Structure of benzene

CIE A-Level Chemistry· Unit 22: Arenes· 25 min read

1. The Kekulé Model of Benzene★★☆☆☆⏱ 10 min

By the mid-19th century, chemists knew benzene had molecular formula $C_{6} H_{6}$ , but could not explain its unusual stability. A highly unsaturated molecule with three double bonds would be expected to react readily via addition, but benzene was far less reactive than predicted. August Kekulé proposed the first widely accepted ring structure in 1865.

📘 Definition

Kekulé Model

$C_{6} H_{6}$

A planar six-membered carbon ring, with alternating single and double carbon-carbon bonds, where each carbon bonds to one hydrogen atom.

Example:

Kekulé later modified the model to suggest double bonds oscillated between two positions to explain why only one 1,2-disubstituted benzene exists.

📐 Worked Example

If the Kekulé model was correct, how many distinct C-C bond lengths would you expect in benzene?

1
Recall the Kekulé model has two types of C-C bonds: single bonds and double bonds.
2
C-C single bonds are longer than C=C double bonds, with different average bond lengths.
3
The Kekulé model therefore predicts two distinct C-C bond lengths in benzene.

Exam tip:

You will often be asked to contrast the Kekulé model with experimental evidence, so memorise its key predictions.

2. Evidence Against the Kekulé Model★★★☆☆⏱ 15 min

Three key pieces of experimental evidence contradict the Kekulé model's prediction of alternating single and double bonds:

X-ray diffraction: All C-C bonds in benzene are the same length (139 pm), between the length of a C-C single (154 pm) and C=C double (134 pm)
Enthalpy of hydrogenation: Actual enthalpy change for hydrogenation of benzene is far less exothermic than predicted for the Kekulé model
Reactivity: Benzene does not undergo electrophilic addition like alkenes, and does not decolorise bromine water

📐 Worked Example

Use the data below to show benzene is more stable than the Kekulé model predicts: Enthalpy of hydrogenation of cyclohexene (one double bond) = -120 kJ mol⁻¹. Actual enthalpy of hydrogenation of benzene = -208 kJ mol⁻¹.

1
The Kekulé model has 3 double bonds, so calculate the predicted total enthalpy of hydrogenation:
2
$3 \times (-120) = -360 \text{ kJ mol}^{-1}$
3
Calculate the difference between actual and predicted enthalpy:
4
$\Delta H_{\text{stability}} = (-208) - (-360) = +152 \text{ kJ mol}^{-1}$
5
A positive value means benzene is 152 kJ mol⁻¹ lower in energy (more stable) than the Kekulé model predicts.

3. Modern Delocalised Model of Benzene★★★☆☆⏱ 15 min

📘 Definition

Modern Delocalised Model

Benzene is a planar, regular six-membered ring of $s p^{2}$ hybridised carbon atoms. Each carbon has one unhybridised p-orbital that overlaps equally with adjacent p-orbitals, forming a delocalised pi system spread evenly over all six carbons, with six delocalised pi electrons.

Example:

This model correctly predicts all C-C bonds are identical, matching experimental X-ray diffraction data.

Each $s p^{2}$ hybridised carbon forms three sigma ( $σ$ ) bonds: one to a hydrogen atom, and one to each adjacent carbon atom. Unhybridised p-orbitals are perpendicular to the plane of the ring, overlapping equally above and below the ring to create a continuous ring of electron density. The six delocalised pi electrons give benzene its characteristic aromatic stability.

📐 Worked Example

Explain why benzene undergoes electrophilic substitution rather than electrophilic addition.

1
In an electrophilic addition reaction, the delocalised pi system of benzene would be permanently broken, losing the 152 kJ mol⁻¹ delocalisation stabilisation energy.
2
In an electrophilic substitution reaction, the delocalised pi system is restored at the end of the reaction, so the stabilisation energy is retained.
3
Substitution is therefore energetically favourable, while addition requires a large input of energy to break the stable delocalised system.

4. Common Pitfalls

Wrong move:

Stating that the enthalpy of hydrogenation of benzene is more exothermic than predicted by the Kekulé model.

Why:

Confusion between actual and predicted values: more stability means lower energy, so actual enthalpy is less exothermic.

Correct move:

State that the actual enthalpy of hydrogenation is 152 kJ mol⁻¹ less exothermic than predicted, so benzene is more stable.

Wrong move:

Claiming all C-C bond lengths in benzene are exactly the average of single and double bond lengths.

Why:

The key exam point is that all bonds are identical, not that the length is an average. The value is between but not exactly the average.

Correct move:

State that all C-C bonds in benzene are the same length, which is between the length of a typical C-C single and C=C double bond.

Wrong move:

Claiming benzene decolorises bromine water with a halogen carrier catalyst.

Why:

Benzene undergoes substitution, not addition, so bromine is not consumed to decolorise the water.

Correct move:

State that benzene does not decolorise bromine water, and reacts with liquid bromine (not bromine water) to form bromobenzene via substitution.

Wrong move:

Drawing the Kekulé structure when asked to draw benzene in an exam answer.

Why:

Unless specifically asked for the Kekulé model, examiners expect the accepted delocalised structure.

Correct move:

Draw benzene as a regular hexagon with a circle inside to represent the delocalised pi system.

5. Quick Reference Cheatsheet

Evidence Type	Experimental Finding	Conclusion Against Kekulé
X-ray diffraction	All C-C bonds = 139 pm, identical	No alternating single/double bonds
Enthalpy of hydrogenation	Predicted: -360 kJ mol⁻¹, Actual: -208 kJ mol⁻¹	Benzene is 152 kJ mol⁻¹ more stable
Reactivity	No addition with bromine water	No reactive localised double bonds
Delocalised model key feature	6 delocalised pi electrons over 6 carbons	Explains identical bonds and stability

6. Frequently Asked

Why is benzene more stable than the Kekulé model predicts?

The delocalisation of pi electrons across the entire ring spreads electron density, lowering the overall energy of the molecule by 152 kJ mol⁻¹ compared to a structure with three localised double bonds.

Why does benzene not decolorize bromine water?

Benzene's delocalised pi system is too stable to undergo the electrophilic addition reaction that would decolorize bromine water; it instead undergoes electrophilic substitution with bromine.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
Compare models of benzene structure
2021 · 1
Bond length evidence question
2023 · 4
Stability from enthalpy data

Going deeper

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What's Next

The structure of benzene is the core foundation for all topics on arenes in CIE A-Level Chemistry. Its delocalised structure directly explains its characteristic reactivity, which is tested heavily in both multiple choice and extended response questions. Mastery of the evidence for the delocalised model is essential for all subsequent topics on aromatic chemistry, including reaction mechanisms and synthesis. The next step is to build on this knowledge to explore the electrophilic substitution reactions that benzene undergoes.