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.2. Reactivity. A molecule with three C=C bonds should behave like an alkene and rapidly decolourise bromine water by electrophilic addition. Benzene does not; it resists addition and reacts by substitution instead.3. Enthalpy of hydrogenation. Adding hydrogen to benzene is much less exothermic than predicted for a molecule with three separate C=C bonds, so benzene is more stable than the Kekulé structure. This evidence is worked through in a later lesson.The Kekulé structure is still sometimes drawn, as a hexagon with three alternate double bonds, because it makes electron counting easy. However, the accepted model of benzene is the delocalised model.
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.2. Reactivity. A molecule with three C=C bonds should behave like an alkene and rapidly decolourise bromine water by electrophilic addition. Benzene does not; it resists addition and reacts by substitution instead.3. Enthalpy of hydrogenation. Adding hydrogen to benzene is much less exothermic than predicted for a molecule with three separate C=C bonds, so benzene is more stable than the Kekulé structure. This evidence is worked through in a later lesson.The Kekulé structure is still sometimes drawn, as a hexagon with three alternate double bonds, because it makes electron counting easy. However, the accepted model of benzene is the delocalised model.