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 : Polyaromatic 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 : Polyaromatic hydrocarbons
From incomplete combustion of
wood, coal, diesel, fat, or tobacco
benzocyclobutadiene
Q.38
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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. New Way Chemistry for Hong Kong A-Level 3A
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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14. New Way Chemistry for Hong Kong A-Level 3A
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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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
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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22. New Way Chemistry for Hong Kong A-Level 3A
Electrophilic Aromatic Substitution Reactions
where E+ denotes an electrophile
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23. New Way Chemistry for Hong Kong A-Level 3A
General Reaction Mechanism
Step 1:
Benzene reacts with the electrophile
A carbocation intermediate is formed
Rate determining step
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24. New Way Chemistry for Hong Kong A-Level 3A
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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25. 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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26. New Way Chemistry for Hong Kong A-Level 3A
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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27. New Way Chemistry for Hong Kong A-Level 3A
1. Halogenation
No apparent reaction
New Way Chemistry for Hong Kong A-Level 3A

28. New Way Chemistry for Hong Kong A-Level 3A
1. Halogenation
fast
slow
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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 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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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
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35. 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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36. New Way Chemistry for Hong Kong A-Level 3A
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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37. New Way Chemistry for Hong Kong A-Level 3A
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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38. New Way Chemistry for Hong Kong A-Level 3A
5. Friedel-Crafts Alkylation
Important starting step in the manufacture of styrene, phenol and detergents
Example 29-6
Check Point 29-6
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39. New Way Chemistry for Hong Kong A-Level 3A
The END
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40. 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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41. 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
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42. 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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43. 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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44. 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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45. 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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46. 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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47. 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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48. 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.
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49. 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)
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50. 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
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51. 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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52. 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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53. 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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54. 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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55. 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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