9aromaticcompounds.ppt

Contents Discovery of Benzene Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C6H6. Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic(芳香性的). The Kekule Structure for benzene 苯的克库勒结构 Modern theories of the structure of benzene Differences in Reactions between alkene and benzene 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 Anti- and Nonaromatic反芳香性和非芳香性 Antiaromatic（反芳香性的） 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（非芳香性的） compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar. Hückel’s Rule （休克尔规则） If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 pi　electrons, it is aromatic. If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic.  [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. MO Derivation of Hückel’s Rule Lowest energy MO has 2 electrons. Each filled shell has 4 electrons. Cyclopentadienyl Ions 环戊二烯离子 Pyridine 吡啶 Nonbonding pair of electrons in sp2 orbital, so weak base, pKb = 8.8. Basic or Nonbasic? 3. Nomenclature of benzene derivatives Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points. Boiling poin