AQA Chem4: Aromatic Chemistry- Benzene

Revision notes on benzene and its reactions from AQA Chem4

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  • Created by: anna
  • Created on: 04-06-13 17:00

Structure of benzene

Evidence for benzenes structure:

  • the molecular formula would suggest a structure with alternate C-C and C=C bonds but  this would mean it would be a hexagon with irregular alternating long and short sides
  • the bond length in benzene is 0.139nm in between the lengths in alkanes (0.154nm) and alkenes (0.134nm)
  • benzene has 6  single covalent bonds all the same between the lengths in alkanes and alkenes
  • benzene is a regular hexagon and is planar
  • there are two rings of delocalised electrons above and below the benzene ring
  • this means it is more stable than alkanes and alkenes and it doesn't readily undergo addition reactions
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Reactions of Benzene

Benzene reacts by the electrophilic substitution mechanism.

Nitration of Benzene:

  • carried out using conc. HNO3 in the presence of conc. H2SO4 at 50C
  • the electrophile NO2+ is produced in the mixture
  • 1) H2SO4 + HNO3 --> HSO4- + H2NO3+
  • 2) H2NO3+  --> H2O + NO2+
  • Together.. 2H2SO4 + HNO3 --> 2HSO4- + H3O+  + NO2+
  • Overall the reaction is C6H6 + HNO3 --> C6H5NO2 + H2O
  • In industry nitration of methylbenzene is used in the production TNT
  • Nitrated arenes can be reduced to amines which are used in the manufacture of dyes
  • C6H5NO2 + 3H2 --> C6H5NH2 + 2H2O
  • In industry this reaction is carried out with a Ni catalyst, in the lab H2 is obtained from the reaction of Sn or Fe with HCl
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More reactions

 

The alkylation of benzene:

  • the introduction of an alkyl group (e.g. -CH3 -CH2CH3) to benzene
  • generally: C6H6 + RCl --> C6H5R + HCl
  • it is carried out in the presence of an AlCl3 catalyst
  • AlCl3 is used to generate an electrophile... R-Cl + AlCl3 --> R+  + AlCl4-
  • The electrophile R+ (a carbocation) attacks the benzene ring via the electrophilic substitution mechanism
  • Finally the catalyst is regenerated AlCl4- + H+ --> AlCl3 + HCl
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More reactions

 

Acylation of benzene:

  • The introduction of an acyl group to benzene e.g. -COCH3
  • An acyl group is derived from the corresponding acyl chloride, R-COCl
  • Generally: C6H6  + R-COCl  --> C6H5COR
  • It's done in the presence of an AlCl3 catalyst which is used to generate an electrophile
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