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Aromatic Hydrocarbons

Chapter 31. Aromatic Hydrocarbons. 31.1 Introduction 31.2 Nomenclature of the Derivatives of Benzene 31.3 The Stability of Benzene 31.4 Physical Properties of Aromatic Hydrocarbons 31.5 Preparation of Benzene 31.6 Reactions of Benzene 31.7 Alkylbenzenes. 31.1 Introduction (SB p.147).

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Aromatic Hydrocarbons

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  1. Chapter 31 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

  2. 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

  3. 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

  4. 31.2 Nomenclature of the Derivatives of Benzene (SB p.148) (b) For other compounds, the substituent and the benzene ring taken together may form a new parent name

  5. 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.

  6. 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.

  7. 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

  8. 31.2 Nomenclature of the Derivatives of Benzene (SB p.149) Solution: (a) (b) (c) 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

  9. 31.2 Nomenclature of the Derivatives of Benzene (SB p.150) (a) 1,2-Dimethylbenzene (b) 1-Methyl-2-nitrobenzene or 2-nitrotoluene (c) 3-Bromo-5-chlorobenzoic acid (d) 4-Bromo-2,6-dinitrophenol Check Point 31-1 Give the IUPAC name for each of the following compounds: (a) (b) (c) (d) Answer

  10. 31.3 The Stability of Benzene (SB p.151) In 1865, Kekule proposed the structure of benzene:

  11. 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!!

  12. 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

  13. 31.3 The Stability of Benzene (SB p.152) Enthalpy Changes of Hydrogenation of Benzene and Cyclohexene Enthalpy change of hydrogenation of cyclohexene = –119.6 kJ mol–1

  14. 31.3 The Stability of Benzene (SB p.152) The enthalpy change of hydrogenation of 1,3-cyclohexadiene is expected to be twicethat of cyclohexene

  15. 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

  16. 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

  17. 31.3 The Stability of Benzene (SB p.153) The Resonance Explanation of the Structure of Benzene 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

  18. 31.3 The Stability of Benzene (SB p.154) All carbon atoms in benzene are sp2-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

  19. 31.3 The Stability of Benzene (SB p.154) The delocalization of  electrons gives benzene extra stability and determines the chemical properties of benzene

  20. 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

  21. 31.3 The Stability of Benzene (SB p.155) Structure of Methylbenzene • All C atoms in the ring is sp2-hybridized • The C atom in the methyl group is sp3-hybridized • The delocalized  electron clouds give rise to extra stability

  22. 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

  23. 31.4 Physical Properties of Aromatic Hydrocarbons (SB p.156)

  24. 31.4 Physical Properties of Aromatic Hydrocarbons (SB p.156)

  25. 31.5 Preparation of Benzene (SB p.157) Industrial Preparation Catalytic Reforming of Alkanes Catalytic reforming converts alkanes and cycloalkanes into aromatic hydrocarbons e.g. Pt C6H14 C6H6 + 4H2 500°C, 10 – 20 atm

  26. 31.5 Preparation of Benzene (SB p.157) Destructive Distillation of Coal • 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 methylbenzenecan be obtained

  27. 31.5 Preparation of Benzene (SB p.158) Laboratory Synthesis Decarboxylation of Sodium Salt of Benzoic Acid When sodium benzoate is fused with sodium hydroxide, the carboxylate group is removed and benzene is formed

  28. 31.5 Preparation of Benzene (SB p.158) Reduction of Phenol Phenol vapour is passed slowly over heated zinc dust to produce benzene and zinc(II) oxide

  29. 31.6 Reactions of Benzene (SB p.158) Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene

  30. 31.6 Reactions of Benzene (SB p.159)

  31. 31.6 Reactions of Benzene (SB p.159) • Methylbenzene is highly unsaturated, but it is resistant to oxidation and addition reactions • The resistance of oxidation and addition reactions of aromatic compounds is used to distinguish from unsaturated alkenes • Methylbenzene reacts with Br2 in the presence of FeBr3. It is through substitution reaction

  32. 31.6 Reactions of Benzene (SB p.160) Electrophilic Aromatic Substitution Reactions Most characteristic reaction of aromatic compounds: Electrophilic 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 (+)

  33. 31.6 Reactions of Benzene (SB p.160) Nitration • Conc. H2SO4increases the rate of reaction by increasing the concentration of the electrophile, NO2+ (nitronium ion)

  34. 31.6 Reactions of Benzene (SB p.160) Sulphonation • Benzene reacts with fuming sulphuric(VI) acid at room temperature to give benzenesulphonic acid • Heating aqueous solution of benzenesulphonic acid above 100°C, benzene and sulphuric(VI) acid are formed

  35. 31.6 Reactions of Benzene (SB p.161) Halogenation Benzene reacts with chlorine and bromine in the presence of catalysts such as AlCl3, FeCl3,FeBr3, to give chlorobenzene and bromobenzene respectively

  36. 31.6 Reactions of Benzene (SB p.161) Alkylation • When benzene is warmed with a haloalkane in the presence of catalysts such as AlCl3, an alkylbenzene is formed • Important step in chemical industry to produce polystyrene, phenol and detergents

  37. 31.6 Reactions of Benzene (SB p.162) Solution: (a) (b) conc. H2SO4, conc. HNO3 (c) fuming H2SO4 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

  38. 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) 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

  39. 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.

  40. 31.7 Alkylbenzenes (SB p.163) This type of oxidation is limited to those molecules with Alkylbenzenes are oxidized to benzoic acid by strong oxidizing agents such as hot alkaline potassium manganate(VII)

  41. 31.7 Alkylbenzenes (SB p.163) Examples:

  42. 31.7 Alkylbenzenes (SB p.164) The C = C double bondand acyl groups in the side chain are oxidized by hot alkaline potassium manganate(VII) e.g.

  43. 31.7 Alkylbenzenes (SB p.164) Example 31-3 State the conditions under which methylbenzene can be converted in the laboratory to (a) C6H5CH2Cl (b) C6H5COOH Answer Solution: (a) Reagent: Cl2 Condition: in the presence of light (b) Reagent: (1) KMnO4–, OH–, (2) H3O+ Condition: heating under reflux

  44. 31.7 Alkylbenzenes (SB p.164) 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

  45. 31.7 Alkylbenzenes (SB p.164) Two different types of chlorination reaction of methylbenzene are: Type I: free radical substitution reaction Type II: electrophilic aromatic substitution reaction

  46. The END

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