BRAINCAKEThe Kekulé Model
In 1865 August Kekulé proposed that benzene is a ring of six carbon atoms joined by alternating single and double bonds, with one hydrogen on each carbon. In modern terms this structure is cyclohexa-1,3,5-triene. Kekulé later suggested the double bonds might flip rapidly between two positions.
The model explained the formula C6H6 and the ring shape, but it fails on three pieces of evidence.
1. Bond lengths. X-ray diffraction shows all six C-C bonds in benzene are the same length, 0.139 nm. A Kekulé ring would have three shorter C=C bonds (0.134 nm) and three longer C-C bonds (0.154 nm), giving a distorted hexagon, not a regular one.
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Key terms in this lesson
- enthalpy of hydrogenation
- The enthalpy change when one mole of hydrogen adds to one mole of a compound containing C=C double bonds.
More in Aromatic Chemistry
- Benzene
- The Delocalised Model of Benzene
- Evidence for Delocalisation - Enthalpy of Hydrogenation
- Naming Aromatic Compounds
- Electrophilic Substitution
- Nitration of Benzene
- Mechanism of Nitration
- Uses of Nitration
All 11 lessons in Aromatic Chemistry · All AQA A-level Chemistry topics