Benzene is described as an aromatic compound. Explain the bonding in benzene and describe two properties that arise from its aromatic nature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Benzene has the molecular formula C₆H₆ and was originally thought to contain alternating single and double carbon–carbon bonds. However, experimental evidence shows that all carbon–carbon bonds in benzene are identical in length and that benzene is less reactive than expected for a molecule containing double bonds.
Model answer (5 marks)
Benzene consists of a six‑membered ring of carbon atoms.
Each carbon atom is bonded to two other carbons and one hydrogen, giving a planar, hexagonal structure.
The six π electrons that arise from the p orbitals on each carbon are not confined to a single C=C bond; they are delocalised over the entire ring.
Because of this delocalisation all C–C bonds are identical, with a length intermediate between a single and a double bond.
The delocalised π system stabilises the molecule, making benzene less reactive than a molecule containing localised double bonds.
Consequently, benzene undergoes electrophilic substitution reactions rather than addition reactions, preserving the aromatic π system.
Each carbon atom is bonded to two other carbons and one hydrogen, giving a planar, hexagonal structure.
The six π electrons that arise from the p orbitals on each carbon are not confined to a single C=C bond; they are delocalised over the entire ring.
Because of this delocalisation all C–C bonds are identical, with a length intermediate between a single and a double bond.
The delocalised π system stabilises the molecule, making benzene less reactive than a molecule containing localised double bonds.
Consequently, benzene undergoes electrophilic substitution reactions rather than addition reactions, preserving the aromatic π system.
Examiner tips
- Use the word ‘delocalised’ and mention the ring of six carbons; this earns the first two points. Show that all C–C bonds are equal – a key feature of aromaticity. Explain the stability and the shift to electrophilic substitution to secure the last two marks.
Common mistakes
- Confusing the delocalised electrons with a single set of alternating double bonds. Forgetting to state that the C–C bonds are equal in length. Claiming benzene reacts by addition rather than substitution.
Mark scheme (5 marks)
- Benzene contains a ring of six carbon atoms
- The electrons are delocalised / spread over the whole ring (rather than fixed between two carbon atoms)
- All carbon–carbon bond lengths in benzene are equal / the same (intermediate between a single and a double bond)
- Benzene is more stable / less reactive than expected (compared to a molecule with localised double bonds / compared to cyclohexadiene)
- Benzene undergoes electrophilic substitution rather than addition (in order to preserve the stable delocalised ring)
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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