Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.
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Transcript of Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.
![Page 1: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/1.jpg)
Benzene animations
• http://www.dnatube.com/video/1418/Vague-Idea-of-Benzene-Resonance
• This resembles the ideas of Kekule who saw benzene as cyclohexatriene.
• http://www.dnatube.com/video/1421/Benzene-Resonance-with-Pi-bonds
• This is an attempt to demonstrate the modern theories of delocalisation of p orbitals.
![Page 2: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/2.jpg)
Structure of Benzene
• Aim:• To compare the Kekulé and
delocalised models for benzene in terms of p-orbital overlap forming Π bonds.
• To review the evidence for a delocalised model of benzene in terms of bond lengths, enthalpy change of hydrogenation and resistance to reaction.
![Page 3: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/3.jpg)
Kekulé Structure
• Molecular formula: C6H6
• Empirical formula CH
![Page 4: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/4.jpg)
• The Kekulé structure is actually cyclohexa-1,3,5 triene.
• Suggests 3 single and 3 double bonds.
• Should behave like cyclohexene and undergo ELECTROPHILIC ADDITION.
• Should have long, single bonds alternating with shorter double bonds.
![Page 5: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/5.jpg)
Faults in Kekulé’s Model
• Structure is regular hexagon with all 6 C-C bonds equal length – 140 pm.
• This lies between C-C single bond length 154 pm and C=C double bond length 134 pm. (pm = picometre = 10-12 m.)
• Benzene does NOT behave like an alkene since it undergoes ELECTROPHILIC SUBSTITUTION.
![Page 6: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/6.jpg)
Enthalpy Change of Hydrogenation• Unsaturated compounds undergo
ELECTROPHILIC ADDITION with hydrogen, under pressure with a nickel catalyst.
• 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
![Page 7: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/7.jpg)
• Since benzene has 3 C=C bonds the enthalpy 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 MORE STABLE than Kekulé structure.
![Page 8: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/8.jpg)
![Page 9: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/9.jpg)
Stabilisation Energy
• The 150 kJ of energy is sometimes described as Stabilisation Energy.
• LESS energy is given out when benzene is hydrogenated than expected so benzene is MORE STABLE than expected and needs MORE ENERGY than expected to break the ring.
![Page 10: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/10.jpg)
Resonance hybrid model of benzene.
This is not a reversible reaction. It is showing a cross between the 2 structures with each C-C bond having some C-C single and some C=C double bond character.
![Page 11: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/11.jpg)
Displayed structure of benzene in which 6 electrons are delocalised and represented by a circle.
![Page 12: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/12.jpg)
Skeletal Formula for Benzene
![Page 13: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/13.jpg)
Molecular Orbital Model For Benzene
![Page 14: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/14.jpg)
Orbital Overlap Model of Benzene
• Each carbon in the hexagonal ring is sp2 hybridised .
• The C-H bonds are sigma bonds formed by the overlap of hydrogen 1s atomic orbital with a carbon sp2 hybrid orbital.
• Each C-C bond is a sigma bond with overlapping sp2 hybridised orbitals. Each H-C-C angle is 120o.The ring is planar.
• Each C has an unhybridised p orbital at 90o to the ring. These overlap laterally and form a delocalised Π orbital above and below the plane of the ring.
![Page 15: Benzene animations e-Idea-of-Benzene-Resonance e-Idea-of-Benzene-Resonance.](https://reader036.fdocuments.net/reader036/viewer/2022071807/56649ea15503460f94ba5002/html5/thumbnails/15.jpg)
Molecular Orbitals For Benzene