Benzene and Electrophilic Aromatic Substitution

1. New Way Chemistry for Hong Kong A-Level 3A Benzene and Electrophilic Aromatic Substitution

2. New Way Chemistry for Hong Kong A-Level 3A C6H5CH3 C6H5C2H5 Aromatic Hydrocarbons (Arenes) • Simplest member : Benzene, C6H6 • Characteristic aroma • Alkylbenzene, Cn+6H2n+6 • E.g.

3. Aromatic Hydrocarbons (Arenes) • Generally less dense than water • Insoluble in water but soluble in many organic solvents New Way Chemistry for Hong Kong A-Level 3A

4. Aromatic Hydrocarbons (Arenes) • PAHs : Polycyclic aromatic hydrocarbons • Fused aromatic rings New Way Chemistry for Hong Kong A-Level 3A

5. Aromatic Hydrocarbons (Arenes) • No heteroatoms Not PAH New Way Chemistry for Hong Kong A-Level 3A

6. Aromatic Hydrocarbons (Arenes) • PAHs : Polycyclic aromatic hydrocarbons Sources : - incomplete combustion of wood, coal, diesel, fat, or tobacco Q.38 benzocyclobutadiene New Way Chemistry for Hong Kong A-Level 3A

7. PAHs : Polyaromatic hydrocarbons Toxic and carcinogenic Benzo(a)pyrene : - first carcinogen discovered Found in : - tobacco smoke, - char-grilled food - burnt toast, - edible oils New Way Chemistry for Hong Kong A-Level 3A

8. Preparation of Benzene New Way Chemistry for Hong Kong A-Level 3A

9. 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 New Way Chemistry for Hong Kong A-Level 3A

10. New Way Chemistry for Hong Kong A-Level 3A 1. Destructive Distillation of Coal A laboratory set-up of the destructive distillation of coal

11. Industrial Preparation 2. Catalytic trimerization of ethyne New Way Chemistry for Hong Kong A-Level 3A

12. Pt C6H14  C6H6 + 4H2 500 oC, 10 – 20 atm Industrial Preparation 3. Catalytic Reforming of Petroleum • Converts alkanes and cycloalkanes into aromatic hydrocarbons New Way Chemistry for Hong Kong A-Level 3A

13. Laboratory Synthesis 1. Decarboxylation of Sodium Salt of Benzoic Acid • When sodium benzoate is fused with sodium hydroxide •  the carboxylate group is removed volatile Benzene is separated by fractional distillation

14. 2. Reduction of Phenol • Passing phenol vapour over heated zinc dust (reducing agent) •  produce benzene and zinc(II) oxide volatile Benzene is separated by fractional distillation

15. Reactions of Benzene New Way Chemistry for Hong Kong A-Level 3A

16. Reactivity of Benzene Unreactive towards addition reactions due to stabilization of the system by delocalization of -electrons New Way Chemistry for Hong Kong A-Level 3A

17. Not oxidized by KMnO4 Reactivity of Benzene New Way Chemistry for Hong Kong A-Level 3A

18. Reactivity of Benzene Resistant to electrophilic addition New Way Chemistry for Hong Kong A-Level 3A

19. Resistant to catalytic hydrogenation Reactivity of Benzene New Way Chemistry for Hong Kong A-Level 3A

20. Addition reactions occur only under drastic conditions. New Way Chemistry for Hong Kong A-Level 3A

21. New Way Chemistry for Hong Kong A-Level 3A Occur only under drastic conditions No reaction

22. The -electron cloud is susceptible to electrophilic attack. Substitution is preferred to addition since the former retains aromaticity. Electrophilic aromatic substitution (SE) New Way Chemistry for Hong Kong A-Level 3A

23. New Way Chemistry for Hong Kong A-Level 3A

24. Electrophilic Aromatic Substitution Reactions E+ : electrophile New Way Chemistry for Hong Kong A-Level 3A

25. General Reaction Mechanism Step 1: • Rate determining step New Way Chemistry for Hong Kong A-Level 3A

26. + General Reaction Mechanism • Stabilized by delocalization of  electrons New Way Chemistry for Hong Kong A-Level 3A

27. General Reaction Mechanism Step 2: • The carbocation loses a hydrogen ion •  forms the substitution product New Way Chemistry for Hong Kong A-Level 3A

28. New Way Chemistry for Hong Kong A-Level 3A 1. Halogenation No apparent reation Catalysts : AlCl3, FeCl3 or FeBr3

29. 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. Step 2: • Formation of carbocation intermediate • Rate determining step Stabilized by delocalization of -electrons New Way Chemistry for Hong Kong A-Level 3A

31. Step 3: • The catalyst (FeBr3) is regenerated Fumes of HBr(g) are produced, indicating substitution rather than addition has occurred. New Way Chemistry for Hong Kong A-Level 3A

32. 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+ New Way Chemistry for Hong Kong A-Level 3A

33. acid base 2HNO3 NO3 + H2O + NO2+ electrophile New Way Chemistry for Hong Kong A-Level 3A

34. Optional meta-directing New Way Chemistry for Hong Kong A-Level 3A

35. New Way Chemistry for Hong Kong A-Level 3A H2SO4 + HNO3NO2+ + H2O + HSO4 Q.39

37. + H2SO4

38. 3. Sulphonation (H2SO4 + SO3) • Benzene reacts with fuming sulphuric acid at room temp •  form benzenesulphonic acid + New Way Chemistry for Hong Kong A-Level 3A

39. heat 3. Sulphonation • Sulphonation is a reversible process • By heating an aqueous solution of benzenesulphonic acid to above 100 oC •  benzene and sulphuric acid are formed New Way Chemistry for Hong Kong A-Level 3A

40. New Way Chemistry for Hong Kong A-Level 3A H2SO4 + H2SO4SO3 + H3O+ + HSO4 Q.40

42. + H2SO4

43. 4. Friedel-Crafts Alkylation • When benzene is warmed with a haloalkane in the presence of AlCl3 as a catalyst •  alkylbenzene is formed New Way Chemistry for Hong Kong A-Level 3A

44. 4. Friedel-Crafts Alkylation Important starting step in the manufacture of styrene, phenol and detergents The alkyl group introduced activates the ring towards further alkylation by +ve I-effect.  Friedel-Crafts alkylation is not a good way to prepare alkylbenzene. New Way Chemistry for Hong Kong A-Level 3A

45. deactivating Friedel-Craft acylation Clemensen reduction + 45 New Way Chemistry for Hong Kong A-Level 3A

46. Devise a reaction pathway of not more than four steps for the following conversion. 6B 09/07/2011 46

47. 47

48. New Way Chemistry for Hong Kong A-Level 3A Chlorination BrominationorFriedel-Craft reaction Soda lime to absorb HBr or HCl fumes Br2, RCl or RCOCl or AlCl3

49. New Way Chemistry for Hong Kong A-Level 3A electrophile Q.41 CH3Cl + AlCl3 [CH3]+[AlCl4]