Benzene and Electrophilic Substitution

1
Benzene and Electrophilic Substitution
29.1 Introduction 29.2 Nomenclature of the
Derivatives of Benzene 29.3 Structure of Benzene
and Aromaticity 29.4 Physical Properties of
Aromatic Compounds 29.5 Preparation of
Benzene 29.6 Reactions of Benzene
2
Introduction
3
29.1 Introduction (SB p.189)
Benzene

• During the latter part of the 19th century,
• ? aromatic compounds are the compounds that
  were fragrant
• ? obtained from balsams, resins or essential
  oils

4
29.1 Introduction (SB p.189)
Benzene

• As parent compound of aromatic compounds
• Highly unsaturated
• Molecular formula C6H6
• Six-membered ring compound

5
29.1 Introduction (SB p.189)
Benzene
Aromatic compounds are mainly benzene and its
related compounds
6
Nomenclature of the Derivatives of Benzene
7
29.2 Nomenclature of the Derivatives of Benzene
(SB p.189)
Derivatives of Benzene

• Formed by replacing hydrogen atom(s) on the
  benzene ring by substituents
• Benzenes that contain one substituent
• ? known as mono-substituted benzenes
• Benzenes that contain more than one substituent
• ? known as poly-substituted benzenes

8
29.2 Nomenclature of the Derivatives of Benzene
(SB p.189)
Mono-substituted Benzenes

• For certain compounds, benzene is the parent name
  
• The substituent is simply indicated by a prefix
• e.g.

9
29.2 Nomenclature of the Derivatives of Benzene
(SB p.190)
Mono-substituted Benzenes
2. For other compounds, the substituent and the
benzene ring taken together may form a new parent
name e.g.
10
29.2 Nomenclature of the Derivatives of Benzene
(SB p.190)
Poly-substituted Benzenes

• 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- and tetra- are used
  to indicate the number of its occurrence

11
29.2 Nomenclature of the Derivatives of Benzene
(SB p.190)
Poly-substituted Benzenes
e.g.
12
29.2 Nomenclature of the Derivatives of Benzene
(SB p.190)
Poly-substituted Benzenes

• When more than one substituent are present and
  the substituents are different
• ? they are listed in alphabetical order

13
29.2 Nomenclature of the Derivatives of Benzene
(SB p.190)
Poly-substituted Benzenes
e.g.
14
29.2 Nomenclature of the Derivatives of Benzene
(SB p.191)
Poly-substituted Benzenes

• 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
• ? the new parent name is used

15
29.2 Nomenclature of the Derivatives of Benzene
(SB p.191)
Poly-substituted Benzenes
e.g.
16
29.2 Nomenclature of the Derivatives of Benzene
(SB p.191)
17
Structure of Benzene and Aromaticity
18
29.3 Structure of Benzene and Aromaticity (SB
p.192)
Kekule Structure of Benzene

• The first structure of benzene was proposed to be
  contained alternate single and double
  carbon-carbon bonds

The Kekule structure of benzene
19
29.3 Structure of Benzene and Aromaticity (SB
p.192)
Observations that Cannot be Explained by the
Kekule Structure

• There should be four isomers of dibromobenzene
• ? but only three isomers exist
• ? 1,2-, 1,3- and 1,4-dibromobenzene
• ? 1,6-dibromobenzene does not exist

20
29.3 Structure of Benzene and Aromaticity (SB
p.193)
Observations that Cannot be Explained by the
Kekule Structure
Only three isomers exist!
21
29.3 Structure of Benzene and Aromaticity (SB
p.193)
Observations that Cannot be Explained by the
Kekule Structure
1,6-Dibromobenzene does not exist!
22
29.3 Structure of Benzene and Aromaticity (SB
p.193)
Observations that Cannot be Explained by the
Kekule Structure

• Ethene and other alkenes are reactive compounds
• ? presence of CC double bonds

23
29.3 Structure of Benzene and Aromaticity (SB
p.193)
Observations that Cannot be Explained by the
Kekule Structure

• Benzene should be a very reactive compound
• ? contains three CC double bonds
• ? benzene reacts only under quite vigorous
  conditions

24
29.3 Structure of Benzene and Aromaticity (SB
p.193)
Observations that Cannot be Explained by the
Kekule Structure
3.
Enthalpy change of hydrogenation of cyclohexene
-119 kJ mol-1
25
29.3 Structure of Benzene and Aromaticity (SB
p.193)
Observations that Cannot be Explained by the
Kekule Structure

• According to the Kekule structure
• ? -358.8 kJ mol-1 (-119.6 ? 3) of energy should
  be evolved
• ? only 208.4 kJ mol-1 of energy is released

26
29.3 Structure of Benzene and Aromaticity (SB
p.194)
Observations that Cannot be Explained by the
Kekule Structure

• The C?C bond lengths in benzene are all found to
  be 0.140 nm
• ? intermediate between the lengths of the
  longer C?C single bond (0.154 nm) and the
  shorter CC double bond (0.134 nm)

27
29.3 Structure of Benzene and Aromaticity (SB
p.194)
Observations that Cannot be Explained by the
Kekule Structure
28
29.3 Structure of Benzene and Aromaticity (SB
p.194)
Delocalized Structure of Benzene

• All carbon atoms in benzene are sp2-hybridized
• Each carbon atom attaches to
• ? two adjacent sp2-hybridized carbon atoms
• ? one hydrogen atom
• ? leaving an unhybridized 2p orbital

29
29.3 Structure of Benzene and Aromaticity (SB
p.194)
Delocalized Structure of Benzene

• Side-way overlap of these 2p orbitals on both
  sides gives a delocalized ? electron cloud
• Delocalization of ? electrons
• ? Imparts extra stability to benzene
• ? Determines the chemical properties of benzene
  and its derivatives

30
29.3 Structure of Benzene and Aromaticity (SB
p.194)
Delocalized Structure of Benzene
? bond skeleton
31
29.3 Structure of Benzene and Aromaticity (SB
p.194)
Delocalized Structure of Benzene
? bond skeleton with ? bonds
32
29.3 Structure of Benzene and Aromaticity (SB
p.194)
Delocalized Structure of Benzene

• The electrons in the ? bonds are free to move
  throughout the entire benzene molecule
• ? said to be delocalized

33
29.3 Structure of Benzene and Aromaticity (SB
p.195)
Delocalized Structure of Benzene

• In order to indicate the delocalized ? electrons,
  the structural formula is represented by

34
29.3 Structure of Benzene and Aromaticity (SB
p.195)
Explanation of Properties of Benzene with the
Delocalized Structure
1. The 1,6-dibromobenzene predicted by Kekulé is
actually identical to the 1,2-isomer
35
29.3 Structure of Benzene and Aromaticity (SB
p.195)
Explanation of Properties of Benzene with the
Delocalized Structure

• Benzene does not contain C C double bonds
• ? it does not react in a similar manner to
  alkenes

36
29.3 Structure of Benzene and Aromaticity (SB
p.195)
Explanation of Properties of Benzene with the
Delocalized Structure

• The delocalized ? electron structure
• ? accounts for the stability of the benzene
  molecule

37
29.3 Structure of Benzene and Aromaticity (SB
p.195)
Explanation of Properties of Benzene with the
Delocalized Structure

• The six delocalized ? electrons are shared
  evenly between the six carbon-carbon bonds
• ? each bond can be thought of as having an
  extra half a bond

38
Physical Properties of Aromatic Hydrocarbons
39
29.4 Physical Properties of Aromatic
Hydrocarbons (SB p.195)
Physical Properties of Aromatic Hydrocarbons

• Most aromatic hydrocarbons have a fragrant smell
• Generally less dense than water
• Immiscible with water
• Soluble in many organic solvents

40
29.4 Physical Properties of Aromatic
Hydrocarbons (SB p.196)
Some physical properties of several aromatic
hydrocarbons
41
29.4 Physical Properties of Aromatic
Hydrocarbons (SB p.196)
Some physical properties of several aromatic
hydrocarbons
42
29.4 Physical Properties of Aromatic
Hydrocarbons (SB p.197)
Polyaromatic Hydrocarbons

• Aromatic hydrocarbons with multiple rings
• Also known as polycyclic aromatic hydrocarbons
• e.g. naphthalene, anthracene and phenanthrene

43
29.4 Physical Properties of Aromatic
Hydrocarbons (SB p.197)
Polyaromatic Hydrocarbons
44
29.4 Physical Properties of Aromatic
Hydrocarbons (SB p.197)
Polyaromatic Hydrocarbons
45
29.4 Physical Properties of Aromatic
Hydrocarbons (SB p.197)
Polyaromatic Hydrocarbons

• Pollutants in the atmosphere
• ? due to incomplete combustion and pyrolysis of
  aromatic hydrocarbons
• Some can be metabolized in human body into
  species that bond to DNA
• ? cause cancer

46
Preparation of Benzene
47
29.5 Preparation of Benzene (SB p.197)
Industrial Preparation
1. Catalytic Reforming of Petroleum

• Converts alkanes and cycloalkanes into aromatic
  hydrocarbons

48
29.5 Preparation of Benzene (SB p.198)
2. Destructive Distillation of Coal

• 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)
• e.g. benzene and methylbenzene can be obtained

49
29.5 Preparation of Benzene (SB p.198)
2. Destructive Distillation of Coal
A laboratory set-up of the destructive
distillation of coal
50
29.5 Preparation of Benzene (SB p.198)
Laboratory Synthesis
1. Decarboxylation of Sodium Salt of Benzoic Acid

• When sodium benzoate is fused with sodium
  hydroxide
• ? the carboxylate group is removed

51
29.5 Preparation of Benzene (SB p.198)
2. Reduction of Phenol

• Passing phenol vapour over heated zinc dust
• ? produce benzene and zinc(II) oxide

52
Reactions of Benzene
53
29.6 Reactions of Benzene (SB p.199)
Comparison of some reactions of cyclohexane,
cyclohexene and methylbenzene
54
29.6 Reactions of Benzene (SB p.199)
Comparison of some reactions of cyclohexane,
cyclohexene and methylbenzene
55
29.6 Reactions of Benzene (SB p.199)
Comparison of some reactions of cyclohexane,
cyclohexene and methylbenzene
56
29.6 Reactions of Benzene (SB p.199)
Comparison of some reactions of cyclohexane,
cyclohexene and methylbenzene
57
29.6 Reactions of Benzene (SB p.200)
Electrophilic Aromatic Substitution Reactions
where E denotes an electrophile
58
29.6 Reactions of Benzene (SB p.200)
Electrophilic Aromatic Substitution Reactions

• Electrophile
• ? an electron-deficient species
• ? can be a positive ion or the positive end of
  a polar molecule
• The substitution product retains the aromatic
  structure

59
29.6 Reactions of Benzene (SB p.200)
1. General Reaction Mechanism

• Step 1
• Benzene reacts with the electrophile
• A carbocation intermediate is formed
• Rate determining step

60
29.6 Reactions of Benzene (SB p.200)
1. General Reaction Mechanism

• The carbocation formed has a positive charge on
  the carbon atom of benzene
• Stabilized by delocalization of ? electrons

61
29.6 Reactions of Benzene (SB p.201)
1. General Reaction Mechanism

• Step 2
• The carbocation loses a hydrogen ion
• ? forms the substitution product

62
29.6 Reactions of Benzene (SB p.201)
2. 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

63
29.6 Reactions of Benzene (SB p.201)
2. Halogenation
64
29.6 Reactions of Benzene (SB p.201)
2. Halogenation

• Step 1
• The catalyst (FeBr3) combines with bromine to
  give a complex

65
29.6 Reactions of Benzene (SB p.201)
2. Halogenation

• Step 2
• Formation of carbocation intermediate
• Rate determining step

66
29.6 Reactions of Benzene (SB p.201)
2. Halogenation

• Step 3
• The loss of a proton from the carbocation
  intermediate
• Forms the bromination product
• The catalyst (FeBr3) is regenerated

67
29.6 Reactions of Benzene (SB p.202)
3. Nitration

• 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

68
29.6 Reactions of Benzene (SB p.202)
4. Sulphonation

• Benzene reacts with fuming sulphuric acid at room
  temp
• ? form benzenesulphonic acid

69
29.6 Reactions of Benzene (SB p.202)
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

70
29.6 Reactions of Benzene (SB p.202)
5. Alkylation

• When benzene is warmed with a haloalkane in the
  presence of AlCl3 as a catalyst
• ? alkylbenzene is formed

71
29.6 Reactions of Benzene (SB p.202)
5. Alkylation

• Important starting step in the manufacture of
  styrene, phenol and detergents

72
The END
73
29.2 Nomenclature of the Derivatives of Benzene
(SB p.191)
Back
Example 29-2
Draw the structural formula for each of the
following compounds (a) 1,3,5-Trichlorobenzene (b
) 2,5-Dibromophenol (c) 2,4-Dinitrobenzoic acid
Answer
74
29.2 Nomenclature of the Derivatives of Benzene
(SB p.192)
Check Point 29-2

• Give the IUPAC name for each of the following
  compounds
• (a)
• (b)

(a) 1,2-Dimethylbenzene (b) 1-Methyl-2-nitrobenzen
e or 2-nitrotoluene
Answer
75
29.2 Nomenclature of the Derivatives of Benzene
(SB p.192)
Check Point 29-2
Back

• Give the IUPAC name for each of the following
  compounds
• (c)
• (d)

(c) 3-Bromo-5-chlorobenzoic acid (d) 4-Bromo-2,6-d
initrophenol
Answer
76
29.3 Structure of Benzene and Aromaticity (SB
p.195)
Let's Think 1
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
77
29.3 Structure of Benzene and Aromaticity (SB
p.195)
Back
Let's Think 1
78
29.4 Physical Properties of Aromatic
Hydrocarbons (SB p.197)
Let's Think 2
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
79
29.4 Physical Properties of Aromatic
Hydrocarbons (SB p.197)
Let's Think 2
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
80
29.6 Reactions of Benzene (SB p.203)
Back
Example 29-6
Complete each of the following by supplying the
missing reactant or product as indicated by the
question mark (a) (b) (c)
Answer
81
29.6 Reactions of Benzene (SB p.203)
Check Point 29-6
(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.
82
29.6 Reactions of Benzene (SB p.203)
Check Point 29-6
(b) Draw the structure of the reaction product in
(a).
Answer
83
29.6 Reactions of Benzene (SB p.203)
Check Point 29-6
(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
84
29.6 Reactions of Benzene (SB p.203)
Back
Check Point 29-6