Structure and stability of benzeneEdexcel International A Level Chemistry: Revision notes
Section 1
The Kekulé structure and its problems
Benzene, C₆H₆, can be drawn as a hexagon of carbon atoms with alternating single and double bonds (the Kekulé structure, cyclohexa-1,3,5-triene), or as a hexagon with a circle inside to show delocalisation. If the Kekulé structure were correct, benzene would behave as a very unsaturated alkene. It does not, and three kinds of evidence show why.
Section 2
Thermochemical evidence
The enthalpy change of hydrogenation of cyclohexene (one C=C) is −120 kJ mol⁻¹. A localised triene should give 3 × (−120) = −360 kJ mol⁻¹.
The measured value for benzene to cyclohexane is only −208 kJ mol⁻¹.
Benzene is therefore 152 kJ mol⁻¹ more stable than the Kekulé structure (−208 − (−360) = +152). This is the delocalisation energy, the energy that would have to be supplied to disrupt the delocalised ring.
Subtracting the wrong way round. The measured value is less exothermic than predicted, so benzene is more stable, not less.
Section 3
X-ray diffraction and infrared evidence
X-ray diffraction shows benzene is a planar hexagon with all C–C–C bond angles 120° and all six carbon–carbon bonds the same length, 0.140 nm. This is between the C–C single bond (0.154 nm, as in cyclohexane) and the C=C double bond (0.134 nm, as in cyclohexene). A Kekulé structure would show alternating long and short bonds.
Infrared spectroscopy shows that benzene has no absorption in the 1620–1680 cm⁻¹ range where alkenes absorb because of C=C. This shows that benzene has no localised C=C bonds.
Section 4
The delocalised model
Each carbon atom in the ring forms three σ bonds (two C–C and one C–H) in the same plane, with 120° angles. Each carbon also has one electron in a p orbital at right angles to the ring.
Adjacent p orbitals overlap sideways to form π bonds, giving a ring of electron density above and below the plane. The six π electrons are delocalised over all six carbon atoms. This lowers the energy of the ring and makes all six bonds identical.
Section 5
Resistance to bromination
Cyclohexene decolourises bromine water at room temperature. The localised π bond has high electron density between two carbons, which induces a dipole in Br₂, so bromine adds across the double bond.
Benzene does not react with bromine water. Its π electrons are delocalised over six carbons, so the electron density at any one place is lower. This cannot polarise Br₂ enough for a reaction. An addition reaction would also destroy the stable delocalised ring.
Contrast localised electron density in the alkene with delocalised electron density in benzene whenever you are asked about bromine.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Structure and stability of benzene
- X-ray diffraction shows that benzene, C₆H₆, is a flat molecule in which all six carbon–carbon bonds have the same length, 0.140 nm. The carbon–carbon bond length is 0.154 nm in cyclohexane and 0.134 nm in cyclohexene.Explain why the carbon–carbon bonds in benzene all have a length of 0.140 nm.2 marks
- The enthalpy change of hydrogenation of cyclohexene to cyclohexane, C₆H₁₀ + H₂ → C₆H₁₂, is −120 kJ mol⁻¹. The enthalpy change of hydrogenation of benzene to cyclohexane, C₆H₆ + 3H₂ → C₆H₁₂, is −208 kJ mol⁻¹.Explain why the hydrogenation of benzene is less exothermic than predicted for cyclohexa-1,3,5-triene.2 marks
- A teacher compares the reaction of bromine water with cyclohexene and with benzene at room temperature. The cyclohexene decolourises the bromine water immediately but the benzene does not. Benzene is a planar ring of six carbon atoms, each bonded to one hydrogen atom.Describe the bonding in the benzene ring using the delocalised model.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).