What would the hydrogenation enthalpy of a Kekulé triene be?

3 × (−120) = −360 kJ mol⁻¹.

What is the delocalisation energy of benzene?

152 kJ mol⁻¹: benzene is that much more stable than the Kekulé structure.

What does X-ray diffraction show about carbon–carbon bonds in benzene?

All six are 0.140 nm, between C–C (0.154 nm) and C=C (0.134 nm).

What bond lengths would a Kekulé structure give?

Alternating 0.154 nm and 0.134 nm.

What does the lack of infrared absorption at 1620–1680 cm⁻¹ show?

Benzene has no localised C=C bonds.

What shape and bond angles does benzene have?

Planar, with all bond angles 120°.

How is the delocalised π system formed?

Sideways overlap of one p orbital on each carbon gives a ring of electron density above and below the plane.

Why does cyclohexene decolourise bromine water?

Its localised π bond has high electron density, which polarises Br₂ and allows addition.

Why does benzene not decolourise bromine water?

Its delocalised π electrons give a lower electron density that cannot polarise Br₂.

Structure and stability of benzeneEdexcel International A Level Chemistry: Flashcards

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What are the two ways of drawing benzene?

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What are the two ways of drawing benzene?: A Kekulé structure with alternating single and double bonds, or a hexagon with a circle to show delocalisation.
What is the enthalpy change of hydrogenation of cyclohexene?: −120 kJ mol⁻¹.
What would the hydrogenation enthalpy of a Kekulé triene be?: 3 × (−120) = −360 kJ mol⁻¹.
What is the measured enthalpy change of hydrogenation of benzene?: −208 kJ mol⁻¹.
What is the delocalisation energy of benzene?: 152 kJ mol⁻¹: benzene is that much more stable than the Kekulé structure.
What does X-ray diffraction show about carbon–carbon bonds in benzene?: All six are 0.140 nm, between C–C (0.154 nm) and C=C (0.134 nm).
What bond lengths would a Kekulé structure give?: Alternating 0.154 nm and 0.134 nm.
What does the lack of infrared absorption at 1620–1680 cm⁻¹ show?: Benzene has no localised C=C bonds.
What shape and bond angles does benzene have?: Planar, with all bond angles 120°.
How is the delocalised π system formed?: Sideways overlap of one p orbital on each carbon gives a ring of electron density above and below the plane.
How many electrons are delocalised in benzene?: Six.
Why does cyclohexene decolourise bromine water?: Its localised π bond has high electron density, which polarises Br₂ and allows addition.
Why does benzene not decolourise bromine water?: Its delocalised π electrons give a lower electron density that cannot polarise Br₂.

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

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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).