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Benzene
and
Electrophilic Aromatic
Substitution
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New Way Chemistry for Hong Kong A-Level Book 3A
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
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Aromatic Hydrocarbons (Arenes)
• PAHs : Polyaromatic hydrocarbons
• Fused aromatic rings
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Aromatic Hydrocarbons (Arenes)
• No heteroatoms
N
B
Not PAH
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Aromatic Hydrocarbons (Arenes)
• PAHs : Polyaromatic hydrocarbons
From incomplete combustion of
wood, coal, diesel, fat, or tobacco
Q.38
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benzocyclobuta
diene
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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
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Preparation of
Benzene
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1. Destructive Distillation of Coal
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•
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
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
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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
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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
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2. Reduction of Phenol
•
Passing phenol vapour over heated zinc
dust
 produce benzene and zinc(II) oxide
Benzene is separated by fractional distillation
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Reactions of
Benzene
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Reactivity of Benzene
Unreactive towards addition reactions due to
stabilization of the system by delocalization
of -electrons
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Reactivity of Benzene
Not oxidized by KMnO4
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Reactivity of Benzene
Br2 / HBr / H2O
Resistant to electrophilic
addition
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Reactivity of Benzene
Resistant to catalytic
hydrogenation
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Addition reactions occur only under drastic
conditions.
excess H2, high T &P
Ni or P t or P d
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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)
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Electrophilic Aromatic Substitution
Reactions
where E+ denotes an electrophile
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General Reaction Mechanism
Step 1:
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•
Benzene reacts with the electrophile
•
A carbocation intermediate is formed
•
Rate determining step
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General Reaction Mechanism
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•
The carbocation formed has a positive
charge on the carbon atom of benzene
•
Stabilized by delocalization of  electrons
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General Reaction Mechanism
Step 2:
•
The carbocation loses a hydrogen ion
 forms the substitution product
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1. Halogenation
•
Benzene does not react with Cl2 and Br2 in
1,1,1-trichloroethane
•
When catalysts (e.g. AlCl3, FeCl3 or FeBr3)
are present
 benzene react readily with Cl2 and Br2
 form chlorobenzene and bromobenzene
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1. Halogenation
No apparent reaction
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1. Halogenation
fast
slow
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1. Halogenation - Mechanism
Step 1:
•
The catalyst (FeBr3) combines with
bromine to give a complex
Br
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Br
FeBr3
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Br [FeBr4]
Step 2:
•
Formation of carbocation intermediate
•
Rate determining step
+
Br [FeBr]
Br
Br
Br
H
H
H
Stabilized by delocalization of -electrons
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Step 3:
•
The loss of a proton from the
carbocation intermediate
•
Forms the bromination product
•
The catalyst (FeBr3) is regenerated
Fumes of HBr(g) are produced, indicating
substitution rather than addition has occurred.
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2. Nitration
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•
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
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NO3 + H2O + NO2+
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NO2
NO2
conc. HNO3
conc. H2SO 4, 95oC
NO2
NO2
NO2
conc. HNO3
conc. H2SO 4, 110oC
O2N
NO2
Optional
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NO2
3. Sulphonation
•
(H2SO4 + SO3)
Benzene reacts with fuming sulphuric
acid at room temp
 form benzenesulphonic acid
+
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4. Sulphonation
•
Sulphonation is a reversible process
•
By heating an aqueous solution of
benzenesulphonic acid to above 100 oC
 benzene and sulphuric acid are formed
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5. Friedel-Crafts Alkylation
•
When benzene is warmed with a
haloalkane in the presence of AlCl3 as a
catalyst
 alkylbenzene is formed
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5. Friedel-Crafts Alkylation
•
Important starting step in the
manufacture of styrene, phenol and
detergents
Example 29-6
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Check Point 29-6
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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)
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(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)
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(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)
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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
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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
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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
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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
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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
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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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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
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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 Cyclohexene (an Methylbenzen
(a saturated
unsaturated
e (an aromatic
alicyclic
alicyclic
hydrocarbon)
hydrocarbon)
hydrocarbon)
Action of
No reaction Bromine is
No reaction
bromine in
decolourized and with bromine
1,1,1no hydrogen
alone. In the
trichlorobromide is
presence of
ethane (in
evolved
iron(III) bromide,
dark)
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
No reaction One mole of
One mole of
hydrogen
cyclohexene
methylbenzene
(with
reacts with one
reacts with
nickel
mole of hydrogen three moles of
catalyst)
at room
hydrogen at
temperature
high
temperature
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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
No reaction Acidified
No reaction
acidified
potassium
potassium
manganate(VII)
manganat
solution is
e(VII)
decolourized
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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
No reaction Cyclohexene is
A yellow liquid
concentrat
oxidized and the
is formed
ed nitric
colour darkens
acid and
concentrat
ed
sulphuric
acid
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