5.4.1Unit 5. Transition Metals and Organic Nitrogen Chemistry
Benzene – Structure and Stability — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 27 of 43 Unit quiz
Section 1 of 5Structure of Benzene
Formula
- Benzene is C₆H₆, a flat ring of six carbon atoms, each bonded to one hydrogen atom.
Bond angles
- All bond angles are 120°, because each carbon uses three sigma bonds arranged in a plane.
Delocalised electrons
- The remaining p orbital on each carbon overlaps sideways to form a ring of electron density above and below the plane.
Bond lengths
- All carbon-carbon bonds are 0.139 nm, between a single (0.154 nm) and a double (0.134 nm) bond.
Section 2 of 5Kekulé and Delocalised Models
Kekulé and Delocalised Models
Kekulé
- A ring with alternating single and double bonds would have three short and three long bonds, which is not found.
Delocalised
- The six pi electrons are shared over the whole ring, so every bond is identical.
Representation
- A hexagon with a circle drawn inside shows the delocalised ring in diagrams.
Section 3 of 5Evidence for Stability
1
Hydrogenation of cyclohexene (one C=C) releases 120 kJ mol⁻¹, so three isolated double bonds would be expected to release 360 kJ mol⁻¹.
2
The measured enthalpy change for hydrogenating benzene to cyclohexane is only -208 kJ mol⁻¹.
3
The difference, 152 kJ mol⁻¹, is the delocalisation energy, showing benzene is more stable than the Kekulé structure.
4
Benzene does not decolourise bromine water or react like an alkene, which supports the absence of localised double bonds.
5
Only one isomer of 1,2-disubstituted benzene exists, which means the bonds around the ring are equivalent.
Section 4 of 5Worked Example
Question
- Use enthalpies of hydrogenation to find the stabilisation energy of benzene.
- Cyclohexene: -120 kJ mol⁻¹.
- Benzene: -208 kJ mol⁻¹.
- Predicted for the Kekulé structure = 3 x (-120) = -360 kJ mol⁻¹.
- Actual = -208 kJ mol⁻¹.
- Difference = -208 - (-360) = +152 kJ mol⁻¹.
Answer Benzene is 152 kJ mol⁻¹ more stable than predicted
Section 5 of 5Exam Checklist & Tips
5.4.1 – Key Points for the Exam
- Delocalisation Sharing of electrons over several atoms rather than between two.
- Delocalisation energy Extra stability compared with the Kekulé structure.
- Pi bond Bond formed by sideways overlap of p orbitals.
- Arene A hydrocarbon containing a benzene ring.
Common Mistakes
- Describing benzene as having alternating single and double bonds.
- Saying that delocalised electrons are free to leave the ring.
- Using the wrong sign for the enthalpy of hydrogenation.
- Forgetting the 120° bond angle for planar carbon atoms.
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