Benzene and chemical reactions

1. THE CHEMISTRY
OF ARENES
A guide for A level students
KNOCKHARDY PUBLISHING
2015
SPECIFICATIONS

2. INTRODUCTION
This Powerpoint show is one of several produced to help students understand
selected topics at AS and A2 level Chemistry. It is based on the requirements of
the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may
be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available
from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
Navigation is achieved by...
either clicking on the grey arrows at the foot of each page
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ARENES
KNOCKHARDY PUBLISHING

3. CONTENTS
• Prior knowledge
• Structure of benzene
• Thermodynamic stability
• Delocalisation
• Electrophilic substitution
• Nitration
• Chlorination
• Friedel-Crafts reactions
• Further substitution
ARENES

4. Before you start it would be helpful to…
• know the functional groups found in organic chemistry
• know the arrangement of bonds around carbon atoms
• recall and explain electrophilic addition reactions of alkenes
ARENES

5. 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

6. 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

7. 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

8. 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.

9. 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.

10. THERMODYNAMIC EVIDENCE FOR STABILITY
2 3
- 120 kJ mol-1
When cyclohexene (one C=C bond) is reduced to
cyclohexane, 120kJ of energy is released per mole.
C6H10(l) + H2(g) ——> C6H12(l)
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.

11. THERMODYNAMIC EVIDENCE FOR STABILITY
2 3
- 120 kJ mol-1
Theoretical
- 360 kJ mol-1
(3 x -120)
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)
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.

12. THERMODYNAMIC EVIDENCE FOR STABILITY
2 3
Experimental
- 208 kJ mol-1
- 120 kJ mol-1
Theoretical
- 360 kJ mol-1
(3 x -120)
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
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.

13. THERMODYNAMIC EVIDENCE FOR STABILITY
2 3
MORE STABLE
THAN EXPECTED
by 152 kJ mol-1
Experimental
- 208 kJ mol-1
- 120 kJ mol-1
Theoretical
- 360 kJ mol-1
(3 x -120)
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.
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.

14. THERMODYNAMIC EVIDENCE FOR STABILITY
2 3
MORE STABLE
THAN EXPECTED
by 152 kJ mol-1
Experimental
- 208 kJ mol-1
- 120 kJ mol-1
Theoretical
- 360 kJ mol-1
(3 x -120)
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.
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.

15. HYBRIDISATION OF ORBITALS - REVISION
The electronic configuration of a
carbon atom is 1s22s22p2
1 1s
2
2s
2p

16. HYBRIDISATION OF ORBITALS - REVISION
The electronic configuration of a
carbon atom is 1s22s22p2
1 1s
2
2s
2p
If you provide a bit of energy you
can promote (lift) one of the s
electrons into a p orbital. The
configuration is now 1s22s12p3
1 1s
2
2s
2p
The process is favourable because of the arrangement of
electrons; four unpaired and with less repulsion is more stable

17. HYBRIDISATION OF ORBITALS - REVISION
The four orbitals (an s and three p’s) combine or HYBRIDISE
to give four new orbitals. All four orbitals are equivalent.
2s22p2 2s12p3 4 x sp3
HYBRIDISE
sp3
HYBRIDISATION

18. HYBRIDISATION OF ORBITALS - REVISION
Alternatively, only three orbitals (an s and two p’s) combine or
HYBRIDISE to give three new orbitals. All three orbitals are
equivalent. The remaining 2p orbital is unchanged.
2s22p2 2s12p3 3 x sp2 2p
HYBRIDISE
sp2
HYBRIDISATION

19. In ALKANES, the four sp3 orbitals
repel each other into a tetrahedral
arrangement.
In ALKENES, the three
sp2 orbitals repel each
other into a planar
arrangement and the
2p orbital lies at right
angles to them
STRUCTURE OF ALKENES - REVISION

20. Covalent bonds are formed
by overlap of orbitals.
An sp2 orbital from each carbon
overlaps to form a single C-C bond.
The resulting bond is called
a SIGMA (δ) bond.
STRUCTURE OF ALKENES - REVISION

21. 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.
STRUCTURE OF ALKENES - REVISION

22. two sp2 orbitals overlap to form a sigma
bond between the two carbon atoms
ORBITAL OVERLAP IN ETHENE - REVIEW
two 2p orbitals overlap to form a pi
bond between the two carbon atoms
s orbitals in hydrogen overlap with the
sp2 orbitals in carbon to form C-H bonds
the resulting shape is planar
with bond angles of 120º

23. 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

24. STRUCTURE OF BENZENE - DELOCALISATION
6 single bonds one way to overlap
adjacent p orbitals
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.

25. STRUCTURE OF BENZENE - DELOCALISATION
6 single bonds one way to overlap
adjacent p orbitals
another
possibility
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.

26. STRUCTURE OF BENZENE - DELOCALISATION
6 single bonds one way to overlap
adjacent p orbitals
delocalised pi
orbital system
another
possibility
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.

27. STRUCTURE OF BENZENE - DELOCALISATION
6 single bonds one way to overlap
adjacent p orbitals
delocalised pi
orbital system
another
possibility
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.
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.

28. STRUCTURE OF BENZENE

29. STRUCTURE OF BENZENE
ANIMATION

30. WHY ELECTROPHILIC ATTACK?
Theory The high electron density of the ring makes it open to attack by electrophiles
HOWEVER...
Because the mechanism involves an initial disruption to the ring,
electrophiles will have to be more powerful than those which react
with alkenes.
A fully delocalised ring is stable so will resist attack.

31. WHY SUBSTITUTION?
Theory Addition to the ring would upset the delocalised electron system
Substitution of hydrogen atoms on the ring does not affect the delocalisation
Overall there is ELECTROPHILIC SUBSTITUTION
ELECTRONS ARE NOT DELOCALISED
AROUND THE WHOLE RING - LESS STABLE
STABLE DELOCALISED SYSTEM

32. ELECTROPHILIC SUBSTITUTION
Theory The high electron density of the ring makes it open to attack by electrophiles
Addition to the ring would upset the delocalised electron system
Substitution of hydrogen atoms on the ring does not affect the delocalisation
Because the mechanism involves an initial disruption to the ring,
electrophiles must be more powerful than those which react with alkenes
Overall there is ELECTROPHILIC SUBSTITUTION

33. ELECTROPHILIC SUBSTITUTION
Theory The high electron density of the ring makes it open to attack by electrophiles
Addition to the ring would upset the delocalised electron system
Substitution of hydrogen atoms on the ring does not affect the delocalisation
Because the mechanism involves an initial disruption to the ring,
electrophiles must be more powerful than those which react with alkenes
Overall there is ELECTROPHILIC SUBSTITUTION
Mechanism
• a pair of electrons leaves the delocalised system to form a bond to the electrophile
• this disrupts the stable delocalised system and forms an unstable intermediate
• to restore stability, the pair of electrons in the C-H bond moves back into the ring
• overall there is substitution of hydrogen ... ELECTROPHILIC SUBSTITUTION

34. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H6 + HNO3 ———> C6H5NO2 + H2O
nitrobenzene

35. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H6 + HNO3 ———> C6H5NO2 + H2O
nitrobenzene
Mechanism

36. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H6 + HNO3 ———> C6H5NO2 + H2O
nitrobenzene
Mechanism
Electrophile NO2
+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...
2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2
+
acid base

37. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H6 + HNO3 ———> C6H5NO2 + H2O
nitrobenzene
Mechanism
Electrophile NO2
+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...
2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2
+
acid base
Use The nitration of benzene is the first step in an historically important chain of
reactions. These lead to the formation of dyes, and explosives.

38. 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 C6H6 + Cl2 ———> C6H5Cl + HCl
Mechanism
Electrophile Cl+ it is generated as follows...
Cl2 + FeCl3 FeCl4¯ + Cl+
a
Lewis Acid

39. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION
Overview Alkylation involves substituting an alkyl (methyl, ethyl) group
Reagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3
Conditions room temperature; dry inert solvent (ether)
Electrophile a carbocation ion R+ (e.g. CH3
+)
Equation C6H6 + C2H5Cl ———> C6H5C2H5 + HCl

40. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION
Overview Alkylation involves substituting an alkyl (methyl, ethyl) group
Reagents a halogenoalkane (RX) and anhydrous aluminium chloride AlCl3
Conditions room temperature; dry inert solvent (ether)
Electrophile a carbocation ion R+ (e.g. CH3
+)
Equation C6H6 + C2H5Cl ———> C6H5C2H5 + HCl
Mechanism
General A catalyst is used to increase the positive nature of the electrophile
and make it better at attacking benzene rings.
AlCl3 acts as a Lewis Acid and helps break the C—Cl bond.

41. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION
Catalyst anhydrous aluminium chloride acts as the catalyst
the Al in AlCl3 has only 6 electrons in its outer shell; a LEWIS ACID
it increases the polarisation of the C-Cl bond in the haloalkane
this makes the charge on C more positive and the following occurs
RCl + AlCl3 AlCl4¯ + R+

42. FRIEDEL-CRAFTS REACTIONS - INDUSTRIAL ALKYLATION
Industrial Alkenes are used instead of haloalkanes but an acid must be present
Phenylethane, C6H5C2H5 is made by this method
Reagents ethene, anhydrous AlCl3 , conc. HCl
Electrophile C2H5
+ (an ethyl carbonium ion)
Equation C6H6 + C2H4 ———> C6H5C2H5 (ethyl benzene)
Mechanism the HCl reacts with the alkene to generate a carbonium ion
electrophilic substitution then takes place as the C2H5
+ attacks the ring
Use ethyl benzene is dehydrogenated to produce phenylethene (styrene);
this is used to make poly(phenylethene) - also known as polystyrene

43. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ACYLATION
Overview Acylation involves substituting an acyl (methanoyl, ethanoyl) group
Reagents an acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3
Conditions reflux 50°C; dry inert solvent (ether)
Electrophile RC+= O ( e.g. CH3C+O )
Equation C6H6 + CH3COCl ———> C6H5COCH3 + HCl
Mechanism
Product A carbonyl compound (aldehyde or ketone)

44. They are STRUCTURAL ISOMERS
1,3-DICHLOROBENZENE
meta dichlorobenzene
RELATIVE POSITIONS ON A BENZENE RING
1,2-DICHLOROBENZENE
ortho dichlorobenzene
1,4-DICHLOROBENZENE
para dichlorobenzene
The compounds have similar chemical properties but different physical properties
FURTHER SUBSTITUTION OF ARENES
4
1
1
1
2
3

45. FURTHER SUBSTITUTION OF ARENES
Theory It is possible to substitute more than one functional group.
But, the functional group already on the ring affects...
• how easy it can be done • where the next substituent goes
Group ELECTRON DONATING ELECTRON WITHDRAWING
Example(s) OH, CH3 NO2
Electron density of ring Increases Decreases
Ease of substitution Easier Harder
Position of substitution 2,4,and 6 3 and 5
2
6
3
5
4
2
6
3
5
4

46. FURTHER SUBSTITUTION OF ARENES
Examples Substitution of nitrobenzene is...
• more difficult than with benzene
• produces a 1,3 disubstituted product

47. FURTHER SUBSTITUTION OF ARENES
Examples Substitution of methylbenzene is…
• easier than with benzene
• produces a mixture of 1,2 and 1,4
isomeric products

48. FURTHER SUBSTITUTION OF ARENES
Examples Some groups (OH) make substitution so much
easier that multiple substitution takes place

49. THE CHEMISTRY
OF ARENES
THE END
© 2015 KNOCKHARDY PUBLISHING