The shape of benzene
Introduced| English | Meaning |
|---|---|
| delocalised/dɪˈlɒkəlaɪzd/ | Describing electrons that are spread over several atoms rather than confined to one bond or atom. |
| hybridised/ˈhaɪbrɪdaɪzd/ | hybridised |
| sigma bond/ˈsɪɡmə bɒnd/ | A covalent bond formed by head-on orbital overlap along the line joining the bonded nuclei. |
| pi system/paɪ ˈsɪstəm/ | pi system |
| hydrogenation/haɪˈdrɒdʒəneɪʃn/ | The addition of hydrogen to a substance, often across a carbon-carbon double bond using a catalyst. |
The puzzle of benzene
- Benzene is a flat, regular hexagon.
- Its electrons are delocalised 离域 into one ring system.
- This makes benzene unusually stable.
Benzene bonding lab
Follow the evidence for a planar delocalised benzene ring.
Benzene is a flat, regular hexagon.
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Its sp² σ framework and delocalised π system make it planar and symmetrical.
Benzene readily undergoes addition reactions because its electrons are localised in three C=C bonds.
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False. Benzene's π electrons are delocalised around the ring, which makes it unusually stable, so it resists addition and prefers substitution.
The bonding
- Each carbon is sp² hybridised 杂化, making sigma bonds σ键 to two carbons and one hydrogen — the flat hexagon.
- Each carbon's leftover p-orbital electron overlaps sideways all the way round, making one delocalised pi system π体系 above and below the ring.
- Because the electrons are shared evenly, all six C–C bonds are the same length.
Benzene: sigma framework plus a delocalised pi system
Each carbon in benzene is:
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sp² carbons give the flat hexagon; the leftover p-orbitals form the delocalised π system.
In benzene, all six C–C bonds are the same length because:
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The single delocalised π system spreads the electrons evenly, so every C–C bond is identical.
Match each term to its meaning.
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Each item links the term to its correct meaning.
Evidence: hydrogenation 氢化
- Adding $\text{H}_2$ to one C=C (cyclohexene) releases about $120\ \text{kJ}/\text{mol}$, so a Kekulé ring of three double bonds should release $\approx 360\ \text{kJ}/\text{mol}$.
- Real benzene releases only $208\ \text{kJ}/\text{mol}$ — about $152\ \text{kJ}/\text{mol}$ more stable than the model predicts. That extra stability is the delocalisation.
The enthalpy of hydrogenation shows benzene is more stable than the Kekulé model because real benzene releases:
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Real benzene releases only 208 kJ/mol, ~152 kJ/mol less than the 360 predicted — the extra stability of delocalisation.
The evidence for delocalisation
- All six C–C bonds in benzene are the same length (between single and double).
- Benzene is more stable than the 'Kekulé' structure predicts — evidence for delocalised electrons.
You've got it
- benzene is a flat, regular hexagon of sp² carbons (a σ framework)
- the p-orbitals overlap into one delocalised π system, so all six C–C bonds are equal
- hydrogenation evidence: real benzene ($-208\ \text{kJ}/\text{mol}$) is far more stable than the Kekulé model ($-360$), by ≈ 152 kJ/mol