Aromatic Hydrocarbons

1
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 Alkylbenzen
es
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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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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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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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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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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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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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31.2 Nomenclature of the Derivatives of Benzene
(SB p.149)
Example 31-1 Draw the structural formula for each
of the following compounds (a) 1,3,5-Trichloroben
zene (b) 2,5-Dibromophenol (c) 2,4-Dinitrobenzoic
acid
Answer
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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) (c) (d
)
Answer
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31.3 The Stability of Benzene (SB p.151)
In 1865, Kekule proposed the structure of benzene
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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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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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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 mol1
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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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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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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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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 gt
0.139 nm gt 0.154 nm C C carbon bond in
benzene C C
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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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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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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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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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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 20C
• usually immiscible with water
• soluble in organic solvents

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31.4 Physical Properties of Aromatic
Hydrocarbons (SB p.156)
Name Formula Boiling point (C) Melting point (C) Density at 20C (g cm3)
Benzene 80.1 5.5 0.878
Methylbenzene 111 95 0.867
Ethylbenzene 136 94 0.867
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31.4 Physical Properties of Aromatic
Hydrocarbons (SB p.156)
Name Formula Boiling point (C) Melting point (C) Density at 20C (g cm3)
1,2-Dimethylbenzene 144 25.2 0.880
1,3-Dimethylbenzene 139 47.4 0.864
1,4-Dimethylbenzene 138 13.3 0.861
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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 500C, 10 20
atm
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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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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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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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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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31.6 Reactions of Benzene (SB p.159)
Reaction Cyclohexane (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon)
Action of acidified KMnO4 No reaction KMnO4 decolourized No reaction
Action of conc. HNO3 and conc. H2SO4 No reaction Cyclohexene oxidized and colour darkens A yellow liquid is formed
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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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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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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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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 100C, benzene and sulphuric(VI) acid are
  formed

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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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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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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 (a) (b) (c)

Answer
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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? (b) Draw the
structure of the reaction product in (a).
Answer
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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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31.7 Alkylbenzenes (SB p.163)
Alkylbenzenes are oxidized to benzoic acid by
strong oxidizing agents such as hot alkaline
potassium manganate(VII)
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31.7 Alkylbenzenes (SB p.163)
Examples
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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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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
Answer
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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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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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