What does the Kekulé model predict about carbon–carbon bond lengths?

Two lengths, alternating: C–C (0.154 nm) and C=C (0.134 nm).

How many σ bonds does each carbon in benzene form?

Three: two to neighbouring carbons and one to hydrogen.

How is the delocalised π system of benzene formed?

Sideways overlap of the six p orbitals (one per carbon), giving π electron density above and below the ring.

What is the length of every carbon–carbon bond in benzene?

0.140 nm, between a C–C (0.154 nm) and a C=C (0.134 nm).

What enthalpy change of hydrogenation does the Kekulé model predict for benzene?

3 × (−120) = −360 kJ mol⁻¹.

What does the 152 kJ mol⁻¹ difference show?

Benzene is more stable than the Kekulé structure; delocalisation energy.

Why does benzene not decolourise bromine water?

Its π electrons are delocalised over six carbons, so electron density is too low to polarise bromine.

Why does cyclohexene decolourise bromine water?

Its π electrons are localised in one C=C, giving high electron density that polarises Br–Br.

Structure and bonding of benzeneEdexcel A-Level Chemistry: Flashcards

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What is the Kekulé model of benzene?

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What is the Kekulé model of benzene?: A planar ring of six carbons with alternating C–C single and C=C double bonds.
What does the Kekulé model predict about carbon–carbon bond lengths?: Two lengths, alternating: C–C (0.154 nm) and C=C (0.134 nm).
How many σ bonds does each carbon in benzene form?: Three: two to neighbouring carbons and one to hydrogen.
What is the shape of benzene and the C–C–C bond angle?: Planar hexagon; 120°.
How is the delocalised π system of benzene formed?: Sideways overlap of the six p orbitals (one per carbon), giving π electron density above and below the ring.
How many electrons are in the delocalised system of benzene?: Six.
What is the length of every carbon–carbon bond in benzene?: 0.140 nm, between a C–C (0.154 nm) and a C=C (0.134 nm).
What enthalpy change of hydrogenation does the Kekulé model predict for benzene?: 3 × (−120) = −360 kJ mol⁻¹.
What is the experimental enthalpy change of hydrogenation of benzene?: −208 kJ mol⁻¹.
What does the 152 kJ mol⁻¹ difference show?: Benzene is more stable than the Kekulé structure; delocalisation energy.
Why does benzene not decolourise bromine water?: Its π electrons are delocalised over six carbons, so electron density is too low to polarise bromine.
Why does cyclohexene decolourise bromine water?: Its π electrons are localised in one C=C, giving high electron density that polarises Br–Br.

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

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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).