Explain benzene’s equal bonds, extra stability and preference for substitution using structural and energetic evidence.
Six carbons share a delocalised π system
Benzene, C₆H₆, is a planar hexagonal ring. Each carbon forms three σ bonds: two to neighbouring carbons and one to hydrogen. Each contributes one electron from a p orbital perpendicular to the ring. Sideways overlap around the complete ring creates a delocalised π system above and below the carbon plane. Bond angles are about 120°.
All six C–C bonds are equivalent and have a length intermediate between an ordinary single and double bond. A hexagon containing a circle represents this delocalisation. Alternating-double-bond drawings can be useful contributors in electron accounting, but benzene does not contain three permanently short and three permanently long C–C bonds or switch back and forth between two separate molecules.
Diagram placeholder
Benzene σ framework and π overlap to add
Labels to include:
- Planar C₆ hexagon with one H on each C
- Three σ bonds at each carbon
- Six parallel p orbitals perpendicular to ring
- Sideways p-orbital overlap around all six carbons
- Delocalised electron density above and below plane
- 120° bond angles; six equivalent C–C bonds
Distinguish the σ framework in the ring plane from the π density above and below it. A circle is a representation of delocalisation, not a seventh atom or a single circular bond.
Compare reactions giving the same final product
Hydrogenating one isolated C=C bond in a cyclohexene-like reference releases about 120 kJ mol⁻¹. A hypothetical cyclohexa-1,3,5-triene with three independent double bonds would therefore be expected to release about 360 kJ mol⁻¹ on forming cyclohexane. Benzene hydrogenation releases only about 208 kJ mol⁻¹ under comparable reference conditions.
Because both comparisons end at cyclohexane, the less exothermic benzene reaction means benzene starts lower in enthalpy. The approximate stabilisation is 360 − 208 = 152 kJ mol⁻¹ relative to the hypothetical localised model. State “less negative” or “less exothermic”; an ambiguous “smaller enthalpy” can hide a sign error. These rounded teaching values support a model comparison rather than three actual independent double bonds in benzene.
Diagram placeholder
Hydrogenation enthalpy comparison to add
Labels to include:
- Vertical enthalpy axis
- Hypothetical localised triene + 3H₂ upper level
- Benzene + 3H₂ lower starting level
- Common cyclohexane final level
- Downward −360 kJ mol⁻¹ reference arrow
- Downward −208 kJ mol⁻¹ benzene arrow
- 152 kJ mol⁻¹ separation between starting levels
Use the same final state for both pathways and put benzene below the hypothetical triene. Do not draw benzene as the higher-energy reactant while calling it more stable.
Substitution restores the aromatic system
The π system attracts electrophiles, but permanently losing delocalisation is energetically costly. In electrophilic substitution, initial attack temporarily disrupts full ring delocalisation, then loss of H⁺ restores it. The final product retains an aromatic ring with a substituent replacing H.
An addition product would usually lose the original aromatic π system. This explains why substitution is favoured under the specified conditions, not why addition is impossible under every condition: benzene can be hydrogenated using sufficiently vigorous catalytic conditions. Benzene does not rapidly decolourise bromine water like an alkene in the ordinary room-temperature test.
Separate an aromatic ring from its side chain
C₆H₅– is a phenyl group. C₆H₅CH₃ is methylbenzene, C₆H₅NO₂ nitrobenzene, C₆H₅NH₂ phenylamine and C₆H₅COOH benzenecarboxylic acid/benzoic acid. “Phenyl” and “benzyl”, C₆H₅CH₂–, are not interchangeable. In a substituted ring, use positions that identify the connectivity unambiguously.
An aromatic molecule can also contain an alkene, alcohol, aldehyde or other group in its side chain. Apply that group’s chemistry without assuming the benzene ring must react. For example, C₆H₅CH₂OH is an alcohol while C₆H₅OH is phenol; moving the oxygen directly onto the ring changes its chemistry. Wider phenol substitution and aromatic directing effects are not required extensions of the monosubstitution scope here.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Why are all the C–C bonds in benzene equal?Show answer
Its six p electrons are delocalised around the ring, making all six C–C bonds equivalent rather than alternating permanently localised single and double bonds.
Q2. Where are the p orbitals relative to the ring plane?Show answer
They are perpendicular to it. Sideways overlap produces delocalised π electron density above and below the plane.
Q3. A supplied isolated-double-bond reference gives −118 kJ mol⁻¹ and benzene gives −206 kJ mol⁻¹. Calculate the inferred stabilisation.Show answer
Three independent bonds predict −354 kJ mol⁻¹. The difference in magnitude is 354 − 206 = 148 kJ mol⁻¹; benzene is lower in enthalpy by this amount relative to that hypothetical model.
Q4. Why does substitution preserve benzene’s stability better than addition?Show answer
The substitution mechanism removes H⁺ after attack and restores full aromatic delocalisation. The usual addition product would permanently disrupt that π system.
Q5. Distinguish C₆H₅– from C₆H₅CH₂–.Show answer
C₆H₅– is phenyl, attached directly through the ring. C₆H₅CH₂– is benzyl, with an extra CH₂ carbon between the ring and the attachment point.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 organic chemistry specification — 3.3.10 coverage and required skills.
- Chemrevise: Aromatic chemistry — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 2 mark scheme — Q03.2 p14 and report Q03.2 p3: electrophilic substitution, arrow to nitronium nitrogen and C–H bond return into the ring.
- AQA June 2023 Paper 2 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
- AQA June 2022 Paper 2 mark scheme — Q08.1–08.3 pp28–29 and report Q08 p6: supplied acylating reagent, AlCl₃, electrophile and ring intermediate.
- AQA June 2022 Paper 2 examiner report — Read with the matching question context described in the mark-scheme source.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.