September 28, 2022
Arene: aromatic hydrocarbon. Aromatic: ring structure
Benzene
Molecular formula: C6H6
Kekule structure: alternate single and double C bonds
In A Level chemistry curriculum, delocalised = Ring of delocalised electrons formed above C planes due to the overlap of p orbitals. Overlap produces a series of pi bonds spread across all 6 C.
Evidence for delocalised structure not kekule in A Level chemistry:
- C-C bond lengths in benzene are all the same intermediate length but C-C and C=C have a different length so can't be kekule (seen in x ray crystallography)
- Benzene does not decolourise Br water, should happen if C=C present
- Hydrogenation for benzene should be 3x Cyclohexene if kekule structure but in reality is is much lower than expected
Benzene's reactions - Electrophilic substitution
Nitration - conc H2SO4 catalyst at 50 degrees/ Halpgenation - halogen carrier (FeCl3/AlCl3/Fe)
Reactivity
There are some highlights of Benzene reactivity in A Level chemistry curriculum:
- Cyclohexane decolourises Br water without heat or a catalyse
- Pi bonds in C=C have a high electron density so can polarise the Br2
- Electron pair from C=C is attracted to polar Br which causes the bind to break & bond to Br.
- Benzene will only decolourise Br water upon heating and in the presence of a catalyst.
- Benzene is unable to react without these conditions due to
- delocalised pi electrons spread across all 6C rather than localised electrons in alkenes
- this gives a lower electron density than alkenes so is unable to polarise Br
- Halogen carrier is required to generate a more powerful electrophile
End of this part! Remember to finish part II !
Drafted by Gina (Chemistry)