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Arenes and Aromaticity

Aromatic Compounds. Arenes: hydrocarbons based on the benzene ring as a structural unit.Aromatic Compounds: Compounds possessing more stability than would be predicted due to having conjugated double bonds.. Benzene. Benzene has been known since 1825. It was discovered by Michael Faraday. The

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Arenes and Aromaticity

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    1. Chapter 11 Arenes and Aromaticity

    2. Aromatic Compounds Arenes: hydrocarbons based on the benzene ring as a structural unit. Aromatic Compounds: Compounds possessing more stability than would be predicted due to having conjugated double bonds.

    3. Benzene Benzene has been known since 1825. It was discovered by Michael Faraday. The empirical formula was determined by Eirlhardt Mitscherlich. Structure worked out by August Kekule in 1866.

    4. Resonance Picture of Benzene The two equivalent Kekule structures represent a resonance hybrid shown by the right hand structure.

    5. Stability of Benzene

    6. The Orbital Hybridization View of Benzene

    7. Molecular Orbitals of Benzene

    8. Nomenclature of Benzene Derivatives Monosubstituted Benzene Compounds are often named by attaching the name of the substitutent as a prefix of benzene. Some Monosubstituted Benzene Derivatives have long standing common names.

    9. Some benzene derivatives have long standing common names.

    10. Disubstituted Benzene Derivatives ortho- 1,2 disubstituted benzene meta- 1,3 disubstituted benzene para- 1,4 disubstituted benzene

    11. Trisubstituted Benzene Derivatives o,m,p are not used when three or more substituents are on the benzene ring. Instead the ring is numbered and the base name is that of the “ranking” substituent. The substituents are named in alphabetical order. The numbering is such that the next substituted position is given the lowest number. Ex: Try pick one p. 467 #27 and p. 468 #28 b.

    12. Benzene as a Substituent Phenyl- = C6H5- Benzyl- = C6H5CH2- Biphenyl

    13. Polycyclic Aromatic Compounds These compounds are composed of benzene rings fused together. They have substantial resonance energy. (See 11.30) Napthalene Anthracene Phenanthrene

    14. Physical Properties of Arenes Arenes are similar to hydrocarbons in physical properties. Nonpolar Insoluble in Water Less Dense than Water

    15. Reactions of Arenes Two main Classes of Reaction: Reactions of the Ring itself: Reduction Electrophilic Aromatic Substitution 2. Reactions in which the ring influences reactivity of a functional unit to which it is attached.

    16. Birch Reduction Reduction of the benzene ring by treatment with sodium and methanol in liquid ammonia. It is used to synthesize dienes.

    17. Birch Reduction Mechanism

    18. Free Radical Halogenation of Alkylbenzenes Benzylic Radicals are especially stable. Therefore free radical attack favors the benzylic position. The benzylic radical is analogous to the allylic radical.

    19. Free Radical Halogenation

    20. Spin Density Representation of Benzyl Radical

    21. lowM.O. View of Benzyl Radical (lowest energy M.O.)

    22. NBS Bromination (lab prep)

    23. Oxidation of Alkyl Benzenes

    24. Nucleophilic Substitution of Benzylic Halides Benzylic halides make ideal substrates for SN2 reactions because they react readily toward good nucleophiles and cannot undergo competing elimination.

    25. Primary Benzylic Halides Mechanism is SN2

    26. Preparation of Alkenylbenzenes Industrial Prep: Dehydrogenation of Ethylbenzene

    27. Laboratory Preparation of Alkenylbenzenes Acid catalyzed dehydration or base promoted elimination reactions of appropriate leaving groups will produce alkneylbenzenes

    28. Dehydration of Benzyl Alcohols

    29. Dehydrohalogenation

    30. Addition Reactions of Alkenylbenzenes Hydrogenation

    31. Addition Reactions of Alkenylbenzenes Halogenation

    32. Addition Reactions of Alkenylbenzenes Hydrohalogenation

    33. Polymerization of Styrene

    34. Polymerization of Styrene (Mechanism)

    35. Huckel’s Rule Planar, monocyclic, fully conjugated polyenes, only with (4n+2) p electrons will have a special stability (be aromatic). Planar, monocyclic, fully conjugated polyenes with (4n) p electrons are especially unstable and are therefore antiaromatic.

    37. Annulenes Annulenes are planar, monocyclic, completely conjugated polyenes. That is, they are the kind of hydrocarbons treated by Hückel's rule. Annulenes are named in this manner: [x] annulene where x = # of carbons. Many have been prepared to test Huckel’s rule.

    43. Aromatic Ions Some nonaromatic compounds will form stabilized aromatic ions.

    44. Aromatic Ions

    45. Heterocyclic Aromatic Compounds

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