In short: Benzene is C₆H₆, a planar ring of six carbon atoms with 120° bond angles and a delocalised π system above and below the ring. All the C–C bonds are the same length, between a single and a double bond, which is the main evidence against Kekulé's structure.
Benzene is a favourite for short exam questions. Each point is simple, but you need all of them, and you need to be able to give the evidence.
The structure of benzene
- Molecular formula: C₆H₆.
- Shape: a flat (planar) hexagonal ring of six carbon atoms, each bonded to one hydrogen atom.
- Bond angle: 120° at every carbon.
- σ bonds: 12 in total: 6 C–C and 6 C–H.
Each carbon has one electron left over, in a p orbital pointing above and below the plane of the ring. These six p orbitals overlap sideways to form a ring of electron density above and below the ring. This is the delocalised π system: the six electrons are not fixed between particular pairs of carbon atoms but are spread over the whole ring.
Because of the delocalised π system, all the C–C bonds are the same length. They are shorter than a C–C single bond but longer than a C=C double bond.
Kekulé's structure and the evidence against it
Kekulé proposed a ring with alternating single and double bonds. The evidence shows that benzene does not behave like that.
- Bond lengths. If Kekulé were right, there would be three longer C–C bonds and three shorter C=C bonds. Measurements show all six bonds are the same length, in between.
- Enthalpy of hydrogenation. Hydrogenating benzene is less exothermic than the value predicted for three separate C=C bonds. That means benzene is lower in energy than Kekulé's structure, so it has extra stability from the delocalisation.
- Reactions. Alkenes readily undergo addition and decolourise bromine water. Benzene does not decolourise bromine water at room temperature. It mainly undergoes electrophilic substitution, which keeps the stable delocalised ring intact.
Rich's tip: When asked for evidence, name the measurement and say what Kekulé would predict. "The bond lengths are all equal, but Kekulé's structure predicts two different lengths."
Common mistakes
- Saying benzene has "three double bonds that move around". It has a delocalised ring, not alternating bonds.
- Writing "the electrons are delocalised" without saying they are in a π system formed by overlap of p orbitals.
- Comparing the enthalpy of hydrogenation but not saying which way. It is less exothermic than predicted, so benzene is more stable.
Five quick questions
Answer these on paper first, then check below. They start simple and build up, in the order a topic is usually taught.
- What is the molecular formula of benzene?
- What is the bond angle between the carbon atoms in the ring?
- How many σ bonds are there in a molecule of benzene?
- Describe the delocalised π system in benzene.
- Give two pieces of evidence against the Kekulé structure.
Answers
- C₆H₆.
- 120°.
- 12 σ bonds: 6 C–C and 6 C–H.
- Each carbon contributes one electron in a p orbital, above and below the plane of the ring. The p orbitals overlap sideways to form a ring of electron density above and below the ring, in which the six electrons are delocalised.
- Any two of: all the C–C bond lengths are equal (Kekulé predicts alternating longer and shorter bonds); the enthalpy of hydrogenation is less exothermic than predicted for three C=C bonds, showing extra stability; benzene does not readily undergo addition and does not decolourise bromine water at room temperature, but undergoes electrophilic substitution.
Mark yourself honestly: tick what you got, cross what you missed, and go back over the crosses. Five quick questions at the end of a topic is a really easy way to consolidate it. For more on checking against a mark scheme, see Learn from a mark scheme.
Dexter, the A Level Chemistry tutor I built, can write quick questions like these at the end of a topic, with a mark scheme to check against.