Kekulé drew benzene with three alternating double bonds. The bond lengths, the enthalpy of hydrogenation and benzene's reluctance to add bromine all say otherwise: the pi electrons are spread round the whole ring.
Each carbon in benzene is sp2 hybridised: it forms three sigma bonds (two to C, one to H) in a plane at 120°, so the ring is planar. The remaining p orbital on each carbon overlaps sideways with both neighbours, forming a delocalised pi system of six electrons in rings above and below the plane. The evidence against the Kekulé structure is threefold: all six C–C bonds are 0.139 nm, between C–C (0.154 nm) and C=C (0.134 nm); the enthalpy of hydrogenation is −208 kJ mol−1, not the −360 kJ mol−1 predicted from three C=C bonds, so benzene is about 152 kJ mol−1 more stable; and benzene resists addition, preferring substitution that keeps the delocalised ring.
Toggle between the Kekulé structure and the delocalised ring in the simulator, and compare the sideways overlap in ethene with the continuous overlap round a benzene ring.
| Feature | Benzene | Reason |
|---|---|---|
| hybridisation of each C | sp2 | three bonding regions around each carbon |
| bond angle | 120° | three regions repel to a trigonal planar arrangement |
| shape | planar hexagon | all twelve atoms lie in one plane |
| sigma bonds | 12 | six C–C plus six C–H |
| pi electrons | 6 | one from the p orbital of each carbon, delocalised above and below the ring |
Three independent observations all point the same way. Learn the numbers.
| Evidence | Kekulé predicts | Observed |
|---|---|---|
| C–C bond lengths | three of 0.154 nm and three of 0.134 nm | all six 0.139 nm |
| enthalpy of hydrogenation | 3 × (−120) = −360 kJ mol−1 | −208 kJ mol−1 |
| reaction with bromine water | rapid decolourisation (addition) | no reaction; substitution needs a FeBr3 catalyst |
The circle does not mean benzene has no double bond character: every C–C bond has some. Stabilisation is 152 kJ mol−1, the gap between −360 and −208, not 208. The pi electrons are above and below the plane of the ring, not in it; the sigma framework lies in the plane.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the hybridisation of each carbon atom in benzene?
What is the C–C–C bond angle in benzene?
Where are the delocalised electrons in benzene?
How long is each carbon-carbon bond in benzene?
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
The enthalpy change of hydrogenation of cyclohexene is −120 kJ mol−1 and of benzene is −208 kJ mol−1. What is the stabilisation energy of benzene?
Which observation is NOT evidence that benzene has a delocalised structure?
Why does benzene undergo substitution rather than addition with bromine?
How many sigma bonds and how many delocalised pi electrons are there in one benzene molecule?
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