1. Polynuclear Aromatic Hydrocarbons
Ref. books
Organic Chemistry, Vol I.L. Finar
Organic Chemistry Morrison and Boyd
Advanced Organic Chemistry – Bahl and Bahl
Organic Chemistry Herbert Meislich

2. Polynuclear Hydrocarbons
Polynuclear aromatic hydrocarbons are composed by two or more benzene rings
Polynuclear Hydrocarbons
Benzenoid
Non- Benzenoid
Isolated
Fused rings
Linear
Angular

3. Benzenoid: Similar to benzene in structure or linkage; having an aromatic ring system.
Fused or condensed ring system: When two rings share a pair of carbon atoms, the rings are said to be fused rings.
Isolated ring o m o m 2 3 3 2 1 1 p p 4 4 5 6 5 6 m o o m
Biphenyl or diphenyl

4. Naphthalene (C10H8) Shows aromatic properties
Satisfy Huckel’s rule (4n+2)
=(4*2+2)=10

5. All C=C are not same (X-ray diffraction study)
Resonance energy of naphthalene is 61 Kcal/mol
Benzene, 36 Kcal/mol
2nd aromatic ring is less stable (61-36)=25 Kcal/mol
Naphthalene is less aromatic (more reactive) than benzene

6. Structure elucidation of naphthalene
1. Molecular Formula: C10H8 2.
So naphthalene contains the skeleton

7. So nitro group is present in benzene ring3.
So nitro group is present in benzene ring 4.
The benzene ring in phthalic acid produced by oxidation of aminonaphthalene is not the same ring is that obtained by oxidation of nitronaphthalene.

8. i.e. Naphthalene contains two benzene rings and we can explain this by this equation

9. The structure of naphthalene is confirmed by method of its synthesis
Howarth method

10. Other way of cyclization

11. The reaction occurs if R is o- or p- directing group such as NH2, NHR, OH, OR, R, halogen.
If R is m- directing group (e.g. NO2, CN, COOH, COCH3, SO3H) no reaction occur.
The above reaction gives -substituted naphthalene.

12. Synthesis of 1-alkyl naphthalene

13. From -benzylidene – propenoic acid

14. Reduction

15. Oxidation

16. Addition of Cl2

17. Electrophilic substitution reaction
Naphthalene undergoes ES mostly at alpha-position
Resonance forms determine higher reactivity at C-1
C-1 attack has 2 resonance structures with benzene rings
C-2 attack has only 1 resonance structure with a benzene ring
The most stable intermediate (C-1 attack) gives faster reaction
Attack at C-1
Attack at C-2

18. At position 1; carbocation intermediate stabilize by two resonance
So carbocation is more stable position 1 than 2

20. Sulfonation
The lower stability of 1-S is attributed to the steric interaction between the sulfonic group and the hydrogen atom in the position.

21. Substituted naphthalene
Activating groups direct the electrophile to the same ring; i.e. Elctrodonating group (EDG): NH2, OH, OR, alkyl
Deactivating groups direct it to the other ring; i.e. Electrowithdrawing group (EWG): NO2, CO, COOH, CN, SO3H

22. Homonuclear attack
Heteronuclear attack

23. Examples

24. Examples

25. Summary of naphthalene reactions

26. Anthracene (C14H10)  8 9 1 7 2 6 3 5 10 4 

27. Anthracene (C14H10)   monosubstitution (C14H9X) = 3 isomers8 2 7 3 6 4 10 5 
monosubstitution (C14H9X) = 3 isomers
Disubstitution (C14H8X2) = 15 isomers

28. Anthracene (C14H10)
C1-C2 bond to have more double bond character (shorter bond length)
C2-C3 bond to have more single bond character (longer bond length)
From X-ray diffraction study: C1-C2 bond = 1.37 Å
C2-C3 bond = 1.42 Å
Resonance energy 84 kcal mol-1, average 28, less aromatic than benzene

29. Synthesis of anthracene
(i) By Friedel Crafts reaction
(a)

30. Synthesis of anthracene
(b)
(c)

31. Synthesis of anthracene
(ii) By Haworth synthesis

32. Synthesis of anthracene
(iii) By Diels-Alder reaction

33. Chemical reactions Attack at C-1 Attack at C-2
Leaves naphthalene intact
Loss of RE=84-61=23 kcal
Attack at C-2

34. Chemical reactions Reactions preferentially occur at C-9 & C-10
Attack at C-9
Leaves two benzene intact
Loss of RE=84-72 =12 kcal
Substitution product
Addition product
Reactions preferentially occur at C-9 & C-10

35. Chemical reactions
Diels Alder reaction
Addition of one molecule of O2

36. [HNO3+H2SO4 is not used, leads formation of 9,10 anthraqunone by oxidation]

37. Phenanthrene C14H10 6 5 7 4 8 3 2 9 1 10

38. Phenanthrene C14H10 3 4 2 5 1 6 10 7 8 9

39. Phenanthrene C14H10 monosubstitution (C14H9X) = 5 isomers
Disubstitution (C14H8X2) = 25 isomers 3 4 2 5 1 6 9 10 10 7 8 1 8 9 7 2 6 5 4 3 8 9

40. Position of double bond3 2 4 1 5 6 10 7 9 8
C9-C10 bond to have more double bond character
RE 92 kcal/mole, 92-72=20 Kcal/mole to remove the aromaticity of the middle ring

41. Preparation of phenanthrene
1) Howrth method

42. 2) Posher synthesis

43. Preparation of 1- alkyl phenanthrene:

44. Oxidation:
Reduction:

46. EAS in anthracene or phenanthrene yields mixtures and is not generally useful. For example, in sulfonation:

47. Diphenyl methane (C13H12) o o m m 2 2 3 3 7 1 1 p p 4 4 6 6 55 o o m m
Biphenyl methane or diphenyl methane

48. Methods of preparation
1. Friedel- Crafte
2. From benzophenone

49. Nitration

50. Halogenation
Oxidation

51. Stilbene
(C6H5-CH=CH-C6H5)
Trans-stilbebe
stable
Cis-stilbebe
unstable

52. Syntheis of trans-stilbene
C6H5CHOHCH2C6H5

53. Syntheis of trans-stilbene
(II)
C6H5CHOHCOC6H5
(III)
-Phenylcinnamic acid

54. Reactions of trans-stilbene
C6H5CH2CH2C6H5
bibenzyl
Stilbebe dibromide
Dphenyl acetylene

55. Synthesis of cis-stilbene
Cis-stilbebe is readily converted into trans-stilbebe under the catalytic influence of traces of hydrogen bromide and peroxides