Structure and bonding of benzeneEdexcel A-Level Chemistry: Revision notes
Section 1
The Kekulé model
Benzene has the molecular formula C₆H₆. Kekulé proposed a planar ring of six carbon atoms with alternating C–C single and C=C double bonds, each carbon bonded to one hydrogen. This model has three localised double bonds, so it would predict that benzene behaves like an alkene and has two different carbon–carbon bond lengths.
Section 2
The delocalised model
In the delocalised model each carbon atom forms three σ bonds: two to neighbouring carbons and one to hydrogen. These lie in one plane with bond angles of 120°, so benzene is a planar hexagon.
Each carbon has one electron left in a p orbital perpendicular to the plane of the ring. The six p orbitals overlap sideways, forming a ring of π electron density above and below the plane. The six π electrons are delocalised: shared equally over all six carbons. All carbon–carbon bonds are therefore identical, with a length between that of a single and a double bond. Benzene is often drawn as a hexagon with a circle inside.
The delocalised electrons do not move around the ring in a circle as separate bonds. Describe them as spread evenly over all six carbons, in rings above and below the plane.
Section 3
Evidence 1: enthalpy changes of hydrogenation
Cyclohexene hydrogenates with ΔH = −120 kJ mol⁻¹. A Kekulé benzene with three C=C bonds should therefore give 3 × (−120) = −360 kJ mol⁻¹. The measured value for benzene is −208 kJ mol⁻¹.
The difference, −208 − (−360) = +152 kJ mol⁻¹, shows that benzene is 152 kJ mol⁻¹ more stable than the Kekulé structure. This extra stability is the delocalisation (stabilisation) energy, and supports the delocalised model.
Always show the prediction first (3 × the cyclohexene value), then state that the actual value is less exothermic, meaning benzene is more stable.
Section 4
Evidence 2: carbon–carbon bond lengths
X-ray diffraction shows all six carbon–carbon bonds in benzene are 0.140 nm long. A C–C single bond is about 0.154 nm and a C=C double bond is about 0.134 nm. The Kekulé model predicts alternating long and short bonds, but only one length is found, intermediate between single and double. This supports the delocalised model, in which the π electrons are shared equally round the ring.
Section 5
Why benzene resists bromination
Cyclohexene readily decolourises bromine water. In an alkene the π electrons are localised between two carbons, giving a high electron density that polarises the Br–Br bond, so the alkene reacts.
Benzene does not decolourise bromine water. Its π electrons are delocalised over six carbons, so the electron density between any two carbons is lower, too low to polarise bromine. A reaction would also destroy the stable delocalised system. The Kekulé model, with three localised double bonds, would wrongly predict benzene reacts like an alkene.
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Exam questions on Structure and bonding of benzene
- Benzene, C₆H₆, is a planar ring of six carbon atoms. Two models are used to describe its bonding: the Kekulé model and the delocalised model.State the shape of a benzene molecule, including the carbon–carbon–carbon bond angle, and the number of electrons in the delocalised π system.2 marks
- X-ray diffraction shows that all six carbon–carbon bonds in a benzene molecule have the same length, 0.140 nm. In other hydrocarbons a typical C–C single bond is 0.154 nm long and a typical C=C double bond is 0.134 nm long.Explain how the bond length data support the delocalised model rather than the Kekulé model.2 marks
- The standard enthalpy change when cyclohexene, C₆H₁₀, is hydrogenated to cyclohexane is −120 kJ mol⁻¹. The enthalpy change when benzene is hydrogenated to cyclohexane is −208 kJ mol⁻¹. A student also shakes bromine water separately with cyclohexene and with benzene, with no catalyst present.Calculate the enthalpy change of hydrogenation predicted for the Kekulé structure of benzene, compare it with the experimental value, and state what the difference shows.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).