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.

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.

Kekulé Structure

• Molecular formula: C6H6

• Empirical formula CH

• 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.

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.

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

• 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.

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.

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.

Displayed structure of benzene in which 6 electrons are delocalised and represented by a circle.

Skeletal Formula for Benzene

Molecular Orbital Model For Benzene

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.

Molecular Orbitals For Benzene