AQA Chem4: Aromatic Chemistry- 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

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

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

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