# Structure of benzene

> CIE A-Level Chemistry · 9701
> Source: https://www.owlsprep.com/study/cie-9701-u22-structure-of-benzene/

This module covers the development of models for benzene's structure, from Kekulé's alternating bond model to the modern delocalised model, including key experimental evidence and implications for reactivity.

**Prerequisites:** Covalent bonding and carbon hybridisation; Bond enthalpy and enthalpy of hydrogenation; Electrophilic addition in alkenes

## Learning objectives

- Compare the historical Kekulé model with the modern delocalised model of benzene
- Interpret bond energy and structural evidence for benzene's structure
- Explain how benzene's structure leads to its characteristic reactivity
- Predict properties of benzene based on the accepted delocalised structure

## The Kekulé Model of Benzene

By the mid-19th century, chemists knew benzene had molecular formula $C_6H_6$, 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.

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

*Notation:* $C_6H_6$

*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.

## Evidence Against the Kekulé Model

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.

## Modern Delocalised Model of Benzene

**Modern Delocalised Model** — Benzene is a planar, regular six-membered ring of $sp^2$ 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 $sp^2$ hybridised carbon forms three sigma ($\sigma$) 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.

> **tip**
>
> Six delocalised pi electrons fit Hückel's rule ($4n+2$, $n=1$), which is required for 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.

## Common pitfalls

- **Wrong:** Stating that the enthalpy of hydrogenation of benzene is more exothermic than predicted by the Kekulé model.
  - Why it fails: Confusion between actual and predicted values: more stability means lower energy, so actual enthalpy is less exothermic.
  - Correct: State that the actual enthalpy of hydrogenation is 152 kJ mol⁻¹ less exothermic than predicted, so benzene is more stable.
- **Wrong:** Claiming all C-C bond lengths in benzene are exactly the average of single and double bond lengths.
  - Why it fails: 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: 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:** Claiming benzene decolorises bromine water with a halogen carrier catalyst.
  - Why it fails: Benzene undergoes substitution, not addition, so bromine is not consumed to decolorise the water.
  - Correct: State that benzene does not decolorise bromine water, and reacts with liquid bromine (not bromine water) to form bromobenzene via substitution.
- **Wrong:** Drawing the Kekulé structure when asked to draw benzene in an exam answer.
  - Why it fails: Unless specifically asked for the Kekulé model, examiners expect the accepted delocalised structure.
  - Correct: Draw benzene as a regular hexagon with a circle inside to represent the delocalised pi system.

## 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 |

## 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.

- [Reactions of arenes](https://www.owlsprep.com/study/cie-9701-u22-reactions-of-arenes/)
- [Amines](https://www.owlsprep.com/study/cie-9701-u23-overview/)
- [Preparation of amines](https://www.owlsprep.com/study/cie-9701-u23-preparation-of-amines/)

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