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A Level Chemistry F324 Unit 4: Arenes I

Benzene

· A Level,Chemistry,A level chemistry,Arenes,Benzene

Arene: aromatic hydrocarbon. Aromatic: ring structure

Benzene

Molecular formula: C6H6

Kekule structure: alternate single and double C bonds

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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.

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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

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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:

  1. 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.
  2. Benzene will only decolourise Br water upon heating and in the presence of a catalyst.
  3. 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 !

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Drafted by Gina (Chemistry)

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