mark:
(a)
(a)
(b)
(b) conc. H2SO4, conc. HNO3
(c) fuming H2SO4
(c)
Answer
80
80
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29.6 Reactions of Benzene (SB p.203)
(a) One mole of benzene reacts with three moles of
chlorine under special conditions. What are the
conditions required for the reaction?
Answer
(a) UV radiation or diffuse sunlight must
be present for the free radical addition
reaction to take place.
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29.6 Reactions of Benzene (SB p.203)
(b) Draw the structure of the reaction product in (a).
Answer
(b)
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82
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29.6 Reactions of Benzene (SB p.203)
(c) Methylbenzene undergoes two different types of
chlorination reaction by different mechanisms.
Compare the two different types of chlorination
reaction in terms of reaction conditions as well as the
products formed.
Answer
83
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29.6 Reactions of Benzene (SB p.203)
Back
(c) The two different types of chlorination reaction of methylbenzene
are:
Type I: free radical substitution reaction
Type II: electrophilic aromatic substitution reaction
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Comparison of some reactions of cyclohexane,
cyclohexene and methylbenzene
Reaction
Cyclohexane (a
saturated
alicyclic
hydrocarbon)
Cyclohexene (an
unsaturated
alicyclic
hydrocarbon)
Methylbenzene
(an aromatic
hydrocarbon)
Action of
bromine in
1,1,1trichloroethane (in
dark)
No reaction
Bromine is
decolourized and no
hydrogen bromide is
evolved
No reaction with
bromine alone. In
the presence of
iron(III) bromide,
bromine is
decolourized and
hydrogen
bromide fumes
are evolved
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Comparison of some reactions of cyclohexane,
cyclohexene and methylbenzene
Reaction Cyclohexane Cyclohexene (an Methylbenzen
(a saturated
unsaturated
e (an aromatic
alicyclic
alicyclic
hydrocarbon)
hydrocarbon)
hydrocarbon)
Action of
hydrogen
(with
nickel
catalyst)
86
86
No reaction
One mole of
cyclohexene
reacts with one
mole of hydrogen
at room
temperature
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One mole of
methylbenzene
reacts with
three moles of
hydrogen at
high
temperature
Comparison of some reactions of cyclohexane,
cyclohexene and methylbenzene
Reaction Cyclohexane Cyclohexene (an Methylbenzen
(a saturated
unsaturated
e (an aromatic
alicyclic
alicyclic
hydrocarbon)
hydrocarbon)
hydrocarbon)
Action of
acidified
potassium
manganat
e(VII)
87
87
No reaction
Acidified
potassium
manganate(VII)
solution is
decolourized
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No reaction
Comparison of some reactions of cyclohexane,
cyclohexene and methylbenzene
Reaction Cyclohexane Cyclohexene (an Methylbenzen
(a saturated
unsaturated
e (an aromatic
alicyclic
alicyclic
hydrocarbon)
hydrocarbon)
hydrocarbon)
Action of
No reaction Cyclohexene is
A yellow liquid
concentrat
oxidized and the
is formed
ed nitric
colour darkens
acid and
concentrat
ed
sulphuric
acid
88
88
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