Describe the structure and bonding of benzene and the evidence supporting this model.

This question has two main parts: structure and the evidence. First, start with the structure. It is always useful to draw a diagram of the structure of benzene to make sure not to miss anything and then start from the basics and move to the more complex concepts. Benzene consists of 6 carbons arranged in a cyclic structure that each have a hydrogen atom bonded to it. There are 6 sigma bonds between carbon atoms in benzene with 3 pi bonds that alternate. Each carbon atom has 4 electrons in bonding electron pairs. 3 of them are in sigma bonds while one contributes to a pi bond. P-orbitals from these pi bonds can overlap above and below the plane of the carbon ring to form a delocalised system above and below the plane containing now free-moving electrons from the p-orbitals. Evidence 1. The length of the bonds in the benzene ring is same for all C-C bonds and has value between standard length of single and double bonds. If it was to be as Kekule model (cyclohex-1,3,5-triene) suggests there should be alternating short and longer bonds. 2. If benzene didn't have delocalised system but would be just a cyclic alkene (Kekule model), its enthalpy change of hydrogenation would be three times the enthalpy change of hydrogenation of cyclohexene (-120 kJ/mol). However, enthalpy change of hydrogenation of benzene is -208 kJ/mol rather than -360 kJ/mol. The delocalised system gives benzene more stability. 3. Benzene's resistance to react with compounds that normally alkenes have a reaction with. For example, benzene doesn't decolourise bromine water when added to solution as a regular alkene would.