Chemistry

1. Chemistry

2. AROMATICS

3. Session objectives • Nomenclature of aromatic hydrocarbon • Structure of benzene • Aromaticity mechanisms of electrophilic of aromatic substituents and orientation • Polynuclear hydrocarbons and their toxicity.

4. Nomenclature of aromatic compound

5. Nomenclature of aromatic compound

6. Structure of benzene Molecular formula C6H6 shows presence of three double bonds But both are same

7. Structure of benzene Support in favour of resonance hybrid of benzene • The heat of hydrogenation of benzene is 36 kcal/mol. • Length of carbon-carbon bond in benzene is (1.39Å), whereas carbon-carbon single bond length is 1.54Å and carbon-carbon double bond is 1.34Å.

8. Structure of benzene Molecular structure of benzene

9. Structure of benzene

10. Aromaticity Class of compounds having a characteristic stability Conditions for aromaticity • Complete delocalization ofp-electrons of the ring system. • Planarity: Delocalization of p-electrons is possible only if the ring is planar to allow cyclic overlap of p-orbitals. • Huckel’s rule for aromaticity:According to Huckel’s rule for a compound to be aromatic, a molecule must have (4n + 2) electrons (n=0 to any number)

11. Aromaticity

12. Aromaticity

13. Preparation of Benzene Hydroformation or catalytic reforming From neptha From neptha From neptha

14. Preparation of Benzene From acetylene From benzenediazonium chloride

15. Preparation of Benzene

16. Double distillation of coal Coal light oil fraction i) washed with conc. H2SO4 ii) washed with NaOH iii) washed with H2O iv) dried and distilled Benzene + Toluene +Thiophene Fractionless distillation Benzene (80 – 820C)

17. Properties Electrophilic aromatic substitution reaction of benzene Halogenation

18. Mechanism of Halogenation

19. Properties Nitration

20. Mechanism of Nitration

21. Mechanism of Nitration

22. Properties Sulphonation

23. Mechanism of Sulphonation

24. Mechanism of Sulphonation

25. Properties Friedel-Craft’s reaction (a) friedel–craft alkylation (a) friedel–craft acylation

26. Mechanism of Friedel–Craft’s reaction

27. Mechanism of Friedel–Craft’s reaction

28. Limitations of Friedel Craft alkylation • The danger of poly substitution. • The possibility that the alkyl group will rearrangement. • Aryl halides cannot take place of alkyl halides. • An aromatic ring less reactive than that of the halobenzenes does not undergo the Friedel Crafts reaction.

29. Limitations of Friedel Craft alkylation • Aromatic rings containing the — NH2, — NHR or — NR2 group do not undergo Friedel Crafts alkylation because the strongly basic nitrogen ties up the Lewis acid.

30. Addition reaction of benzene Addition of hydrogen Addition of halogens

31. Oxidation

32. Oxidation

33. Oxidation

34. Ozonolysis of benzene

35. Influence of Substituents on Elecrophilic Aromatic Substitution Activates Deactivates

36. Influence of Substituents on Elecrophilic Aromatic Substitution or or

37. .. .. : : Influence of Substituents on Elecrophilic Aromatic Substitution

38. – – – + + + Influence of Substituents on Elecrophilic Aromatic Substitution

39. Ortho–Para and meta directors

40. Polynuclear Hydrocarbon and their Toxicity

41. Class exercise

42. Class exercise 1 In the reaction of C6H5Y, the major product (> 60%) is m-isomer, so the group Y is (a) — COOH (b) — Cl (c) — OH (d) — NHCOCH3 Solution: — COOH is m-directing group. Hence, the answer is (a)

43. Class exercise 2 Which of the following is the strongest o, p-directing group? (a) –OH (b) –Cl (c) –OCH3 (d) –CH3 Solution: Because the lone pair of electrons of OCH3 takes part in resonance with benzene ring. Hence it is stronger ortho-para directing group. Hence, the answer is (c)

44. Class exercise 3 • Aromatic compounds burn with a sooty flame because • they have a ring • they have a relatively high percentage of hydrogen • they have a relatively high percentage of carbon • Of structure of carbon atoms Solution: Aromatic compounds have relativelyhigh percentage of carbon. Hence, the answer is (c)

45. Class exercise 4 • Benzene is less reactive than ethene and ethyne towards addition reactions due to • the presence of • the cyclic nature • the sp2 hybridization of carbon atoms • the delocalization of Delocalization of reduce the reactivity of benzene. Solution: Hence, the answer is (d)

46. Class exercise 5 Which of the following has the highest melting point? (a) o-xylene (b) m-xylene(c) p-xylene (d) toluene Solution: p-xylene being symmetrical packs closely in the crystal lattice. Hence, the answer is (c)

47. Class exercise 6 Which of the following types of compounds are expected to have the highest octane number? (a) Straight chain alkanes (b) Cycloalkanes(c) Branched chain alkanes(d) Aromatic hydrocarbons Solution: Aromatic hydrocarbons have high octane number. Hence, the answer is (d)

48. Class exercise 7 The chemical system that is non-aromatic is (a) (b) (c) (d) The compound B has no but other are aromatic compounds. Solution: Hence, the answer is (b)

49. Class exercise 8 What is the ratio of bonds in benzene? (a) 1 : 4 (b) 2 : 1 (c) 1 : 1 (d) 2 : 2 In benzene Number of -bonds = 3 Number of s-bonds = 12 Ratio = 3 : 12 = 1 : 4 Solution: Hence, the answer is (a)

50. Class exercise 9 Which one of the following is not compatible with arenes? (a) Greater stability (b) Delocalization of (c) Electrophilic addition(d) Resonance Solution: Arenes do not give electrophilic addition because they have stable compounds. Hence, the answer is (c)