OCR F324 (Chains, rings and spectroscopy) 28/06/10 Watch
This is probably the exam I'm most worried about at the moment
I attempted to start revising this morning but I only got as far as aldehydes and ketones before ending up on here lol.
We basically need to know the whole organic bit from AS as well in terms of reactions.
cgp is awesome.
Ok, I've now gone through the text book and revision guide for the first module (up to amines) so one down two to go
Hate that unit
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
Benzene + Halogen >>> Halogenobenzene + Hydrogen Halide
The mechanisms for the nitration and halogenation of benzene are attached.