What is the carbon–carbon bond length in benzene?

0.140 nm for all six bonds, between C–C (0.154 nm) and C=C (0.134 nm).

What is the Kekulé structure of benzene?

Cyclohexa-1,3,5-triene, a ring with alternating single and double bonds.

Why does the Kekulé structure not fit the bond length evidence?

It predicts two different bond lengths, but all the C–C bonds in benzene are the same length.

How is the delocalised π system in benzene formed?

Each carbon's p orbital overlaps sideways with its neighbours above and below the ring, delocalising six p electrons.

What enthalpy change of hydrogenation is predicted for cyclohexa-1,3,5-triene?

3 × (−120) = −360 kJ mol⁻¹.

How much more stable is benzene than the Kekulé structure?

152 kJ mol⁻¹, from −360 − (−208).

What name is given to this extra stability?

Delocalisation energy (stabilisation energy).

Why is benzene's hydrogenation less exothermic than predicted?

Benzene is lower in energy because of delocalisation, so less energy is released.

Why does benzene not decolourise bromine water?

Its delocalised electron density is too low between any two carbons to polarise bromine.

Why does benzene undergo substitution in preference to addition?

Addition would destroy the delocalised ring and lose the 152 kJ mol⁻¹ stability; substitution keeps the ring.

Bonding and stability of benzeneAQA A-Level Chemistry: Flashcards

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What is the shape of the benzene molecule and its bond angle?

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What is the shape of the benzene molecule and its bond angle?: A planar regular hexagon with bond angles of 120°.
What is the carbon–carbon bond length in benzene?: 0.140 nm for all six bonds, between C–C (0.154 nm) and C=C (0.134 nm).
What is the Kekulé structure of benzene?: Cyclohexa-1,3,5-triene, a ring with alternating single and double bonds.
Why does the Kekulé structure not fit the bond length evidence?: It predicts two different bond lengths, but all the C–C bonds in benzene are the same length.
How is the delocalised π system in benzene formed?: Each carbon's p orbital overlaps sideways with its neighbours above and below the ring, delocalising six p electrons.
What enthalpy change of hydrogenation is predicted for cyclohexa-1,3,5-triene?: 3 × (−120) = −360 kJ mol⁻¹.
What is the measured enthalpy change of hydrogenation of benzene?: −208 kJ mol⁻¹.
How much more stable is benzene than the Kekulé structure?: 152 kJ mol⁻¹, from −360 − (−208).
What name is given to this extra stability?: Delocalisation energy (stabilisation energy).
Why is benzene's hydrogenation less exothermic than predicted?: Benzene is lower in energy because of delocalisation, so less energy is released.
Why does benzene not decolourise bromine water?: Its delocalised electron density is too low between any two carbons to polarise bromine.
Why does benzene undergo substitution in preference to addition?: Addition would destroy the delocalised ring and lose the 152 kJ mol⁻¹ stability; substitution keeps the ring.

Exam questions on Bonding and stability of benzene

A research chemist uses X-ray diffraction to measure the carbon–carbon bond lengths in solid benzene, C₆H₆. For comparison, the C–C bond length in alkanes is 0.154 nm and the C=C bond length in alkenes is 0.134 nm. All six carbon atoms of the benzene ring and all six hydrogen atoms are found to lie in one plane.
Explain why all the carbon–carbon bonds in benzene are the same length.2 marks
A student compares the enthalpy changes when cyclohexene and benzene are each hydrogenated to cyclohexane using hydrogen gas and a nickel catalyst. The enthalpy change of hydrogenation of cyclohexene is −120 kJ mol⁻¹ and that of benzene is −208 kJ mol⁻¹. Kekulé proposed that benzene is cyclohexa-1,3,5-triene, with three localised C=C double bonds.
Calculate the difference between the predicted and measured enthalpy changes of hydrogenation of benzene, and state what this difference shows about benzene.2 marks
A technician shakes cyclohexene with bromine water and the orange colour disappears at once. When benzene is shaken with bromine water under the same conditions, the orange colour remains. Benzene does react with bromine in the presence of anhydrous aluminium bromide, AlBr₃, forming bromobenzene, C₆H₅Br, and hydrogen bromide. The enthalpy change of hydrogenation of benzene is −208 kJ mol⁻¹, whereas that predicted for cyclohexa-1,3,5-triene is −360 kJ mol⁻¹.
Explain why cyclohexene decolourises bromine water but benzene does not.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).