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New Way Chemistry for Hong Kong A-Level Book 3A
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Benzene Benzene and and
Electrophilic AromatElectrophilic Aromatic Substitutionic Substitution
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Aromatic Hydrocarbons (Arenes)Aromatic Hydrocarbons (Arenes)
• Simplest member : Benzene, C6H6
• Characteristic aroma
• Alkylbenzene, Cn+6H2n+6
E.g.CH3
C6H5CH3 C6H5C2H5
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Aromatic Hydrocarbons (Arenes)Aromatic Hydrocarbons (Arenes)
• Generally less dense than water
• Insoluble in water but soluble in many organic solvents
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Aromatic Hydrocarbons (Arenes)Aromatic Hydrocarbons (Arenes)
• PAHs : Polyaromatic hydrocarbons
• Fused aromatic rings
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Aromatic Hydrocarbons (Arenes)Aromatic Hydrocarbons (Arenes)
• No heteroatoms
N
B
Not PAH
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Aromatic Hydrocarbons (Arenes)Aromatic Hydrocarbons (Arenes)
• PAHs : Polyaromatic hydrocarbons
From incomplete combustion of
wood, coal, diesel, fat, or tobacco
Q.38benzocyclobutadiene
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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 Preparation of BenzeneBenzene
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1. Destructive Distillation of Coal1. 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 Coal1. Destructive Distillation of Coal
A laboratory set-up of the destructive
distillation of coal
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Industrial Industrial PreparationPreparation
2. Catalytic trimerization of ethyne2. Catalytic trimerization of ethyne
HC CH3organonickel catalyst
70oC, under pressure
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Industrial Industrial PreparationPreparation
3. Catalytic Reforming of Petroleum3. Catalytic Reforming of Petroleum• Converts alkanes and cycloalkanes
into aromatic hydrocarbons
C6H14 C6H6 + 4H2500 oC, 10 – 20 atm
Pt
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Laboratory Laboratory SynthesisSynthesis1. Decarboxylation of Sodium Salt of Benzoic Acid1. 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 Phenol2. 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 Reactions of BenzeneBenzene
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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
Resistant to electrophilic addition
Br2 / HBr / H2O
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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 Pt or Pd
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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 Electrophilic Aromatic Substitution ReactionsReactions
where E+ denotes an electrophile
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General Reaction MechanismGeneral Reaction MechanismStep 1:
• Benzene reacts with the electrophile
• A carbocation intermediate is formed
• Rate determining step
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General Reaction MechanismGeneral 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 MechanismGeneral Reaction MechanismStep 2:
• The carbocation loses a hydrogen ion
forms the substitution product
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1. Halogenation1. 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. Halogenation1. Halogenation
No apparent reaction
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1. Halogenation1. Halogenation
fast
slow
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1. Halogenation - Mechanism1. Halogenation - Mechanism
Step 1:
• The catalyst (FeBr3) combines with bromine to give a complex
Br BrFeBr3
Br [FeBr4]
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Step 2:
• Formation of carbocation intermediate
• Rate determining step
Br [FeBr]
Br
H
+
Br
H
Br
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. Nitration2. 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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H2SO4 + N
O
O
HO O N
O
O
H
H
O N O + H2O
acid base
electrophile
2HNO3 NO3 + H2O + NO2
+
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NO2
conc. HNO3
conc. H2SO4, 110oC
NO2
NO2NO2O2N
NO2
conc. HNO3
conc. H2SO4, 95oC
NO2
NO2
Optional
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3. Sulphonation3. Sulphonation• Benzene reacts with fuming sulphuric
acid at room temp
form benzenesulphonic acid
+
(H2SO4 + SO3)
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4. Sulphonation4. 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 Alkylation5. 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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• Important starting step in the manufacture of styrene, phenol and detergents
Example 29-6Example 29-6 Check Point 29-6Check Point 29-6
5. Friedel-Crafts Alkylation5. Friedel-Crafts Alkylation
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The END
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29.2 Nomenclature of the Derivatives of Benzene (SB p.191)
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
Back
(a) (b) (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)
Answer
(a) 1,2-Dimethylbenzene
(b) 1-Methyl-2-nitrobenzene or 2-
nitrotoluene
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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:
(c)
(d)
Answer
Back
(c) 3-Bromo-5-chlorobenzoic acid
(d) 4-Bromo-2,6-dinitrophenol
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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 inthe 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)
Complete each of the following by supplying the missing reactant or product as indicated by the question mark:
(a)
(b)
(c)
Answer
Back
(a)
(b) conc. H2SO4, conc. HNO3
(c) fuming H2SO4
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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?
(a) UV radiation or diffuse sunlight must
be present for the free radical addition
reaction to take place.
Answer
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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)
(c) The two different types of chlorination reaction of methylbenzene
are:
Type I: free radical substitution reaction
Type II: electrophilic aromatic substitution reaction
Back
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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
Comparison of some reactions of cyclohexane, cyclohexene and methylbenzene
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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
Comparison of some reactions of cyclohexane, cyclohexene and methylbenzene
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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
Comparison of some reactions of cyclohexane, cyclohexene and methylbenzene
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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
Comparison of some reactions of cyclohexane, cyclohexene and methylbenzene