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Benzene Animations

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    Benzene animations

    http://www.dnatube.com/video/1418/Vagu

    e-Idea-of-Benzene-Resonance

    This resembles the ideas of Kekule who saw

    benzene as cyclohexatriene.

    http://www.dnatube.com/video/1421/Benz

    ene-Resonance-with-Pi-bonds

    This is an attempt to demonstrate the modern

    theories of delocalisation of p orbitals.

    http://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bondshttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonancehttp://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonance
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    Structure of Benzene

    Aim:

    To compare the Kekul and delocalisedmodels for benzene in terms of p-orbital overlap forming bonds.

    To review the evidence for a delocalisedmodel of benzene in terms of bondlengths, enthalpy change of

    hydrogenation and resistance toreaction.

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    Kekul Structure

    Molecular formula: C6H

    6 Empirical formula CH

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    The Kekul structure is actuallycyclohexa-1,3,5 triene.

    Suggests 3 single and 3 double bonds.

    Should behave like cyclohexene andundergo ELECTROPHILIC ADDITION.

    Should have long, single bonds

    alternating with shorter double bonds.

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    Faults in Kekuls Model

    Structure is regular hexagon with all 6C-C bonds equal length 140 pm.

    This lies between C-C single bond length154 pm and C=C double bond length 134pm. (pm = picometre = 10-12 m.)

    Benzene does NOT behave like analkene since it undergoesELECTROPHILIC SUBSTITUTION.

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    Enthalpy Change of Hydrogenation

    Unsaturated compounds undergo

    ELECTROPHILIC ADDITION withhydrogen, under pressure with a nickelcatalyst.

    C2H4 + H2

    C2H6

    Bonds broken = 1 C=C + 1 H-H Bonds made = 1 C-C + 2 C-H Enthalpy change of hydrogenation

    should be about -120kJmol-1

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    Since benzene has 3 C=C bonds theenthalpy change of hydrogenation

    should be about -360 kJmol-1. Experimental data shows it to be -

    208kJmol-1.

    Conclusion: The ring does NOT contain C=C bonds

    and is more stable than expected.

    Benzene is about 150 kJ mol-1 MORESTABLE than Kekul structure.

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    Stabilisation Energy

    The 150 kJ of energy is sometimesdescribed as Stabilisation Energy.

    LESS energy is given out when benzeneis hydrogenated than expected sobenzene is MORE STABLE thanexpected and needs MORE ENERGYthan expected to break the ring.

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    Resonance hybrid model of benzene.

    This is not a reversible reaction. It is

    showing a cross between the 2structures with each C-C bond havingsome C-C single and some C=C double

    bond character.

    http://en.citizendium.org/wiki/Image:Chemistry_-_Benzene_-_Kekule_Structure.png
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    Displayed structure of benzene in which 6electrons are delocalised and representedbya

    circle.

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    Skeletal Formula for Benzene

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    Molecular Orbital Model For Benzene

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    Orbital Overlap Model of Benzene

    Each carbon in the hexagonal ring is sp2 hybridised .

    The C-H bonds are sigma bonds formed by theoverlap of hydrogen 1s atomic orbital with a carbonsp2 hybrid orbital.

    Each C-C bond is a sigma bond with overlapping sp2

    hybridised orbitals. Each H-C-C angle is 120o.The ringis planar.

    Each C has an unhybridised p orbital at 90o to thering. These overlap laterally and form a delocalised

    orbital above and below the plane of the ring.

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    Molecular Orbitals For Benzene

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