THE CHEMISTRY OF ARENES A guide for A level students

1. THE CHEMISTRY OF ARENES A guide for A level students

2. ARENES • CONTENTS • Prior knowledge • Structure of benzene • Thermodynamic stability • Delocalisation • Electrophilic substitution • Nitration • Chlorination • Friedel-Crafts reactions • Further substitution • The chemistry of phenol

3. ARENES • Before you start it would be helpful to… • know the functional groups found in organic chemistry • know the arrangement of bonds around carbon atoms • recall and explain electrophilic addition reactions of alkenes

4. STRUCTURE OF BENZENE Primary analysis revealed benzene had... an empirical formula of CHand a molecular mass of 78 and a formula of C6H6

5. STRUCTURE OF BENZENE Primary analysis revealed benzene had... an empirical formula of CHand a molecular mass of 78 a formula of C6H6 Kekulésuggested that benzene was... PLANAR CYCLIC and HADALTERNATING DOUBLE AND SINGLE BONDS

6. STRUCTURE OF BENZENE HOWEVER... • it did not readily undergo electrophilic addition - no true C=C bond • only one 1,2 disubstituted product existed • all six C—C bond lengths were similar; C=C bonds are shorter than C-C • the ring was thermodynamically more stable than expected

7. STRUCTURE OF BENZENE HOWEVER... • it did not readily undergo electrophilic addition - no true C=C bond • only one 1,2 disubstituted product existed • all six C—C bond lengths were similar; C=C bonds are shorter than C-C • the ring was thermodynamically more stable than expected To explain the above, it was suggested that the structure oscillated between the two Kekulé forms but was represented by neither of them. It was a RESONANCE HYBRID.

8. 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.

9. 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

10. 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

11. 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

12. 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 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

13. 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 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

14. 2p 2 2s 1 1s HYBRIDISATION OF ORBITALS - REVISION The electronic configuration of a carbon atom is 1s22s22p2

15. 2p 2p 2 2 2s 2s 1 1 1s 1s HYBRIDISATION OF ORBITALS - REVISION The electronic configuration of a carbon atom is 1s22s22p2 If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3 The process is favourable because of the arrangement of electrons; four unpaired and with less repulsion is more stable

16. HYBRIDISATION OF ORBITALS - REVISION The four orbitals (an s and three p’s) combine or HYBRIDISE to give four new orbitals. All four orbitals are equivalent. 2s22p2 2s12p3 4 x sp3 HYBRIDISE sp3HYBRIDISATION

17. HYBRIDISATION OF ORBITALS - REVISION Alternatively, only three orbitals (an s and two p’s) combine or HYBRIDISE to give three new orbitals. All three orbitals are equivalent. The remaining 2p orbital is unchanged. 2s22p2 2s12p3 3 x sp2 2p HYBRIDISE sp2HYBRIDISATION

18. STRUCTURE OF ALKENES - REVISION In ALKANES, the four sp3 orbitals repel each other into a tetrahedral arrangement. In ALKENES, the three sp2 orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to them

19. STRUCTURE OF ALKENES - REVISION Covalent bonds are formed by overlap of orbitals. An sp2 orbital from each carbon overlaps to form a single C-C bond. The resulting bond is called a SIGMA (δ) bond.

20. STRUCTURE OF ALKENES - REVISION The two 2p orbitals also overlap. This forms a second bond; it is known as a PI (π) bond. For maximum overlap and hence the strongest bond, the 2p orbitals are in line. This gives rise to the planar arrangement around C=C bonds.

21. ORBITAL OVERLAP IN ETHENE - REVIEW two sp2 orbitals overlap to form a sigma bond between the two carbon atoms two 2p orbitals overlap to form a pi bond between the two carbon atoms s orbitals in hydrogen overlap with the sp2 orbitals in carbon to form C-H bonds the resulting shape is planar with bond angles of 120º

22. 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 planarstructure. 6 single bonds

23. 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 planarstructure. 6 single bonds one way to overlap adjacent p orbitals

24. 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 planarstructure. 6 single bonds one way to overlap adjacent p orbitals another possibility

25. 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 planarstructure. 6 single bonds one way to overlap adjacent p orbitals another possibility delocalised pi orbital system

26. 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 planarstructure. 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.

27. STRUCTURE OF BENZENE

28. STRUCTURE OF BENZENE ANIMATION The animation doesn’t work on early versions of Powerpoint

29. WHY ELECTROPHILIC ATTACK? TheoryThe 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.

30. WHY SUBSTITUTION? TheoryAddition to the ring would upset the delocalised electron system Substitution of hydrogen atoms on the ring does not affect the delocalisation Overall there isELECTROPHILIC SUBSTITUTION STABLE DELOCALISED SYSTEM ELECTRONS ARE NOT DELOCALISED AROUND THE WHOLE RING - LESS STABLE

31. ELECTROPHILIC SUBSTITUTION TheoryThe 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

32. ELECTROPHILIC SUBSTITUTION TheoryThe 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

33. H H E H + H H H H H H H + E+ + H+ H H H E H H H H

34. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagentsconc. nitric acid and conc. sulphuric acid(catalyst) Conditionsreflux at 55°C EquationC6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzene

35. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagentsconc. nitric acid and conc. sulphuric acid(catalyst) Conditions reflux at 55°C EquationC6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzene Mechanism

36. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagentsconc. nitric acidand conc. sulphuric acid(catalyst) Conditionsreflux at 55°C EquationC6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzene Mechanism ElectrophileNO2+ ,nitronium ionor nitryl cation; it is generated in an acid-base reaction... 2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2+ acid base

37. ELECTROPHILIC SUBSTITUTION REACTIONS - NITRATION Reagentsconc. nitric acid and conc. sulphuric acid(catalyst) Conditions reflux at 55°C EquationC6H6 + HNO3 ———> C6H5NO2 + H2O nitrobenzene Mechanism ElectrophileNO2+ ,nitronium ionor nitryl cation; it is generated in an acid-base reaction... 2H2SO4 + HNO3 2HSO4¯ + 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.

38. ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION Reagentschlorine and a halogen carrier(catalyst) Conditionsreflux 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 EquationC6H6 + Cl2 ———> C6H5Cl + HCl Mechanism ElectrophileCl+it is generated as follows... Cl2 + FeCl3 FeCl4¯ + Cl+ a Lewis Acid

39. ELECTROPHILIC SUBSTITUTION REACTIONS - HALOGENATION Reactivity of Halogenarenes Compared with halogenalkanes (e.g. chloroethane), the C-X bond In halogenarenes is shorter and thus stronger: This is because the lone-pair on the halogen overlap with the p System of the aromatic ring and delocalisation occurs between the C-Cl atoms too; thus partial double bond character. Nucleophilic substitution is thus more difficult [requires 600K and 2 x 104 kPa pressure with NaOH(aq)].

40. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION OverviewAlkylation 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+) EquationC6H6 + C2H5Cl ———> C6H5C2H5 + HCl

41. FRIEDEL-CRAFTS REACTIONS OF BENZENE - ALKYLATION OverviewAlkylation 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+) EquationC6H6 + C2H5Cl ———> C6H5C2H5 + HCl Mechanism GeneralA 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.

42. 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 + AlCl3 AlCl4¯ + R+

43. FURTHER SUBSTITUTION OF ARENES TheoryIt 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, CH3 NO2 Electron density of ring Increases Decreases Ease of substitution Easier Harder Position of substitution 2,4,and 6 3 and 5

44. FURTHER SUBSTITUTION OF ARENES ExamplesSubstitution 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

45. FURTHER SUBSTITUTION OF ARENES Methylbenzene undergoes (a) electrophilic substitution reactions in the ring or (b) free-radical substitution in the methyl group (the alkyl substituent). (a)Electrophilic Substitution reaction: chlorination reaction With methylbenzene, a mixture of 2-chloromethylbenzene and 4-chloromethylbenzene is mostly; only 5% is 3-chloromethylbenzene. The methyl group is electron releasing (+I effect) and helps to stabilize the positive charge further during the reaction.

46. FURTHER SUBSTITUTION OF ARENES Methylbenzene undergoes (a) electrophilic substitution reactions in the ring or (b) free-radical substitution in the methyl group (the alkyl substituent). (b) Free Radical Substitution reaction: chlorination reaction Similar to methane, methylbenzene with chlorine in the presence of U.V. light will give mostly chloromethylbenzene and a mixture of further substitution products:

47. 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

48. b) The mixture is acidified to form benzoic acid which is only slightly soluble in aqueous solution. C6H5COO-(aq) + H+(aq) C6H5COOH(S) OXIDATION OF THE SIDE CHAIN In addition to substitution reactions, the methyl group can be oxidised to benzoic acid In the presence of hot alkaline potassium manganate(VII), the methyl side-chain can be oxidised to benzoic acid: • The KMnO4 is made alkaline using aqueous sodium carbonate. The purple colour of aqueous • manganate (VII) becomes a cloudy brown as MnO2 is formed (and can be removed by filtration).

49. CHEMISTRY OF PHENOL • (d) Recall the chemistry of phenol, as exemplified by the following reactions: • With bases • With sodium • Bromination of, and nitration of, the aromatic ring • (e) Explain the relative acidities of water, phenol and ethanol

50. CHEMISTRY OF PHENOL Extra delocalisation in phenol