1. Arenes
Textbook p4-5

2. Explain the terms: arene and aromatic
Explain the terms: arene and aromatic. Describe and explain the models used to describe the structure of benzene.

3. Some examples of arenes or aromatic compounds and how they are named

4. Structures of some common benzene derivatives
Week 1
Structures of some common benzene derivatives
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5. Week 1 Three isomers of C7H7Br 5 © Pearson Education Ltd 2009
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6. So arene or aromatic means the compound contains a benzene ring
Is aspirin - on the next slide an arene?

7. Week 1 Structure of aspirin 7 © Pearson Education Ltd 2009
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8. Benzene is classified as a carcinogen
Week 1
Benzene is classified as a carcinogen
Imagine putting sodium on your chips if sodium chloride if it did react like sodium metal!
Most arenes are not carenogenic – just as sodium in sodium chloride does not behave like sodium metal on its own.
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9. models used to describe the structure of benzene
Benzene has the molecular formula C6H6
What is its emprical formula?
Answer Q1 in your pack

10. Suggested linear structure for benzene, with several double bonds
Week 1
Suggested linear structure for benzene, with several double bonds
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11. Kekulé structure of benzene
Week 1
Kekulé structure of benzene
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12. Now try Q1-5 on p5 of the text

13. The Structure of Benzene Textbook p6-7
Review the evidence for a delocalised model of benzene.

14. a molecular formula of C6H6
STRUCTURE OF BENZENE
Primary analysis revealed benzene had...
an empirical formula of CH and
a molecular mass of 78 and
a molecular formula of C6H6

15. a molecular formula of C6H6
STRUCTURE OF BENZENE
Primary analysis revealed benzene had...
an empirical formula of CH and
a molecular mass of 78
a molecular formula of C6H6
Kekulé suggested that benzene was...
PLANAR
CYCLIC and
HAD ALTERNATING DOUBLE AND SINGLE BONDS

16. STRUCTURE OF BENZENE
HOWEVER...
• it did not readily undergo electrophilic addition - no true C=C bond
• only one 1,2 disubstituted product existed
• all six C—C bond lengths were similar; C=C bonds are shorter than C-C
• the ring was thermodynamically more stable than expected

17. To explain the above, it was suggested that the structure oscillated
STRUCTURE OF BENZENE
HOWEVER...
• it did not readily undergo electrophilic addition - no true C=C bond
• only one 1,2 disubstituted product existed
• all six C—C bond lengths were similar; C=C bonds are shorter than C-C
• the ring was thermodynamically more stable than expected
To explain the above, it was suggested that the structure oscillated
between the two Kekulé forms but was represented by neither of
them. It was a RESONANCE HYBRID.

18. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.

19. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l) 2 3
- 120 kJ mol-1

20. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
- 120 kJ mol-1

21. THERMODYNAMIC EVIDENCE FOR STABILITY
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
Experimental
- 208 kJ mol-1
- 120 kJ mol-1

22. THERMODYNAMIC EVIDENCE FOR STABILITY MORE STABLE THAN EXPECTED
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale
It is 152kJ per mole more stable than expected.
This value is known as the RESONANCE ENERGY.
MORE STABLE THAN EXPECTED
by 152 kJ mol-1
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
Experimental
- 208 kJ mol-1
- 120 kJ mol-1

23. THERMODYNAMIC EVIDENCE FOR STABILITY MORE STABLE THAN EXPECTED
When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured.
When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane
C6H6(l) + 3H2(g) ——> C6H12(l)
Actual benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale
It is 152kJ per mole more stable than expected.
This value is known as the RESONANCE ENERGY.
MORE STABLE THAN EXPECTED
by 152 kJ mol-1
Theoretical
- 360 kJ mol-1
(3 x -120) 2 3
Experimental
- 208 kJ mol-1
- 120 kJ mol-1

24. Now try textbook p7 Q1-3

25. Week 1
Compare the Kekulé and delocalised models of benzene in terms of p-orbital overlap forming π-bonds.
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26. The delocalised model of benzene p8-9

27. Bonding around a carbon atom in a section of a benzene ring or alkene
Week 1
Bonding around a carbon atom in a section of a benzene ring or alkene
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28. STRUCTURE OF ALKENES - REVISION
The two 2p orbitals also overlap. This forms a second bond; it is known as a PI (π) bond.
For maximum overlap and hence the strongest bond, the 2p orbitals are in line.
This gives rise to the planar arrangement around C=C bonds.

29. Week 1
The delocalised structure of benzene forms when the p-orbitals overlap sideways
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30. STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds

31. STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals

32. STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
another
possibility

33. STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
another
possibility
delocalised pi
orbital system

34. STRUCTURE OF BENZENE - DELOCALISATION
The theory suggested that instead of three localised (in one position) double bonds,
the six p (p) electrons making up those bonds were delocalised (not in any one
particular position) around the ring by overlapping the p orbitals. There would be no
double bonds and all bond lengths would be equal. It also gave a planar structure.
6 single bonds
one way to overlap
adjacent p orbitals
another
possibility
delocalised pi
orbital system
This final structure was particularly stable and
resisted attempts to break it down through normal
electrophilic addition. However, substitution of any
hydrogen atoms would not affect the delocalisation.

35. STRUCTURE OF BENZENE

36. STRUCTURE OF BENZENE
ANIMATION

37. Now try Textbook p9 Q1-4

38. Benzene and its reactions
Textbook p10-11

39. Week 1
Describe the electrophilic substitution of arenes with concentrated nitric acid in the presence of concentrated sulfuric acid.
Describe the electrophilic substitution of arenes with halogens in the presence of a suitable halogen carrier.
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40. Equation for the preparation of nitrobenzene
Week 1
Equation for the preparation of nitrobenzene
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41. Preparing nitrobenzene by heating benzene
Week 1
Preparing nitrobenzene by heating benzene
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42. Benzene reacts with chlorine to produce chlorobenzene
Week 1
Benzene reacts with chlorine to produce chlorobenzene
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43. Equation for the preparation of bromobenzene
Week 1
Equation for the preparation of bromobenzene
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44. Substitution reactions of benzene
Textbook p12-13

45. Outline the mechanism of electrophilic substitution in arenes.
Week 1
Outline the mechanism of electrophilic substitution in arenes.
Outline the mechanism for the mononitration and monohalogenation of benzene.
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46. Electrophilic substitution mechanism (Nitration)
1. Formation of NO2+
the nitronium ion
HNO3
+ H2SO4
NO2+
+ HSO4-
+ H2O
2. Electrophilic attack on benzene
NO2+ +
NO2 H
NO2
+ H+
3. Re-forming the catalyst
HSO4- + H+
H2SO4
reaction equation

47. Electrophilic substitution in benzene
Week 1
Electrophilic substitution in benzene
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48. Curly arrows are used to represent the movement of electron pairs
Week 1
Curly arrows are used to represent the movement of electron pairs
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49. Halogenation
C6H6 + Cl2 → C6H5 Cl HCl
Substitution of a halogen in the presence of a halogen carrier;
Halogen carriers include iron, iron halides and aluminium halides. (FeCl3 + Cl2 → FeCl4- Cl+ )
Cl+ is the electrophile

50. ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION
Reagents chlorine and a halogen carrier (catalyst)
Conditions reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3)
chlorine is non polar so is not a good electrophile
the halogen carrier is required to polarise the halogen
Equation C6H Cl2 ———> C6H5Cl HCl
Mechanism
Electrophile Cl+ it is generated as follows...
Cl FeCl FeCl4¯ Cl+

51. Keywords Arene Addition reaction Mononitration Aromatic
Substitution reaction
Monohalogenation
Delocalisation
Electrophile
Benzene
π-electron
Halogen carrier
Orbital overlap
π-bond
Reaction mechanism
Catalyst
Curly arrow
Electrophilic substitution

52. Now try Textbook p13 Q1-3

53. The Reactivity of Alkenes and Benzene
Textbook p14-15
Explain the relative resistance to bromination of benzene compared with alkenes.

54. Reaction of cyclohexene with bromine
Week 2
Reaction of cyclohexene with bromine
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55. A double bond in cyclohexene induces a dipole in a bromine molecule
Week 2
A double bond in cyclohexene induces a dipole in a bromine molecule
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56. Electrophilic addition in cyclohexene
Week 2
Electrophilic addition in cyclohexene
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57. Relative resistance to bromination of benzene compared with cyclohexene
Alkenes - add Br2 - room temp
C6H Br2 → C6H10Br2
Electrophilic addition
Arenes – add Br2 – halogen carrier needed
C6H6 + Br2 → C6H5Br
Electrophilic substitution

58. Benzene needs a halogen carrier to produce a more powerful electrophile Br+ – alkenes do not need a halogen carrier for reaction.
Benzene has a lower electron density between two carbon atoms compared to alkenes.
There is insufficient pi electron density between and two carbon atoms to polarise non-polar molecules – unlike an alkene.

59. Now try Textbook p15 Q1-2