1. Electrophilic Aromatic Substitution
Benzene
and
Electrophilic Aromatic Substitution
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2. New Way Chemistry for Hong Kong A-Level 3A
Aromatic Hydrocarbons (Arenes)
Simplest member : Benzene, C6H6
Characteristic aroma
Alkylbenzene, Cn+6H2n+6
E.g.
C6H5CH3
C6H5C2H5
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3. New Way Chemistry for Hong Kong A-Level 3A
Aromatic Hydrocarbons (Arenes)
Generally less dense than water
Insoluble in water but soluble in many organic solvents
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4. New Way Chemistry for Hong Kong A-Level 3A
Aromatic Hydrocarbons (Arenes)
PAHs : Polycyclic aromatic hydrocarbons
Fused aromatic rings
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5. New Way Chemistry for Hong Kong A-Level 3A
Aromatic Hydrocarbons (Arenes)
No heteroatoms
Not PAH
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6. New Way Chemistry for Hong Kong A-Level 3A
Aromatic Hydrocarbons (Arenes)
PAHs : Polycyclic aromatic hydrocarbons
Sources : - incomplete combustion of
wood, coal, diesel, fat, or tobacco
Q.38
benzocyclobutadiene
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7. New Way Chemistry for Hong Kong A-Level 3A
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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8. Preparation of Benzene
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9. New Way Chemistry for Hong Kong A-Level 3A
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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10. 1. Destructive Distillation of Coal
A laboratory set-up of the destructive distillation of coal
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11. New Way Chemistry for Hong Kong A-Level 3A
Industrial Preparation
2. Catalytic trimerization of ethyne
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12. New Way Chemistry for Hong Kong A-Level 3A
Industrial Preparation
3. Catalytic Reforming of Petroleum
Converts alkanes and cycloalkanes into aromatic hydrocarbons
C6H14  C6H6 + 4H2
500 oC, 10 – 20 atm Pt
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13. Laboratory Synthesis
1. Decarboxylation of Sodium Salt of Benzoic Acid
When sodium benzoate is fused with sodium hydroxide
 the carboxylate group is removed
volatile
Benzene is separated by fractional distillation

14. Passing phenol vapour over heated zinc dust (reducing agent)
2. Reduction of Phenol
Passing phenol vapour over heated zinc dust (reducing agent)
 produce benzene and zinc(II) oxide
volatile
Benzene is separated by fractional distillation

15. New Way Chemistry for Hong Kong A-Level 3A
Reactions of Benzene
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16. New Way Chemistry for Hong Kong A-Level 3A
Reactivity of Benzene
Unreactive towards addition reactions due to
stabilization of the system by delocalization of -electrons
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17. New Way Chemistry for Hong Kong A-Level 3A
Reactivity of Benzene
Not oxidized by KMnO4
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18. New Way Chemistry for Hong Kong A-Level 3A
Reactivity of Benzene
Resistant to electrophilic addition
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19. New Way Chemistry for Hong Kong A-Level 3A
Reactivity of Benzene
Resistant to catalytic hydrogenation
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20. New Way Chemistry for Hong Kong A-Level 3A
Addition reactions occur only under drastic conditions.
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21. New Way Chemistry for Hong Kong A-Level 3A
No reaction
Occur only under drastic conditions
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22. New Way Chemistry for Hong Kong A-Level 3A
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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23. New Way Chemistry for Hong Kong A-Level 3A

24. New Way Chemistry for Hong Kong A-Level 3A
Electrophilic Aromatic Substitution Reactions
E+ : electrophile
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25. New Way Chemistry for Hong Kong A-Level 3A
General Reaction Mechanism
Step 1:
Rate determining step
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26. New Way Chemistry for Hong Kong A-Level 3A
General Reaction Mechanism
Stabilized by delocalization of  electrons +
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27. New Way Chemistry for Hong Kong A-Level 3A
General Reaction Mechanism
Step 2:
The carbocation loses a hydrogen ion
 forms the substitution product
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28. New Way Chemistry for Hong Kong A-Level 3A
1. Halogenation
No apparent reation
Catalysts : AlCl3, FeCl3 or FeBr3
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29. New Way Chemistry for Hong Kong A-Level 3A
1. Halogenation - Mechanism
Step 1:
The catalyst (FeBr3) combines with bromine to give a complex
New Way Chemistry for Hong Kong A-Level 3A

30. New Way Chemistry for Hong Kong A-Level 3A
Step 2:
Formation of carbocation intermediate
Rate determining step
Stabilized by delocalization of -electrons
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31. New Way Chemistry for Hong Kong A-Level 3A
Step 3:
The catalyst (FeBr3) is regenerated
Fumes of HBr(g) are produced, indicating substitution rather than addition has occurred.
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32. New Way Chemistry for Hong Kong A-Level 3A
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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33. New Way Chemistry for Hong Kong A-Level 3A
acid
base
electrophile
2HNO NO3 + H2O + NO2+
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34. New Way Chemistry for Hong Kong A-Level 3A
Optional
meta-directing
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35. New Way Chemistry for Hong Kong A-Level 3A
Q.39
H2SO4 + HNO NO2+ + H2O + HSO4
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37. + H2SO4

38. New Way Chemistry for Hong Kong A-Level 3A
3. Sulphonation
(H2SO4 + SO3)
Benzene reacts with fuming sulphuric acid at room temp
 form benzenesulphonic acid +
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39. New Way Chemistry for Hong Kong A-Level 3A
3. Sulphonation
Sulphonation is a reversible process
By heating an aqueous solution of benzenesulphonic acid to above 100 oC
 benzene and sulphuric acid are formed
heat
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40. New Way Chemistry for Hong Kong A-Level 3A
Q.40
H2SO4 + H2SO SO3 + H3O+ + HSO4
New Way Chemistry for Hong Kong A-Level 3A

42. + H2SO4

43. New Way Chemistry for Hong Kong A-Level 3A
4. 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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44. New Way Chemistry for Hong Kong A-Level 3A
4. Friedel-Crafts Alkylation
Important starting step in the manufacture of styrene, phenol and detergents
The alkyl group introduced activates the ring towards further alkylation by +ve I-effect.
 Friedel-Crafts alkylation is not a good way to prepare alkylbenzene.
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45. Friedel-Craft acylation Clemensen reduction
deactivating +
Friedel-Craft acylation
Clemensen reduction 45
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46. Devise a reaction pathway of not more than four steps for the following conversion.
6B 09/07/2011 46

47. 47

48. New Way Chemistry for Hong Kong A-Level 3A
Chlorination
Bromination or Friedel-Craft reaction
Soda lime to absorb HBr or HCl fumes
Br2, RCl or RCOCl
or AlCl3
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49. New Way Chemistry for Hong Kong A-Level 3A
Q.41
electrophile
CH3Cl + AlCl3  [CH3]+[AlCl4]
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51. New Way Chemistry for Hong Kong A-Level 3A
Q.42(a)
2C6H X C6H5CH2C6H HCl
AlCl3
X : CH2Cl2
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52. New Way Chemistry for Hong Kong A-Level 3A
-CH2Cl deactivate the ring
Susceptibility to SE reaction :
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53. New Way Chemistry for Hong Kong A-Level 3A
Q.42(b)
3C6H Y (C6H5)3CH + 3HCl
AlCl3
Y : CHCl3
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54. New Way Chemistry for Hong Kong A-Level 3A

55. New Way Chemistry for Hong Kong A-Level 3A
Q.42(c)
2C6H Z C6H5CH2CH2C6H HCl
AlCl3
Z : CH2ClCH2Cl
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56. New Way Chemistry for Hong Kong A-Level 3A

57. New Way Chemistry for Hong Kong A-Level 3A
Industrial preparation of toluene
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58. New Way Chemistry for Hong Kong A-Level 3A
Laboratory Preparation of Methylbenzene
1. Friedel-Crafts acylation of benzene followed by Clemensen reduction.
2. Friedel-Crafts alkylation of benzene is not recommended as further alkylation will take place giving a mixture of alkylbenzenes that are difficult to be separated.
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59. Reactions of methylbenzene and Other Alkylbenzenes
1. Electrophilic aromatic substitution rxs
more susceptible to electrophilic attacks ∵ alkyl groups activate the ring by positive inductive effect.

60. New Way Chemistry for Hong Kong A-Level 3A
2. Reactions of the side chain
(a) Free radical substitution vs electrophilic substitution at the ring
Free radical substitution
Electrophilic substitution
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61. New Way Chemistry for Hong Kong A-Level 3A
Benzylic hydrogens are much more susceptible to free radical substitution than hydrogens at other positions.
Major products
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62. New Way Chemistry for Hong Kong A-Level 3A
(b) Oxidation at the benzylic carbon
Absence of benzylic H
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63. New Way Chemistry for Hong Kong A-Level 3A
If alkaline KMnO4 is used, the resulting mixture must be acidified in order to obtain the benzoic acid.
heat
Or,
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64. New Way Chemistry for Hong Kong A-Level 3A
Structural determination of isomeric alkylbenzene
alkylbenzenes with very close boiling points
C8H10
acids with different melting points
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65. New Way Chemistry for Hong Kong A-Level 3A
Structural determination of isomeric alkylbenzene
alkylbenzenes with very close boiling points
acids with different melting points
New Way Chemistry for Hong Kong A-Level 3A

66. New Way Chemistry for Hong Kong A-Level 3A
Q.43(a) 66
New Way Chemistry for Hong Kong A-Level 3A

67. New Way Chemistry for Hong Kong A-Level 3A
Q.43(b)
Terminal alkene gives CO2
+ H2O 67
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68. New Way Chemistry for Hong Kong A-Level 3A
Q.43(b) 68
New Way Chemistry for Hong Kong A-Level 3A

69. New Way Chemistry for Hong Kong A-Level 3A
Q.43(b) 69
New Way Chemistry for Hong Kong A-Level 3A

70. New Way Chemistry for Hong Kong A-Level 3A
Q.43(b) 70
New Way Chemistry for Hong Kong A-Level 3A

71. New Way Chemistry for Hong Kong A-Level 3A
Q.43(c)
+ H2O 71
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72. New Way Chemistry for Hong Kong A-Level 3A
The END
New Way Chemistry for Hong Kong A-Level 3A

73. New Way Chemistry for Hong Kong A-Level 3A
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
(a) (b) (c)
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74. New Way Chemistry for Hong Kong A-Level 3A
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-nitrobenzene or 2-nitrotoluene
Answer
New Way Chemistry for Hong Kong A-Level 3A

75. New Way Chemistry for Hong Kong A-Level 3A
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-dinitrophenol
Answer
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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
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77. 29.3 Structure of Benzene and Aromaticity (SB p.195)
Back
Let's Think 1
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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78. New Way Chemistry for Hong Kong A-Level 3A
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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79. New Way Chemistry for Hong Kong A-Level 3A
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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80. New Way Chemistry for Hong Kong A-Level 3A
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)
(a)
conc. H2SO4, conc. HNO3
fuming H2SO4
Answer
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81. New Way Chemistry for Hong Kong A-Level 3A
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.
New Way Chemistry for Hong Kong A-Level 3A

82. New Way Chemistry for Hong Kong A-Level 3A
29.6 Reactions of Benzene (SB p.203)
Check Point 29-6
(b) Draw the structure of the reaction product in (a).
Answer
(b)
New Way Chemistry for Hong Kong A-Level 3A

83. New Way Chemistry for Hong Kong A-Level 3A
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
New Way Chemistry for Hong Kong A-Level 3A

84. New Way Chemistry for Hong Kong A-Level 3A
29.6 Reactions of Benzene (SB p.203)
Check Point 29-6
Back
(c) The 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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85. Cyclohexane (a saturated alicyclic hydrocarbon)
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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86. Cyclohexane (a saturated alicyclic hydrocarbon)
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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87. Cyclohexane (a saturated alicyclic hydrocarbon)
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
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88. Cyclohexane (a saturated alicyclic hydrocarbon)
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
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