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  1. 1. Chapter 17 Spring 2009
  2. 2. Benzene <ul><li>Six-membered ring </li></ul><ul><li>Is planar </li></ul><ul><li>All C-C bond lengths are equal </li></ul>Kekulé structure Resonance description of benzene Implication of the geometry of the structure above Shorter isolated double bonds Longer single bonds Common representation of benzene
  3. 3. Benzene <ul><li>Benzene is the parent aromatic molecule. </li></ul><ul><li>Benzene has the following characteristics that are common among all aromatic compounds </li></ul><ul><li>Benzene is cyclic and conjugated </li></ul><ul><li>Benzene is unusually stable in comparison to a molecule with three isolated pi bonds </li></ul><ul><li>Benzene is planar and has the shape of a regular hexagon. All bond angles are 120º, all carbon atoms are sp 2 hybridized, and all carbon-carbon bond lengths are intermediate in length between a single and double bond. </li></ul><ul><li>Benzene undergoes substitution reactions rather than electrophilic addition reactions that would destroy its conjugation. </li></ul>
  4. 4. Larger aromatic hydrocarbons
  5. 5. Pictures of Benzene Electrostatic potential surface Benzene molecular orbital In top picture notice a p orbital on each C atom. Bottom picture show the overlap of the p orbitals.
  6. 6. Orbital picture of benzene each carbon atom is sp 2 hybridized The resonance picture indicates why each C-C bond is the same length.
  7. 7. Electrostatic potential surface Electron-rich portion represents π electron cloud
  8. 8. For a compound to be classified as aromatic, it must fulfill both of the of the following criteria: <ul><li>It must have an uninterrupted cyclic cloud of π electrons above and below the plane of the molecule (this is often called the π cloud. Also, the π cloud must be cyclic , uninterrupted , and planar . </li></ul><ul><li>The p cloud must contain an odd number of pairs of π electrons. (4n+2 rule) </li></ul>
  9. 9. Common names for aromatic compounds
  10. 10. IUPAC names <ul><li>To name benzene ring with one substituent, name the substituent and add the word benzene. </li></ul>ethyl benzene ?
  11. 11. <ul><li>If the alkane chain is longer than six carbon atoms name the benzene ring as a phenyl substituent. </li></ul>
  12. 12. Benzyl Benzyl substituent
  13. 13. Disubstituted benzenes 1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene IUPAC names
  14. 14. Same substituent meta-dichlorobenzene or, 1,3-dichlorobenzene
  15. 15. Different substituents These two have common root names
  16. 16. Different substituents These two have common root names toluene analine
  17. 17. Different substituents These two have common root names meta-chlorotoluene ortho-chloroanaline
  18. 18. Different substituents These don’t have a common root name so alphabetize the name of the substituents.
  19. 19. Different substituents These don’t have a common root name so alphabetize the name of the substituents. ortho-bromochlorobenzene para-bromonitrobenzene
  20. 20. Heat of hydrogenation data The low heat of hydrogenation means that benzene is especially stable – this extra stability is characteristic of all aromatic compounds.
  21. 21. Reaction of aromatic compounds <ul><li>Benzene does not undergo addition reactions typical of other unsaturated hydrocarbons. </li></ul>
  22. 22. What makes a compound aromatic? <ul><li>A molecule must be cyclic </li></ul><ul><li>Planar </li></ul><ul><li>Completely conjugated (alternating single-double bonds) </li></ul><ul><li>Have a particular number of π electrons </li></ul>
  23. 23. This molecule is non-aromatic It is cyclic, completely conjugated, non-planar, and contains 8 π electrons
  24. 24. This molecule is anti-aromatic Cyclic, planar, completely conjugated, and contain four π electrons.
  25. 25. Aromatic, anti-aromatic, or non-aromatic <ul><li>Aromatic: A cyclic planar, completely conjugated compound with 4n+2 π electrons </li></ul><ul><li>Anti-aromatic: A cyclic, planar, completely conjugated compound with 4n π electrons </li></ul><ul><li>Non-aromatic: A compound that lack one (or more) of the following requirements for aromaticity. </li></ul>
  26. 26. Aromatic compounds can have more than one ring.
  27. 27. Aromatic rings can contain a single ring with lots of alternating single-double bonds
  28. 28. atomic orbitals molecular orbitals
  29. 29. Anti-aromatic
  30. 30. The inscribed polygon method <ul><li>Draw the polygon in question inside a circle with its vertices touching the circle and one of the vertices pointing down. Mark the points at which the polygon intersects the circle. </li></ul><ul><li>Draw a line horizontally through the center of the circle and label MOs as bonding, nonbonding, or antibonding. </li></ul><ul><li>Add the electrons, beginning with the lowest energy MO. </li></ul>
  31. 31. Label the cation, radical, and anion as aromatic, antiaromatic, or nonaromatic.
  32. 32. Heterocyclic aromatic compounds
  33. 34. Nucleic acids
  34. 35. Thymine
  35. 37. Aromatic Question Thiamin, or vitamin B1, contains a five-membered, nitrogen-sulfur heterocycle called a thiazalium ring. Explain why the thiazolium ring is aromatic.
  36. 38. Cycloheptatrienyl cation, radical and anion problem