Chapter 31
Aromatic Hydrocarbons
31.1 Introduction
31.2 Nomenclature of the Derivatives of Benzene
31.3 The Stability of Benzene
31.4 Physical Properties of Aromatic Hydrocarbons
31.5 Preparation of Benzene
31.6 Reactions of Benzene
31.7 Alkylbenzenes
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New Way Chemistry for Hong Kong A-Level Book 3A
31.1 Introduction (SB p.147)
Benzene
• Highly unsaturated
• Six-membered ring compound with alternative single
and double bonds between adjacent carbon atoms
• Chemically unreactive compared to alkenes
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New Way Chemistry for Hong Kong A-Level Book 3A
31.2 Nomenclature of the Derivatives of Benzene (SB p.148)
1. Monosubstituted benzenes
(a) For certain compounds, benzene is the parent name
and the substituent is simply indicated by a prefix
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New Way Chemistry for Hong Kong A-Level Book 3A
31.2 Nomenclature of the Derivatives of Benzene (SB p.148)
(b) For other compounds, the substituent and the benzene
ring taken together may form a new parent name
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New Way Chemistry for Hong Kong A-Level Book 3A
31.2 Nomenclature of the Derivatives of Benzene (SB p.148)
2. Polysubstituted benzenes
(a) If more than one substituent are present and the substituents
are identical, their relative positions are indicated by the use
of numbers assigned on the ring. The prefixes ‘di-’, ‘tri-’,
‘tetra-’, and so on are used.
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New Way Chemistry for Hong Kong A-Level Book 3A
31.2 Nomenclature of the Derivatives of Benzene (SB p.149)
(b) When more than one substituent are present and the
substituents are different, they are listed in alphabetical
order.
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New Way Chemistry for Hong Kong A-Level Book 3A
31.2 Nomenclature of the Derivatives of Benzene (SB p.149)
(c) When a substituent is one that when taken together with the
benzene ring gives a new parent name, that substituent is
assumed to be in position 1 and the new parent name is used
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New Way Chemistry for Hong Kong A-Level Book 3A
31.2 Nomenclature of the Derivatives of Benzene (SB p.149)
Example 31-1
Draw the structural formula for each of the following compounds:
Solution:
(a) 1,3,5-Trichlorobenzene
(a)
(b)
(b) 2,5-Dibromophenol
(c) 2,4-Dinitrobenzoic acid
Answer
(c)
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New Way Chemistry for Hong Kong A-Level Book 3A
31.2 Nomenclature of the Derivatives of Benzene (SB p.150)
Check Point 31-1
Give the IUPAC name for each of the following compounds:
(a)
(b)
(a) 1,2-Dimethylbenzene
(c)
(b) 1-Methyl-2-nitrobenzene
or 2-nitrotoluene
(d)
(c) 3-Bromo-5-chlorobenzoic acid
(d) 4-Bromo-2,6-dinitrophenol
Answer
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.151)
In 1865, Kekule proposed the structure of benzene:
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.151)
According to the Kekulé structure, there should be two
different 1,2-dibromobenzenes:
Only one 1,2-dibromobenzene has been found!!
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.151)
According to the Kekulé structure, benzene should
• undergo addition reactions readily
• it gave substitution reaction products rather than
addition reaction products
 Kekulé structure cannot explain the behaviour
of benzene
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.152)
Enthalpy Changes of Hydrogenation of Benzene
and Cyclohexene
Enthalpy change of hydrogenation of cyclohexene
= –119.6 kJ mol–1
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.152)
The enthalpy change of hydrogenation of 1,3-cyclohexadiene
is expected to be twice that of cyclohexene
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.152)
If benzene has the structure of 1,3,5-cyclohexatriene, The
enthalpy change of hydrogenation is expected to be three
times as much as that of cyclohexene
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.153)
• Benzene is more stable than Kekulé structure
• The energy difference for the stabilization of benzene
is called resonance energy of benzene
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.153)
The Resonance Explanation of the Structure
of Benzene
From X-ray crystallography,
The length of carbon-carbon bond in benzene is
intermediate between C – C bond and C = C bond
0.134 nm
C=C
17
>
0.139 nm
> 0.154 nm
carbon bond in benzene
C–C
New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.154)
All carbon atoms in benzene are sp2-hybridized
The side-way overlap of unhybridized 2p orbitals on
both sides gives a delocalized  electron cloud above
and below the plane of the ring
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.154)
The delocalization of  electrons gives benzene extra
stability and determines the chemical properties of benzene
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.154)
Structural formula of benzene:
The circle represents the six electrons that are delocalized
about the six carbon atoms of the benzene ring
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New Way Chemistry for Hong Kong A-Level Book 3A
31.3 The Stability of Benzene (SB p.155)
Structure of Methylbenzene
• All C atoms in the ring is sp2-hybridized
• The C atom in the methyl group is sp3-hybridized
• The delocalized  electron clouds give rise to extra
stability
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New Way Chemistry for Hong Kong A-Level Book 3A
31.4 Physical Properties of Aromatic Hydrocarbons (SB p.155)
Physical properties of aromatic hydrocarbons:
• have a fragrant smell
• generally less dense than water at 20°C
• usually immiscible with water
• soluble in organic solvents
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New Way Chemistry for Hong Kong A-Level Book 3A
31.4 Physical Properties of Aromatic Hydrocarbons (SB p.156)
Name
23
Formula
Boiling
point (°C)
Melting
Density at
point (°C) 20°C (g cm–3)
Benzene
80.1
5.5
0.878
Methylbenzene
111
–95
0.867
Ethylbenzene
136
–94
0.867
New Way Chemistry for Hong Kong A-Level Book 3A
31.4 Physical Properties of Aromatic Hydrocarbons (SB p.156)
Name
24
Formula
Boiling
Melting
Density at
point (°C) point (°C) 20°C (g cm–3)
1,2Dimethylbenzene
144
–25.2
0.880
1,3Dimethylbenzene
139
–47.4
0.864
1,4Dimethylbenzene
138
13.3
0.861
New Way Chemistry for Hong Kong A-Level Book 3A
31.5 Preparation of Benzene (SB p.157)
Industrial Preparation
Catalytic Reforming of Alkanes
Catalytic reforming converts alkanes and cycloalkanes
into aromatic hydrocarbons
e.g.
Pt
C6H14  C6H6 + 4H2
500°C, 10 – 20 atm
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New Way Chemistry for Hong Kong A-Level Book 3A
31.5 Preparation of Benzene (SB p.157)
Destructive Distillation of Coal
• Heating coal in the absence of air gives out coal gas,
ammoniacal liquor, coal tar and coke
• Coal tar is a mixture of many
organic compounds, mainly
aromatic ones
• Benzene and methylbenzene
can be obtained
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New Way Chemistry for Hong Kong A-Level Book 3A
31.5 Preparation of Benzene (SB p.158)
Laboratory Synthesis
Decarboxylation of Sodium Salt of Benzoic Acid
When sodium benzoate is fused with sodium hydroxide, the
carboxylate group is removed and benzene is formed
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New Way Chemistry for Hong Kong A-Level Book 3A
31.5 Preparation of Benzene (SB p.158)
Reduction of Phenol
Phenol vapour is passed slowly over heated zinc dust to
produce benzene and zinc(II) oxide
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New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.158)
Comparative Investigation of Chemical Properties of
Cyclohexane, Cyclohexene and Benzene
Reaction
Cyclohexane (a
saturated alicyclic
hydrocarbon)
Cyclohexene (an
unsaturated
alicyclic
hydrocarbon)
Methylbenzene (an
aromatic
hydrocarbon)
Action of Br2
in CH3Cl3 (in
dark)
No reaction
Br2 decolourized
and no HBr
evolved
No reaction with Br2
alone
In the presence of
FeBr3, Br2
decolourized and HBr
fumes evolved
Action of H2
(with Ni
catalyst)
No reaction
1 mole of
cyclohexene reacts
with 1 mole of H2
at room
temperature
1 mole of
methylbenzene reacts
3 moles of H2 at high
temperature and
pressure
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New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.159)
Reaction
Action of
acidified
KMnO4
Cyclohexane (a
saturated alicyclic
hydrocarbon)
No reaction
Action of conc. No reaction
HNO3 and
conc. H2SO4
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Cyclohexene (an
unsaturated
alicyclic
hydrocarbon)
KMnO4
decolourized
Methylbenzene (an
aromatic
hydrocarbon)
No reaction
Cyclohexene
A yellow liquid is
oxidized and colour formed
darkens
New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.159)
• Methylbenzene is highly unsaturated, but it is
resistant to oxidation and addition reactions
• The resistance of oxidation and addition reactions of
aromatic compounds is used to distinguish from
unsaturated alkenes
• Methylbenzene reacts with Br2 in the presence of
FeBr3. It is through substitution reaction
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New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.160)
Electrophilic Aromatic Substitution Reactions
Most characteristic reaction of aromatic compounds:
Electrophilic substitution reactions
• The electrophiles attack the benzene ring, replacing one of
the hydrogen atoms in the reaction
• Electrophiles are either a positive ion (E+) or some other
electron-deficient species with a partial positive charge (+)
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New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.160)
Nitration
• Conc. H2SO4 increases the rate of reaction by
increasing the concentration of the electrophile, NO2+
(nitronium ion)
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New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.160)
Sulphonation
• Benzene reacts with fuming sulphuric(VI) acid at room
temperature to give benzenesulphonic acid
• Heating aqueous solution of benzenesulphonic acid above
100°C, benzene and sulphuric(VI) acid are formed
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New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.161)
Halogenation
Benzene reacts with chlorine and bromine in the presence of
catalysts such as AlCl3, FeCl3, FeBr3, to give
chlorobenzene and bromobenzene respectively
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New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.161)
Alkylation
• When benzene is warmed with a haloalkane in the presence
of catalysts such as AlCl3, an alkylbenzene is formed
• Important step in chemical industry to produce
polystyrene, phenol and detergents
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New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.162)
Example 31-2
Complete each of the following by supplying the missing
reactant or product as indicated by the question mark:
Solution:
(a)
(a)
(b)
(b) conc. H2SO4, conc. HNO3
(c)
(c) fuming H2SO4
Answer
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New Way Chemistry for Hong Kong A-Level Book 3A
31.6 Reactions of Benzene (SB p.162)
Check Point 31-2
(a) One mole of benzene reacts with three moles of chlorine
under special conditions. What is the reaction condition
required for the reaction?
(a) UV radiation or diffuse sunlight must be
(b) Draw the structure
of the reaction product in (a).
present for the free radical addition
reaction to take place.
(b)
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Answer
31.7 Alkylbenzenes (SB p.162)
• Alkylbenzenes are a group of aromatic hydrocarbons in
which an alkyl group is bonded directly to a benzene
ring
• also known as arenes
e.g.
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New Way Chemistry for Hong Kong A-Level Book 3A
31.7 Alkylbenzenes (SB p.163)
Alkylbenzenes are oxidized to benzoic acid by strong oxidizing
agents such as hot alkaline potassium manganate(VII)
This type of oxidation is limited to those molecules with
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New Way Chemistry for Hong Kong A-Level Book 3A
31.7 Alkylbenzenes (SB p.163)
Examples:
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New Way Chemistry for Hong Kong A-Level Book 3A
31.7 Alkylbenzenes (SB p.164)
The C = C double bond and acyl groups in the side chain
are oxidized by hot alkaline potassium manganate(VII)
e.g.
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New Way Chemistry for Hong Kong A-Level Book 3A
31.7 Alkylbenzenes (SB p.164)
Example 31-3
State the conditions under which methylbenzene can be
converted in the laboratory to
(a) C6H5CH2Cl
(b) C6H5COOH
Solution:
(a) Reagent: Cl2
Condition: in the presence of light
(b) Reagent: (1) KMnO4–, OH–, (2) H3O+
Condition: heating under reflux
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New Way Chemistry for Hong Kong A-Level Book 3A
Answer
31.7 Alkylbenzenes (SB p.164)
Check Point 31-3
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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New Way Chemistry for Hong Kong A-Level Book 3A
31.7 Alkylbenzenes (SB p.164)
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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The END
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New Way Chemistry for Hong Kong A-Level Book 3A