2. New Way Chemistry for Hong Kong A-Level Book 3A1 Aromatic Hydrocarbons 31.1Introduction 31.2Nomenclature of the Derivatives of Benzene 31.3The Stability of Benzene 31.4Physical Properties of Aromatic Hydrocarbons 31.5Preparation of Benzene 31.6Reactions of Benzene 31.7Alkylbenzenes Chapter 31

3. New Way Chemistry for Hong Kong A-Level Book 3A2 31.1 Introduction (SB p.147) Benzene Highly unsaturated Six-membered ring compound with alternative single and double bonds between adjacent carbon atoms Chemically unreactive compared to alkenes

4. New Way Chemistry for Hong Kong A-Level Book 3A3 31.2 Nomenclature of the Derivatives of Benzene (SB p.148) 1.Monosubstituted benzenes (a)For certain compounds, benzene is the parent name and the substituent is simply indicated by a prefix

5. New Way Chemistry for Hong Kong A-Level Book 3A4 (b)For other compounds, the substituent and the benzene ring taken together may form a new parent name 31.2 Nomenclature of the Derivatives of Benzene (SB p.148)

6. New Way Chemistry for Hong Kong A-Level Book 3A5 31.2 Nomenclature of the Derivatives of Benzene (SB p.148) 2.Polysubstituted benzenes (a) If more than one substituent are present and the substituents are identical, their relative positions are indicated by the use of numbers assigned on the ring. The prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on are used.

7. New Way Chemistry for Hong Kong A-Level Book 3A6 31.2 Nomenclature of the Derivatives of Benzene (SB p.149) (b)When more than one substituent are present and the substituents are different, they are listed in alphabetical order.

8. New Way Chemistry for Hong Kong A-Level Book 3A7 31.2 Nomenclature of the Derivatives of Benzene (SB p.149) (c)When a substituent is one that when taken together with the benzene ring gives a new parent name, that substituent is assumed to be in position 1 and the new parent name is used

9. New Way Chemistry for Hong Kong A-Level Book 3A8 Example 31-1 Draw the structural formula for each of the following compounds: (a)1,3,5-Trichlorobenzene (b)2,5-Dibromophenol (c)2,4-Dinitrobenzoic acid Answer 31.2 Nomenclature of the Derivatives of Benzene (SB p.149) Solution: (a)(b) (c)

10. New Way Chemistry for Hong Kong A-Level Book 3A9 Check Point 31-1 Give the IUPAC name for each of the following compounds: (a)(b) (c)(d) 31.2 Nomenclature of the Derivatives of Benzene (SB p.150) Answer (a)1,2-Dimethylbenzene (b)1-Methyl-2-nitrobenzene or 2-nitrotoluene (c)3-Bromo-5-chlorobenzoic acid (d)4-Bromo-2,6-dinitrophenol

11. New Way Chemistry for Hong Kong A-Level Book 3A10 31.3 The Stability of Benzene (SB p.151) In 1865, Kekule proposed the structure of benzene:

12. New Way Chemistry for Hong Kong A-Level Book 3A11 31.3 The Stability of Benzene (SB p.151) According to the Kekulé structure, there should be two different 1,2-dibromobenzenes: Only one 1,2-dibromobenzene has been found!!

13. New Way Chemistry for Hong Kong A-Level Book 3A12 31.3 The Stability of Benzene (SB p.151) According to the Kekulé structure, benzene should undergo addition reactions readily it gave substitution reaction products rather than addition reaction products  Kekulé structure cannot explain the behaviour of benzene

14. New Way Chemistry for Hong Kong A-Level Book 3A13 31.3 The Stability of Benzene (SB p.152) Enthalpy change of hydrogenation of cyclohexene = –119.6 kJ mol –1 Enthalpy Changes of Hydrogenation of Benzene and Cyclohexene

15. New Way Chemistry for Hong Kong A-Level Book 3A14 31.3 The Stability of Benzene (SB p.152) The enthalpy change of hydrogenation of 1,3-cyclohexadiene is expected to be twice that of cyclohexene

16. New Way Chemistry for Hong Kong A-Level Book 3A15 31.3 The Stability of Benzene (SB p.152) If benzene has the structure of 1,3,5-cyclohexatriene, The enthalpy change of hydrogenation is expected to be three times as much as that of cyclohexene

17. New Way Chemistry for Hong Kong A-Level Book 3A16 31.3 The Stability of Benzene (SB p.153) Benzene is more stable than Kekulé structure The energy difference for the stabilization of benzene is called resonance energy of benzene

18. New Way Chemistry for Hong Kong A-Level Book 3A17 31.3 The Stability of Benzene (SB p.153) From X-ray crystallography, The length of carbon-carbon bond in benzene is intermediate between C – C bond and C = C bond 0.134 nm > 0.139 nm > 0.154 nm C = C carbon bond in benzene C – C The Resonance Explanation of the Structure of Benzene

19. New Way Chemistry for Hong Kong A-Level Book 3A18 31.3 The Stability of Benzene (SB p.154) All carbon atoms in benzene are sp 2 -hybridized The side-way overlap of unhybridized 2p orbitals on both sides gives a delocalized  electron cloud above and below the plane of the ring

20. New Way Chemistry for Hong Kong A-Level Book 3A19 31.3 The Stability of Benzene (SB p.154) The delocalization of  electrons gives benzene extra stability and determines the chemical properties of benzene

21. New Way Chemistry for Hong Kong A-Level Book 3A20 31.3 The Stability of Benzene (SB p.154) Structural formula of benzene: The circle represents the six electrons that are delocalized about the six carbon atoms of the benzene ring

22. New Way Chemistry for Hong Kong A-Level Book 3A21 31.3 The Stability of Benzene (SB p.155) All C atoms in the ring is sp 2 -hybridized The C atom in the methyl group is sp 3 -hybridized The delocalized  electron clouds give rise to extra stability Structure of Methylbenzene

23. New Way Chemistry for Hong Kong A-Level Book 3A22 31.4 Physical Properties of Aromatic Hydrocarbons (SB p.155) Physical properties of aromatic hydrocarbons: have a fragrant smell generally less dense than water at 20°C usually immiscible with water soluble in organic solvents

24. New Way Chemistry for Hong Kong A-Level Book 3A23 31.4 Physical Properties of Aromatic Hydrocarbons (SB p.156) NameFormula Boiling point (°C) Melting point (°C) Density at 20°C (g cm –3 ) Benzene80.15.50.878 Methylbenzene111–950.867 Ethylbenzene136–940.867

25. New Way Chemistry for Hong Kong A-Level Book 3A24 31.4 Physical Properties of Aromatic Hydrocarbons (SB p.156) NameFormula Boiling point (°C) Melting point (°C) Density at 20°C (g cm –3 ) 1,2- Dimethylbenzene 144–25.20.880 1,3- Dimethylbenzene 139–47.40.864 1,4- Dimethylbenzene 13813.30.861

26. New Way Chemistry for Hong Kong A-Level Book 3A25 31.5 Preparation of Benzene (SB p.157) Catalytic reforming converts alkanes and cycloalkanes into aromatic hydrocarbons e.g. Industrial Preparation Catalytic Reforming of Alkanes Pt C 6 H 14  C 6 H 6 + 4H 2 500°C, 10 – 20 atm

27. New Way Chemistry for Hong Kong A-Level Book 3A26 31.5 Preparation of Benzene (SB p.157) Heating coal in the absence of air gives out coal gas, ammoniacal liquor, coal tar and coke Coal tar is a mixture of many organic compounds, mainly aromatic ones Benzene and methylbenzene can be obtained Destructive Distillation of Coal

28. New Way Chemistry for Hong Kong A-Level Book 3A27 31.5 Preparation of Benzene (SB p.158) When sodium benzoate is fused with sodium hydroxide, the carboxylate group is removed and benzene is formed Decarboxylation of Sodium Salt of Benzoic Acid Laboratory Synthesis

29. New Way Chemistry for Hong Kong A-Level Book 3A28 31.5 Preparation of Benzene (SB p.158) Phenol vapour is passed slowly over heated zinc dust to produce benzene and zinc(II) oxide Reduction of Phenol

30. New Way Chemistry for Hong Kong A-Level Book 3A29 31.6 Reactions of Benzene (SB p.158) Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene Reaction Cyclohexane (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon) Action of Br 2 in CH 3 Cl 3 (in dark) No reactionBr 2 decolourized and no HBr evolved No reaction with Br 2 alone In the presence of FeBr 3, Br 2 decolourized and HBr fumes evolved Action of H 2 (with Ni catalyst) No reaction1 mole of cyclohexene reacts with 1 mole of H 2 at room temperature 1 mole of methylbenzene reacts 3 moles of H 2 at high temperature and pressure

31. New Way Chemistry for Hong Kong A-Level Book 3A30 31.6 Reactions of Benzene (SB p.159) Reaction Cyclohexane (a saturated alicyclic hydrocarbon) Cyclohexene (an unsaturated alicyclic hydrocarbon) Methylbenzene (an aromatic hydrocarbon) Action of acidified KMnO 4 No reactionKMnO 4 decolourized No reaction Action of conc. HNO 3 and conc. H 2 SO 4 No reactionCyclohexene oxidized and colour darkens A yellow liquid is formed

32. New Way Chemistry for Hong Kong A-Level Book 3A31 Methylbenzene is highly unsaturated, but it is resistant to oxidation and addition reactions 31.6 Reactions of Benzene (SB p.159) The resistance of oxidation and addition reactions of aromatic compounds is used to distinguish from unsaturated alkenes Methylbenzene reacts with Br 2 in the presence of FeBr 3. It is through substitution reaction

33. New Way Chemistry for Hong Kong A-Level Book 3A32 31.6 Reactions of Benzene (SB p.160) Electrophilic Aromatic Substitution Reactions The electrophiles attack the benzene ring, replacing one of the hydrogen atoms in the reaction Electrophiles are either a positive ion (E + ) or some other electron-deficient species with a partial positive charge (  +) Most characteristic reaction of aromatic compounds: Electrophilic substitution reactions

34. New Way Chemistry for Hong Kong A-Level Book 3A33 31.6 Reactions of Benzene (SB p.160) Conc. H 2 SO 4 increases the rate of reaction by increasing the concentration of the electrophile, NO 2 + (nitronium ion) Nitration

35. New Way Chemistry for Hong Kong A-Level Book 3A34 31.6 Reactions of Benzene (SB p.160) Benzene reacts with fuming sulphuric(VI) acid at room temperature to give benzenesulphonic acid Sulphonation Heating aqueous solution of benzenesulphonic acid above 100°C, benzene and sulphuric(VI) acid are formed

36. New Way Chemistry for Hong Kong A-Level Book 3A35 31.6 Reactions of Benzene (SB p.161) Benzene reacts with chlorine and bromine in the presence of catalysts such as AlCl 3, FeCl 3, FeBr 3, to give chlorobenzene and bromobenzene respectively Halogenation

37. New Way Chemistry for Hong Kong A-Level Book 3A36 31.6 Reactions of Benzene (SB p.161) When benzene is warmed with a haloalkane in the presence of catalysts such as AlCl 3, an alkylbenzene is formed Alkylation Important step in chemical industry to produce polystyrene, phenol and detergents

38. New Way Chemistry for Hong Kong A-Level Book 3A37 Example 31-2 Complete each of the following by supplying the missing reactant or product as indicated by the question mark: (a) (b) (c) Answer 31.6 Reactions of Benzene (SB p.162) Solution: (a) (b)conc. H 2 SO 4, conc. HNO 3 (c)fuming H 2 SO 4

39. New Way Chemistry for Hong Kong A-Level Book 3A38 Check Point 31-2 (a)One mole of benzene reacts with three moles of chlorine under special conditions. What is the reaction condition required for the reaction? (b)Draw the structure of the reaction product in (a). Answer 31.6 Reactions of Benzene (SB p.162) (a)UV radiation or diffuse sunlight must be present for the free radical addition reaction to take place. (b)

40. New Way Chemistry for Hong Kong A-Level Book 3A39 31.7 Alkylbenzenes (SB p.162) Alkylbenzenes are a group of aromatic hydrocarbons in which an alkyl group is bonded directly to a benzene ring also known as arenes e.g.

41. New Way Chemistry for Hong Kong A-Level Book 3A40 31.7 Alkylbenzenes (SB p.163) Alkylbenzenes are oxidized to benzoic acid by strong oxidizing agents such as hot alkaline potassium manganate(VII) This type of oxidation is limited to those molecules with

42. New Way Chemistry for Hong Kong A-Level Book 3A41 31.7 Alkylbenzenes (SB p.163) Examples:

43. New Way Chemistry for Hong Kong A-Level Book 3A42 31.7 Alkylbenzenes (SB p.164) The C = C double bond and acyl groups in the side chain are oxidized by hot alkaline potassium manganate(VII) e.g.

44. New Way Chemistry for Hong Kong A-Level Book 3A43 Example 31-3 State the conditions under which methylbenzene can be converted in the laboratory to (a)C 6 H 5 CH 2 Cl (b)C 6 H 5 COOH Answer Solution: (a)Reagent: Cl 2 Condition: in the presence of light (b)Reagent: (1) KMnO 4 –, OH –, (2) H 3 O + Condition: heating under reflux 31.7 Alkylbenzenes (SB p.164)

45. New Way Chemistry for Hong Kong A-Level Book 3A44 Check Point 31-3 Methylbenzene undergoes two different types of chlorination reaction by different mechanisms. Compare the two different types of chlorination reaction in terms of reaction conditions as well as the products formed. Answer 31.7 Alkylbenzenes (SB p.164)

46. New Way Chemistry for Hong Kong A-Level Book 3A45 Two different types of chlorination reaction of methylbenzene are: Type I: free radical substitution reaction Type II: electrophilic aromatic substitution reaction 31.7 Alkylbenzenes (SB p.164)

47. New Way Chemistry for Hong Kong A-Level Book 3A46 The END