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Terrence P. Sherlock Burlington County College 2004

CHE 242 Unit VI The Study of Conjugated Systems, Aromaticity, and Reactions of Aromatic Compounds CHAPTER SIXTEEN. Terrence P. Sherlock Burlington County College 2004. =>. Resonance Structure.

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Terrence P. Sherlock Burlington County College 2004

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  1. CHE 242Unit VIThe Study of Conjugated Systems, Aromaticity, and Reactions of Aromatic CompoundsCHAPTER SIXTEEN Terrence P. Sherlock Burlington County College 2004

  2. => Resonance Structure Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. Chapter 16

  3. Unusual Reactions • Alkene + KMnO4 diol (addition)Benzene + KMnO4 no reaction. • Alkene + Br2/CCl4  dibromide (addition)Benzene + Br2/CCl4  no reaction. • With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain. => Chapter 16

  4. Unusual Stability Hydrogenation of just one double bond in benzene is endothermic! => Chapter 16

  5. Annulenes • All cyclic conjugated hydrocarbons were proposed to be aromatic. • However, cyclobutadiene is so reactive that it dimerizes before it can be isolated. • And cyclooctatetraene adds Br2 readily. • Look at MO’s to explain aromaticity. => Chapter 16

  6. MO Rules for Benzene • Six overlapping p orbitals must form six molecular orbitals. • Three will be bonding, three antibonding. • Lowest energy MO will have all bonding interactions, no nodes. • As energy of MO increases, the number of nodes increases. => Chapter 16

  7. Aromatic Requirements • Structure must be cyclic with conjugated pi bonds. • Each atom in the ring must have an unhybridized p orbital. • The p orbitals must overlap continuously around the ring. (Usually planar structure) • Compound is more stable than its open-chain counterpart. => Chapter 16

  8. Anti- and Nonaromatic • Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart. • Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar. => Chapter 16

  9. Hückel’s Rule • If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 electrons, it is aromatic. • If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic. => Chapter 16

  10. [N]Annulenes • [4]Annulene is antiaromatic (4N e-’s) • [8]Annulene would be antiaromatic, but it’s not planar, so it’s nonaromatic. • [10]Annulene is aromatic except for the isomers that are not planar. • Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar. => Chapter 16

  11. Cyclopentadienyl Ions • The cation has an empty p orbital, 4 electrons, so antiaromatic. • The anion has a nonbonding pair of electrons in a p orbital, 6 e-’s, aromatic. => Chapter 16

  12. => Pyridine • Heterocyclic aromatic compound. • Nonbonding pair of electrons in sp2 orbital, so weak base, pKb = 8.8. Chapter 16

  13. Other Heterocyclics => Chapter 16

  14. Fused Heterocyclic Compounds Common in nature, synthesized for drugs. => Chapter 16

  15. => Allotropes of Carbon • Amorphous: small particles of graphite; charcoal, soot, coal, carbon black. • Diamond: a lattice of tetrahedral C’s. • Graphite: layers of fused aromatic rings. Chapter 16

  16. => Some New Allotropes • Fullerenes: 5- and 6-membered rings arranged to form a “soccer ball” structure. • Nanotubes: half of a C60 sphere fused to a cylinder of fused aromatic rings. Chapter 16

  17. Common Names of Benzene Derivatives => Chapter 16

  18. Disubstituted Benzenes The prefixes ortho-, meta-, and para- are commonly used for the 1,2-, 1,3-, and 1,4- positions, respectively. => Chapter 16

  19. 3 or More Substituents Use the smallest possible numbers, but the carbon with a functional group is #1. => Chapter 16

  20. Common Names forDisubstituted Benzenes => Chapter 16

  21. Phenyl and Benzyl Phenyl indicates the benzene ring attachment. The benzyl group has an additional carbon. => Chapter 16

  22. Physical Properties • Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points. • Boiling points: Dependent on dipole moment, so ortho > meta > para, for disubstituted benzenes. • Density: More dense than nonaromatics, less dense than water. • Solubility: Generally insoluble in water. => Chapter 16

  23. IR and NMR Spectroscopy • C=C stretch absorption at 1600 cm-1. • sp2 C-H stretch just above 3000 cm-1. • 1H NMR at 7-8 for H’s on aromatic ring. • 13C NMR at 120-150, similar to alkene carbons. => Chapter 16

  24. Mass Spectrometry => => Chapter 16

  25. POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5TH EDITION”L.G. WADEALL MATERIALS USED WITH PERMISSION OF AUTHORPRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGEORGANIC CHEMISTRY COURSEBY:ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN Chapter 16

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