I've started my revision. Just thought I'd share what I've learnt so far on Benzene:
Problems with the Kekule structure of benzene:
1) Low reactivity - C=C present so should react in similar way to alkenes
- C=C should decolourise Bromine water.
But this does not happen, does not undergo electrophilic addition, undergoes electrophilic substitution.
2) Bond length - C-C and C=C have different bond lengths.
But this is not the case, according to xray studies all C-C bonds are the same, equal length.
3) Hydrogenation - expected enthalpy change of hydrogenation of benzene is -360kj mol-1, but the actual enthalpy change of hydrogenation of benzene is -208kj mol-1
>>> Therefore the actual structure of benzene is more stable than kekule's.
Delocalised model of benzene
- Trigonal Planar 120 degrees
- p-orbitals overlap around the whole of the ring forming a system of pie bonds.
- electron spread all the whole of the ring >>> delocalised >>> more stable.
- All C-C bonds are identical.
Nitration of benzene:
Uses: Dyes and pharmaceuticals.
Conditions: Conc. H2SO4, conc. HNO3, 50 degrees C
C6H6 + HNO3 >>> C6H5NO2 + H20
Halogenation of benzene
- Only reacts in the presence of a halogen carrier - AlCl3, FeCl3