Chapter 14 (lecture 6)

Chapter 14 (lecture 6) - The Kekule Structure for Benzene H...

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The Kekule' Structure for Benzene C C C C C C H H H H H H or Kekule' structure for benzene, C 6 H 6
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Resonance Theory Description of Benzene According to Resonance Theory , the two equivalent Kekule' structures of benzene are resonance structures that contribute to the hybrid .The hybrid is 36.0 kcal/mol more stable than the Kekule' structures. Kekule' structures hybrid Resonance Energy The hexagon with the circle inside is widely used to represent the hybrid of benzene.
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Bond Equivalency in Benzene Spectroscopic studies indicate that benzene is planar, with the geometry of a regular hexagon, and carbon-carbon bond lengths of 1.39 A. o H H H H H H 120 o 1.39 A o 1.09 A o a comparison of C-C bond lengths H 3 C CH 3 H 2 C=CH 2 benzene 1.54 A 1.34 A 1.39 A o o o The equivalent lengths of the C-C bonds in benzene are consistent with the hybrid of resonance theory, which describes each C-C bond in the hybrid as a mixture of 50% single bond and 50% double bond character . hybrid
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Molecular Orbital Description of the ! -Electronic Structure of Benzene Consider the ! -bond of ethene formed by overlap of p-orbitals on adjacent carbons. The wave functions of the combining p-orbitals may be in-phase (bonding molecular orbital) or out-of-phase (antibonding molecular orbital). Energy C C isolated p-orbitals combine ! bonding ! antibonding * " (in-phase) (out-of-phase) The quantity ! is the stabilization energy of the electrons when a " -bond is formed. It measures the stabilization of the two electrons in a bonding " molecular orbital relative to two electrons in localized p-orbitals.
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Benzene: a 6 ! -Electron System In a similar way, the energies of the six electrons in the bonding ! -molecular orbitals of benzene (a regular hexagon) are compared with the energies of six electrons in localized p-orbitals. Energy six isolated p-orbitals combine ! " 2 " 3 " 1 2 ! " 4 " 5 " 6 Bonding MOs Antibonding MOs In the ground electronic state of benzene, the six ! -electrons are in the three low lying bonding molecular orbitals: " 1 , " 2 , " 3 . The antibonding molecular orbitals are unfilled. Benzene has a closed bonding shell of ! -electrons, which is a stable configuration. The total stabilization energy is (2 x 2 # ) + (4 x # ) = 8 # .
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π -bonding π -anti-bonding
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Huckel's Rule: The (4n + 2) ! -Electron Rule for Predicting Aromaticity : In 1931, the German physicist Erich Huckel carried out molecular orbital calculations on planar carbocycles in which each carbon has a p-orbital. His calculations showed that monocyclic systems containing (4n + 2) ! -electrons , where n = 0, 1, 2, 3. .., have closed shells of ! -electrons like benzene. These systems have a single
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This note was uploaded on 01/20/2012 for the course CHEM 325BL at USC.

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Chapter 14 (lecture 6) - The Kekule Structure for Benzene H...

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