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Polynuclear Aromatic Hydrocarbons

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

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Polynuclear Aromatic Hydrocarbons

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

  2. 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 m o 2 3 3 2 1 1 p p 4 4 5 5 6 6 m o m o 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) • C1=C2=1.36 Å • C2=C3=1.40 Å • 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. 3. • 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

  19. Sulfonation • The lower stability of 1-S is attributed to the steric interaction between the sulfonic group and the hydrogen atom in the 8-position.

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

  21. Homonuclear attack Heteronuclear attack

  22. Examples

  23. Examples

  24. Summary of naphthalene reactions

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

  26. Anthracene (C14H10)  1 8 9 2 7 6 3 4 5 10  • monosubstitution (C14H9X) = 3 isomers • Disubstitution (C14H8X2) = 15 isomers

  27. 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

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

  29. Synthesis of anthracene (b) (c)

  30. Synthesis of anthracene (ii) By Haworth synthesis

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

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

  33. Chemical reactions 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

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

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

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

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

  38. Position of double bond 3 2 4 1 5 6 10 9 7 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

  39. Preparation of phenanthrene 1) Howrth method

  40. 2) Posher synthesis

  41. Preparation of 1- alkyl phenanthrene: Preparation of 2- alkyl phenanthrene:

  42. Oxidation: Reduction:

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

  44. Diphenyl methane (C13H12) o o m m 2 2 3 3 7 1 1 p 4 p 4 5 6 6 5 m o o m Biphenyl methane or diphenyl methane

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

  46. Nitration

  47. Halogenation Oxidation

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

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