1. Reactions of arenes

2. Benzene and bromine can react together in an electrophilic substitution reaction.

3. Iron filings or iron(III) bromide are needed as a catalyst. If iron filings are used they react with bromine to form iron(III) bromide.

4. When benzene reacts with bromine, bromobenzene and fumes of hydrogen bromide are produced. The bromine is decolourised in the reaction.

5. Let’s look at the mechanism of the reaction.

6. The bromine molecule becomes polarised when it comes near to the benzene ring.

7. The iron(III) bromide accepts a pair of electrons from the polarised bromine molecule.

8. The covalent bond in the bromine molecule breaks, forming Br + and FeBr 4 -.

9. Br + is an electrophile. It forms a covalent bond with benzene.

10. An unstable intermediate ion is formed.

11. A hydrogen ion H + is eliminated from the unstable intermediate, forming bromobenzene.

12. The hydrogen ion reacts with FeBr 4 - to form hydrogen bromide, HBr.

13. Iron(III) bromide is regenerated, so it acts as a catalyst in the reaction.

14. Benzene and chlorine can react together in a substitution reaction.

15. Aluminium chloride is used as a catalyst. The reaction must be carried out in anhydrous conditions because aluminium chloride reacts violently with water.

16. Chlorobenzene and fumes of hydrogen chloride are produced.

17. Let’s look at the mechanism of the reaction.

18. The chlorine molecule becomes polarised when it comes near to the benzene ring.

19. The aluminium chloride accepts a pair of electrons from the polarised chlorine molecule.

21. The covalent bond in the chlorine molecule breaks, forming Cl + and AlCl 4 -.

23. Cl + is an electrophile. It forms a covalent bond with benzene.

24. An unstable intermediate is formed.

25. A hydrogen ion H + is eliminated from the unstable intermediate, forming chlorobenzene.

26. The hydrogen ion reacts with AlCl 4 - to form hydrogen chloride, HCl.

27. Aluminium chloride is regenerated, so it acts as a catalyst in the reaction.

28. Benzene reacts with a mixture of concentrated nitric acid and sulfuric acid at around 50°C.

29. In the reaction sulfuric acid acts as a homogeneous catalyst (it is in the same phase as the reactants).

30. Nitrobenzene and water are produced. This is a nitration reaction and an example of an electrophilic substitution reaction.

31. Sulfuric acid is a stronger acid than nitric acid and donates a proton to it.

33. The protonated nitric acid is unstable and breaks down to form the nitronium ion, NO 2 +.

34. The nitronium ion is the electrophile that attacks the benzene molecule in nitration.

35. This is the overall equation for the reaction that forms the nitrating mixture.

37. Let’s look at the mechanism of nitration of benzene.

38. Remember that the nitronium ion is the electrophile.

39. A pair of electrons from the delocalised electrons in benzene forms a covalent bond with the nitronium ion.

40. An unstable intermediate ion is formed.

41. A hydrogen ion H + is eliminated from the unstable intermediate.

44. Nitrobenzene is formed. Nitration of benzene and related compounds is an important step in the manufacture of explosives and dyes.

45. Benzene reacts with concentrated sulfuric acid when heated under reflux conditions for several hours.

47. Water and benzenesulfonic acid are formed.

48. What is the electrophile in this reaction?

49. The electrophile in sulfonation is SO 3.

50. Oxygen is more electronegative than sulfur and so the S=O bonds are polar.

51. The sulfur atom has a large partial positive charge allowing the SO 3 to act as an electrophile.

52. Benzenesulfonic acid is a strong acid and will react with alkalis to form salts.

53. It reacts with sodium hydroxide to form sodium benzenesulfonate.

54. Most solid detergents comprise salts like this, with an alkyl chain attached to the benzene ring.

55. Friedel-Crafts reactions can be used to react benzene with halogenoalkanes.

56. Aluminium chloride is used as a catalyst.

57. On warming, the alkyl group from the halogenoalkane is substituted for a hydrogen atom in benzene.

58. For example, if chloromethane is used a methyl group CH 3 can be introduced.

60. The organic product of this reaction is methylbenzene, often called toluene.

62. Let’s look at the mechanism for the reaction.

63. The aluminium atom in aluminium chloride has a vacant orbital. It can form a dative covalent bond with a chloride ion, formed when the bond in the halogenoalkane breaks.

64. A carbocation R + forms, which can act as an electrophile.

67. A pair of electrons from the delocalised electrons in benzene forms a covalent bond with the electrophile.

69. An unstable intermediate ion is formed.

70. A hydrogen ion H + is eliminated from the unstable intermediate.

71. The hydrogen ion reacts with the AlCl 4 - ion.

72. This forms hydrogen chloride and regenerates the aluminium chloride catalyst.

73. Friedel-Crafts reactions can be used to react benzene with acyl chlorides.

74. Aluminium chloride is used as a catalyst.

75. On warming, an aromatic ketone is formed.

76. For example, if ethanoyl chloride is used a COCH 3 group can be introduced.

79. The organic product of this reaction is phenylethanone.

81. Let’s look at the mechanism for the reaction.

82. The aluminium atom in aluminium chloride has a vacant orbital. It can form a dative covalent bond with a chloride ion, formed when the bond in the acyl chloride breaks.

84. An acylium ion forms, which can act as an electrophile.

85. A pair of electrons from the delocalised electrons in benzene forms a covalent bond with the electrophile.

87. An unstable intermediate ion is formed.

88. A hydrogen ion H + is eliminated from the unstable intermediate.

89. The hydrogen ion reacts with the AlCl 4 - ion.

90. This forms hydrogen chloride and regenerates the aluminium chloride catalyst.

91. Friedel-Crafts acylation reactions can also be carried out using ionic liquids as a combined catalyst and solvent system.

92. The Q in the formula represents one of a range of organic cations, such as the 1-ethyl-3-methylimidazolium ion.

94. Ionic liquids have low volatility, so emissions are reduced.

95. They have low flammability and toxicity, so they are safer to use.

96. Friedel-Crafts reactions can be carried out at lower temperatures, saving energy.

97. Ionic liquids are easily recycled, saving money and resources.