Bonding and stability of benzeneAQA A-Level Chemistry: Revision notes
Section 1
The structure of benzene
Benzene, C₆H₆, is a planar ring of six carbon atoms, each bonded to one hydrogen atom. All the bond angles are 120°, so the molecule is a regular hexagon.
X-ray diffraction shows that every carbon–carbon bond has the same length, 0.140 nm. This is between a C–C single bond (0.154 nm) and a C=C double bond (0.134 nm).
Kekulé proposed cyclohexa-1,3,5-triene, with alternating single and double bonds. That structure would have two different bond lengths and would be a lopsided hexagon, so it does not fit the evidence.
Quote the numbers: 0.140 nm is between 0.154 nm (C–C) and 0.134 nm (C=C).
Section 2
Delocalised π electrons
Each carbon atom uses three of its four outer electrons in σ bonds (to two carbons and one hydrogen). The fourth electron is in a p orbital at right angles to the plane of the ring.
Adjacent p orbitals overlap sideways above and below the plane, forming a ring of electron density. The six p electrons are delocalised over all six carbon atoms, rather than held between two atoms.
Because the electron density is shared evenly, all six C–C bonds are identical in length and in strength.
Do not say the bonds 'keep switching' between single and double. There are no single or double bonds in benzene at all.
Section 3
Enthalpy of hydrogenation as evidence
Hydrogenation adds hydrogen across C=C bonds. With a nickel catalyst:
- cyclohexene + H₂ → cyclohexane, ΔH = −120 kJ mol⁻¹
- cyclohexa-1,3,5-triene would add three H₂, so predicted ΔH = 3 × (−120) = −360 kJ mol⁻¹
- benzene + 3H₂ → cyclohexane, measured ΔH = −208 kJ mol⁻¹
The measured value is less exothermic by −360 − (−208) = 152 kJ mol⁻¹.
Less energy is released because benzene starts at a lower energy than the hypothetical triene. Benzene is therefore more stable than the Kekulé structure by 152 kJ mol⁻¹. This is called the delocalisation energy (or stabilisation energy).
Work it out in three steps: predict (× 3), compare with the measured value, then state the difference and what it means.
A less exothermic value means benzene is MORE stable, not less.
Section 4
Why benzene is stable
The delocalised π system lowers the energy of benzene. To react by addition, a benzene molecule would have to break this delocalised system, which means giving up the extra 152 kJ mol⁻¹ of stability.
This explains why benzene does not decolourise bromine water as an alkene does. In alkenes the C=C bond is a localised region of high electron density that polarises bromine. In benzene the π electrons are spread over six carbon atoms, so the electron density between any two carbons is lower and bromine is not polarised enough to react.
Section 5
Substitution rather than addition
Benzene reacts mainly by substitution, where a hydrogen atom is replaced by another atom or group, for example C₆H₆ + Br₂ → C₆H₅Br + HBr with an AlBr₃ catalyst.
Substitution keeps the delocalised ring intact, so the stability is retained. An addition reaction would destroy the delocalised system and lose the delocalisation energy, so it is energetically less favourable.
Benzene still has a high electron density in the ring, so it reacts with electrophiles, which are attracted to the π electrons. These reactions are covered in the next topic.
Explain 'why substitution' in two parts: addition would break the delocalised ring and lose stability, while substitution keeps it.
Must know
- Benzene is planar, with 120° bond angles and six identical C–C bonds of 0.140 nm
- Each carbon contributes one p electron to a delocalised π system above and below the ring
- Hydrogenation: predicted −360, measured −208 kJ mol⁻¹, so benzene is 152 kJ mol⁻¹ more stable than cyclohexa-1,3,5-triene
- Benzene resists addition because it would lose the delocalisation energy, so it undergoes substitution
That's the notes covered.
Carry on to the next subtopic.
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
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).