Ch16Su08

Ch16Su08 - Aromatic Compounds Historically, benzene and its...

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Aromatic Compounds Historically, benzene and its first derivatives had pleasant aromas, and were called aromatic compounds. Structure of Benzene Kekulé Structure Kekulé (1866) bravely proposed that benzene had a cyclic structure with three alternating C=C double and three C-C single bonds. Whilst this is reasonably close to accurate, it cannot be exactly correct since this would require that 1,2-dichlorobenzene existed as two isomeric forms, yet it was known that it did not. Resonance Structure The Kekulé structure would have the single bonds of longer length than the double bonds, and thus an irregular hexagonal shape. But spectroscopy had shown that benzene had a planar ring, with all the carbon-carbon bond distances the same 1.397Å (C-C typically 1.48Å, C=C typically 1.34Å). ce6f31d9117f0c97ee1ad8847c5b650529337a35.doc page 1 C C C C C C H H H H H H Cl Cl Cl Cl
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Since the atoms are the same distance apart, and the only difference is the location of the π electrons in the two Kekulé structures, they are in fact resonance structures of one another. This implies that the bond order should be 1.5, and that the π electrons are delocalized around the ring. Because of the delocalization of the π electrons, often the double bonds are represented by a circle in the middle of the hexagon. This resonance description lets us draw a more realistic representation of benzene, with 6 sp 2 hybrid carbons, each bonded to one hydrogen atom. All the carbon-carbon bonds are of equal length, and all the bond angles are 120°. Each carbon has an unhybridized p orbital, which lies perpendicular to the plane of the ring. These p orbitals each have 1 electron inside. There are therefore 6 electrons in the circle of p orbitals. ce6f31d9117f0c97ee1ad8847c5b650529337a35.doc page 2 = all CC bonds = 1.397A bond order = 1.5
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(In simple terms, an aromatic compound can be defined as a cyclic compound, containing a certain number of conjugated double bonds, and being especially stable due to resonance). Unusual Behavior of Benzene Benzene has much more stability than predicted by the simple resonance delocalized structure. For example, we know alkenes can be oxidized to syn diols (KmnO 4 ) and undergo electrophilic additions with halogens (Br 2 ). ce6f31d9117f0c97ee1ad8847c5b650529337a35.doc page 3 H H KMnO 4 , H 2 O OH OH H H KMnO 4 , H 2 O No Reaction H H Br 2 H Br H Br Br 2 No Reaction
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Yet the same reactions do not work with benzene. Benzene does not react - benzene is more stable than normal cyclo-alkenes. When a catalyst is added to the benzene bromination reaction, reaction does occur, but the reaction is not an addition , but rather a substitution (a ring hydrogen is substituted for a ring bromine). All three double bonds are retained in the product.
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This note was uploaded on 02/23/2010 for the course CHEM 202 taught by Professor Smith during the Spring '10 term at Duke.

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Ch16Su08 - Aromatic Compounds Historically, benzene and its...

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