Benzene

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Benzene and Electrophilic Aromatic Substitution
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Aromatic Hydrocarbons (Arenes)

• Simplest member Benzene, C6H6
• Characteristic aroma
• Alkylbenzene, Cn6H2n6
• E.g.

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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

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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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

• 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
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Industrial Preparation
3. Catalytic Reforming of Petroleum

• Converts alkanes and cycloalkanes into aromatic
  hydrocarbons

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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
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Reactivity of Benzene
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Reactivity of Benzene
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Addition reactions occur only under drastic
conditions.
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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
• Benzene reacts with the electrophile
• A carbocation intermediate is formed
• Rate determining step

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General Reaction Mechanism

• 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
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1. Halogenation - Mechanism

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

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

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

• 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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electrophile
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Optional
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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

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The END
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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
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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
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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
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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
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29.3 Structure of Benzene and Aromaticity (SB
p.195)
Back
Let's Think 1
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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
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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
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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
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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.
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29.6 Reactions of Benzene (SB p.203)
Check Point 29-6
(b) Draw the structure of the reaction product in
(a).
Answer
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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
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29.6 Reactions of Benzene (SB p.203)
Back
Check Point 29-6
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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 1,1,1-trichloro-ethane (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 (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon)
Action of hydrogen (with nickel catalyst) No reaction One mole of cyclohexene reacts with one mole of hydrogen at room temperature One mole of methylbenzene reacts with three moles of hydrogen at high temperature
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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 acidified potassium manganate(VII) No reaction Acidified potassium manganate(VII) solution is decolourized No 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 concentrated nitric acid and concentrated sulphuric acid No reaction Cyclohexene is oxidized and the colour darkens A yellow liquid is formed