1. A2 Chemistry F324 - Rings, Acids and Analysis
Arenes
Plymstock School
P.J.McCormack

2. Empirical & Molecular Formulae
Lesson 1
Empirical & Molecular Formulae
25 February 2019
A hydrocarbon is found to contain 92.3% carbon and has a molecular mass of 78. Work out the empirical and molecular formula of the hydrocarbon. Draw this structure.
Empirical formula = CH
Molecular formula = C6H6
C6H6 = benzene
P.J.McCormack

3. Arenes
25 February 2019
Arenes are compounds that contain a benzene ring or derivatives of benzene obtained by replacing hydrogen’s by other groups or fusing benzene rings together, e.g.
2,4,6 Trinitrotoluene
(TNT)
P.J.McCormack

4. Learning Objectives By the end of the lesson I will be able to…
25 February 2019
Learning Objectives
By the end of the lesson I will be able to…
Draw the skeletal formula of benzene;
Describe the bonding with benzene;
Compare and contrast the Kekule structure of benzene with the delocalised model
Key words: Delocalised, Kekule, orbitals, electrophilic
P.J.McCormack

5. The Structure of Benzene
25 February 2019
Benzene Animation
P.J.McCormack

6. Kekulé Structure.
25 February 2019
The molecular formula was found to be C6H6 therefore the structure must be unsaturated. In 1865 Kekulé proposed a ring structure and drew the structure cyclohexa-1,3,5-triene.
Kekulé structure suggests that benzene;
Has alternating double and single bonds
Would react in the same way as cyclohexene (undergo electrophilic addition).
Would have three short bonds (double) and three longer bonds (single) bonds.
P.J.McCormack

7. 25 February 2019
P.J.McCormack

8. Problems with Kekulé’s Model.
25 February 2019
Problems with Kekulé’s Model.
X-ray diffraction shows that benzene is planar which concurs with Kekulé’s structure but:-
The molecule is a regular hexagon of carbon atoms, with six equal bond lengths.
C-C in cyclohexane = 0.154nm
C=C in cyclohexene = 0.133nm
C-C bond in benzene = 0.139nm
We find that benzene has a constant bond length, somewhere between a single and double bond length.
Benzene does not behave like an alkene. It reacts by electrophilic substitution rather than by electrophilic addition, even though it is unsaturated.
P.J.McCormack

9. Problems with Kekulé’s Model.
25 February 2019
KeKulé Benzene (cyclohexa-1,3,5-triene) was drawn as, a highly symmetrical hexagon, but using the x-ray diffraction evidence is should be
Benzene is drawn to represents the delocalisation of electrons within the ring system.
P.J.McCormack

10. Problems with Kekulé’s Model.
25 February 2019
Problems with Kekulé’s Model.
The theoretical heat of formation of benzene is +252 kJmol-1 (Kekulé’s structure).
The experimental value is +82 kJmol-1. The structure is more stable (endothermic) than Kekulé’s structure. The extra stability and equivalent carbon-carbon bond length can be explained by delocalisation.
P.J.McCormack

11. Properties of Benzene. Planar Highly symmetrical
25 February 2019
Properties of Benzene.
Planar
Highly symmetrical
Non-polar (evenly distributed charge).
Immiscible with water.
Boiling point 80C
Melting point 6C
Reacts by electrophilic substitution.
Carcinogenic.
Charge Density
P.J.McCormack

12. Benzene and its Derivatives
25 February 2019
Benzene and its Derivatives
Give three properties of benzene.
What is the empirical formula of benzene?
What evidence suggests that the structure of benzene is not the Kekulé structure?
What is the main mechanism of reaction of benzene?
P.J.McCormack

13. 25 February 2019
Bonding in Benzene.
The carbon atoms are bonded to one another and to their hydrogen atoms by sigma bonds.
This leaves one unused p orbital on each carbon, each containing a single electron. These p orbitals are perpendicular to the plane of the ring, with one lobe above and one below the plane.
P.J.McCormack

14. Delocalisation
25 February 2019
Each p orbital overlaps sideways with two neighbouring orbitals to form a single  bond that extends as a ring of charge above and below the plane of the molecule.
P.J.McCormack

15. Bonding in Arenes.
25 February 2019
The carbon atoms in the ring are bonded to one another and to their hydrogen atoms by  bonds.
This leaves one unused p orbital on each carbon, each containing a single electron.
The p orbitals are perpendicular to the plane of the ring above and below the plane of the molecule.
Each p orbital overlaps sideways with the two neighbouring orbitals to form a single bond that extends as a ring.
P.J.McCormack

16. 25 February 2019
Delocalisation.
Electrons tend to repel one another, so a system when they are far apart as possible will involve minimum repulsion and will therefore be stabilised.
Delocalisation of the  electrons has a profound affect on the both physical and chemical properties.
P.J.McCormack

17. Bonding in Benzene. 6 electrons in a delocalised  bond C
25 February 2019
Bonding in Benzene. C H
6 electrons in a delocalised  bond
above and below the plane of the atoms
P.J.McCormack

18. delocalisation energy
Energy Diagram
25 February 2019
-152 kJmol-1
delocalisation energy
Enthalpy
(kJ mol-1)
+ 3H2
-360
theoretical
value
+ 2H2
-240
+ 3H2
+ H2
+ H2
-120
real value
-208
-120
P.J.McCormack

19. Hydrogenation of Benzene
25 February 2019
Hydrogenation of Benzene
Hydrogenation of cyclohexene – one double bond
H = -120 KJmol-1
Hydrogenation of Kekule’s benzene – three double bonds
Expected value H = -360 KJmol-1 (theoretical value)
Hydrogenation benzene – delocalised system
Hrxn = -208 KJmol-1
P.J.McCormack

20. 25 February 2019
Benzene is more stable than the Kekule model as less energy is given out
P.J.McCormack

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) 2 3
- 120 kJ mol-1

22. 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

23. 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

24. 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

25. 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

26. Reactions of Arenes.
25 February 2019
An arene has a percentage composition of 94.38% carbon and 5.62% hydrogen. The molecular mass of this compound is 178. Determine the empirical, molecular masses and the structure of the arene.
Answers;
Empirical formula = C7H5
Molecular formula = C14H10
P.J.McCormack

27. Learning Objectives. By the end of the lesson I will be able to....
25 February 2019
Learning Objectives.
By the end of the lesson I will be able to....
All..
State the name of the mechanism by which benzene reacts;
Draw the mechanism for the reactivity of benzene;
State the equation for the formation of a nitronium ion
Most..
Explain what a halogen carrier is
Some..
State the equation with conditions for the alkylation of benzene
Low
High
Key Words: Halogenation, benzene, halogen carrier, electrophile
P.J.McCormack

28. General Electrophilic Substitution Mechanism.
25 February 2019
General Electrophilic Substitution Mechanism.
E+ is the electrophile (an electron deficient species). The attack comes from the benzene ring (centre of high electron density). The mechanism is firstly an addition reaction then followed by an elimination reaction.
P.J.McCormack

29. Key Terminology What is a nitronium ion
25 February 2019
Key Terminology
What is a nitronium ion
What the nitration mixture required for nitration of benzene?
What are the conditions required for nitration of benzene?
Give the equation (2) for the formation of the nitronium ion.
What is the mechanism for nitration of benzene?
NO2+
C. H2SO4 + c. HNO3
50 degree C
H2SO4 + HNO3 = HSO4- + H2NO3+
H2NO3+ = NO2+ + H2O
Electrophilic substitution
P.J.McCormack

30. Electrophilic Substitution.
Nitration of benzene
C6H6
+ HNO3
C6H5NO2
+ H2O
Conditions / Reagents
concentrated HNO3
and concentrated H2SO4
50oC
Mechanism
25 February 2019
P.J.McCormack

31. Electrophilic Substitution Mechanism (nitration)
1. Formation of NO2 +
the nitronium ion
HNO3
+ 2H2SO4
NO2 +
+ 2HSO4-
+ H3O+
2. Electrophilic attack on benzene
NO2 + +
NO2 H
O SO3H-
NO2
3. Forming the product
and re-forming the catalyst
H O SO3H
25 February 2019
Reaction Equation
P.J.McCormack

32. Nitration of Benzene.
Benzene undergoes electrophilic substitution reactions. The substitution of a hydrogen atom for a nitronium ion (NO2+).
This reaction occurs in several steps. First the nitronium ion is formed.
HNO3 + H2SO4  H2NO3+ + HSO4-
H2NO3+  NO H2O
The reaction mechanism is:
Arenium ion
25 February 2019
P.J.McCormack

33. Friedel-Crafts Reactions.
Alkylarenes are made by using a halogen carrier and an halogenoalkane to bring about the substitution reaction of a delocalised ring.
As in the reaction with halogens, the halogen carrier (aluminium chloride) accepts an electron pair from the chlorine atom, polarising the chloromethane molecule.
CH3---Cl:---AlCl3
 -
The positively charged methyl group attacks the delocalised ring and electrophilic substitution occurs. This is an example of a Friedel-Crafts reaction.
25 February 2019
P.J.McCormack

34. Reaction Mechanism. Reflux A carbocation is formed CH3+ - +
25 February 2019
Reaction Mechanism. -
Reflux +
A carbocation is formed CH3+
P.J.McCormack

35. 25 February 2019
Reactions of Arenes.
What are the conditions and reagents needed for the following:
Nitration of benzene?
Alkylation of benzene
2. Give the reactions to show the formation of a nitronium ion.
3. Show the reaction sequence for the formation of propylbenzene.
4. What is a halogen carrier?
P.J.McCormack

36. Alkenes & Benzene.
25 February 2019
Alkylbenzene can be made from a halogenoalkane using the Freidel-Crafts catalyst. A cheaper method is to use an alkene.
Phenylethene is a very useful monomer. What is the industrial name for phenylethene?
styrene
P.J.McCormack

37. Arenes Revisited. Show the mechanism for the nitration of benzene.
25 February 2019
Show the mechanism for the nitration of benzene.
Give a Freidel-Crafts reaction to show the alkylation of benzene using a halogenoalkane.
Explain the relative stability of benzene compared to cyclohexene.
Name the following arenes.
P.J.McCormack

38. Learning Objectives. By the end of the lesson I will be able to....
25 February 2019
Learning Objectives.
By the end of the lesson I will be able to....
All..
State the mechanism for the monohalogenation of benzene;
State a suitable halogen carrier required to form chlorobenzene and bromobenzene;
Most..
Explain the function of a halogen carrier
Some..
Explain the resistance of benzene to halogenation
Low
High
Key Words: Halogenation, benzene, halogen carrier, electrophile
P.J.McCormack

39. Reactivity of Benzene and Alkenes
25 February 2019
Reactivity of Benzene and Alkenes
Give the mechanism for the reaction of cyclohexene with bromine water.
Give the mechanism for the reaction of benzene with bromine
Explain the relative resistance of benzene to halogenation in terms of its structure compared to the halogenation of cyclohexene
P.J.McCormack

40. Halogenation of Benzene
25 February 2019
Halogenation of Benzene
Benzene does not react with halogens on its own, but will react in the presence of a halogen carrier. Examples of halogen carriers catalysts are FeCl3, FeBr3, AlCl3, AlBr3. The reaction is an electrophilic substitution reaction as with the nitration of benzene where one halogen replaced a hydrogen on the benzene ring.
P.J.McCormack

41. Halogenation of Benzene
25 February 2019
Halogenation of Benzene
Benzene will react with bromine in a similar way except the halogen carrier catalyst is FeBr3, AlBr3.
P.J.McCormack

42. Halogenation of Benzene
25 February 2019
Halogenation of Benzene
The halogen carrier generates the bromonium ion Br+, which is a more powerful electrophile than Br2. Br2 + FeBr3  Br+ + FeBr4-
H+ + FeBr4-  FeBr3 + HBr
P.J.McCormack

43. Learning Objectives. All.. Draw the structure of phenol; Most..
25 February 2019
Learning Objectives.
By the end of the lesson I will be able to....
All..
Draw the structure of phenol;
Most..
Describe the reactions of phenol with aqueous alkalis and with sodium to form salts;
Describe the reaction of phenol with bromine to form 2,4,6-tribromophenol
Some..
Compare and contrast the reaction of bromine with phenol and benzene;
Low
High
Key Words: Phenol, phenoxide ion, bromine, TCP
P.J.McCormack

44. Phenol & Sodium. Phenol reacts vigorously with sodium. What is formed?
25 February 2019
Phenol reacts vigorously with sodium. What is formed?
The greater reactivity of phenol compared to ethanol is due to the weak acidity of phenol.
P.J.McCormack

45. Electrophilic Reactions of Phenol.
25 February 2019
Electrophilic Reactions of Phenol.
Phenol (in sodium hydroxide) reacts with aqueous bromine room temperature to give 2,4,6-tribromophenol, which is a white precipitate. The reaction does not require a catalyst.
The reason why phenol reacts much faster than benzene is that the lone pair of electrons on the oxygen atom becomes part of the delocalised  system. The  system is extended over seven atoms rather than six, therefore electron density is increased making the ring more reactive.
Highly activating
P.J.McCormack

46. Phenol.
Phenol is a weak acid but neutralises strong alkalis. What is the product of phenol and sodium hydroxide?
Sodium phenoxide is an ionic compound. The phenol dissolves completely in NaOH, but is only sparingly soluble in water.
What is formed when an acid is added to sodium phenoxide?
25 February 2019
P.J.McCormack

47. Acidity of Phenol.
Phenol is slightly acidic, considerably more so than water and aliphatic alcohols. If we consider the following equilibrium we can explain why phenols are acidic.
In the case of the phenoxide ion, the negative charge on the oxygen can to some extent be delocalised round the ring.
One of the p orbitals on the oxygen atom, which contains two electrons, interacts slightly with the singly occupied p orbital on the adjacent carbon atom.
Phenol C6H5OH
Mpt. 43C
Bpt 181C
Mpt. ethanol = -117  C
25 February 2019
P.J.McCormack

48. 25 February 2019
Phenol
Phenols are a class of organic compounds where a hydroxyl group (-OH) is attached directly to the benzene ring.
Phenol is a solid at room temperature and pressure
It is slightly soluble in water as the –OH group can hydrogen bond with water but the benzene ring makes it less soluble than alcohols
P.J.McCormack

49. Relative Ease of Bromination of Phenol
25 February 2019
Relative Ease of Bromination of Phenol
The reaction of bromine with phenol does not involve a halogen carrier like benzene.
This means that something in its structure must make in more reactive than benzene.
The electron density in phenol is greater than that of benzene
The difference between benzene and phenol is the OH group.
The oxygen atom in the OH group has a lone pair of p orbital electrons.
These are able to delocalise with the p electrons in the benzene ring:
P.J.McCormack

50. 25 February 2019
Acidity of Phenol.
This reduces the tendency of the phenoxide ion to attract protons, in other words it reduces its strength as a base.
Consequently, phenol is a stronger acid Ka = 1.3x10-10) than aliphatic alcohols, though it is still weak. (Weak acids have low Ka values).
Acidic strength:
Phenol (more acidic) > water > ethanol
Ka = x10-10 mol dm > 1x10-14 mol dm > x10-18 mol dm-3
P.J.McCormack

51. Delocalised Phenol.
25 February 2019
Delocalisation also occurs in this ion. One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring.
This overlap leads to a delocalisation which extends from the ring out over the oxygen atom. As a result, the negative charge is no longer entirely localised on the oxygen, but is spread out around the whole ion.
P.J.McCormack

52. 25 February 2019
Solubility of Phenol.
Phenol is sparingly soluble in water. The –OH hydrogen bonds to water, whilst the benzene ring reduces the solubility because it only forms weak Van der Waals’ forces to other molecules.
P.J.McCormack

53. Recap. What is a phenoxide ion?
25 February 2019
What is a phenoxide ion?
Why is phenol more acidic than ethanol?
Why is phenol more reactive than benzene with Br2?
Describe the reactivity of phenol and benzene with aqueous bromine.
Explain, and show how you would make cumene from benzene.
P.J.McCormack

54. 25 February 2019
Task 1
Produce a mind map summarising all the reactions of benzene and phenol and the bonding within each molecule.
P.J.McCormack

55. 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

56. 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

57. 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

58. 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

59. 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.

60. STRUCTURE OF BENZENE

61. STRUCTURE OF BENZENE
ANIMATION

62. 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.

63. 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
STABLE DELOCALISED SYSTEM
ELECTRONS ARE NOT DELOCALISED
AROUND THE WHOLE RING - LESS STABLE

64. 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

65. 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

66. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO3 ———> C6H5NO H2O
nitrobenzene

67. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO3 ———> C6H5NO H2O
nitrobenzene
Mechanism

68. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO3 ———> C6H5NO H2O
nitrobenzene
Mechanism
Electrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...
2H2SO HNO HSO4¯ H3O NO2+
acid base

69. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION
Reagents conc. nitric acid and conc. sulphuric acid (catalyst)
Conditions reflux at 55°C
Equation C6H HNO3 ———> C6H5NO H2O
nitrobenzene
Mechanism
Electrophile NO2+ , nitronium ion or nitryl cation; it is generated in an acid-base reaction...
2H2SO HNO HSO4¯ 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.

70. 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+ a
Lewis Acid

71. 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 C6H C2H5Cl ———> C6H5C2H HCl

72. 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 C6H C2H5Cl ———> C6H5C2H 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.

73. 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 AlCl AlCl4¯ R+

74. 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 C6H 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

75. 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 C6H CH3COCl ———> C6H5COCH HCl
Mechanism
Product A carbonyl compound (aldehyde or ketone)

76. 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, CH NO2
Electron density of ring Increases Decreases
Ease of substitution Easier Harder
Position of substitution 2,4,and and 5

77. FURTHER SUBSTITUTION OF ARENES
Examples Substitution of nitrobenzene is...
• more difficult than with benzene
• produces a 1,3 disubstituted product
Substitution of methylbenzene is…
• easier than with benzene
• produces a mixture of 1,2 and 1,4
isomeric products
Some groups (OH) make substitution so much
easier that multiple substitution takes place

78. RELATIVE POSITIONS ON A BENZENE RING ortho dichlorobenzene
STRUCTURAL ISOMERISM
RELATIVE POSITIONS ON A BENZENE RING
1,2-DICHLOROBENZENE
ortho dichlorobenzene
1,3-DICHLOROBENZENE
meta dichlorobenzene
1,4-DICHLOROBENZENE
para dichlorobenzene
Compounds have similar chemical properties but different physical properties