Reaction with oxygen

• Hydrocarbons will burn in air or oxygen to produce carbon dioxide and water providing sufficient oxygen is available
• Benzene reacts will also follow this pattern

2C6H6?(l) + 15O2?(g) → 12CO2?(g) + 6H2O (g)

• Given that a large volume of oxygen is required for this reaction, incomplete combustion could occur
  • Therefore unreacted benzene may remain
• This would lead to a smokey yellow flame as there would be insufficient oxygen available

Halogenation?

• The nature of benzene is different to other unsaturated compounds such as alkenes and halogenation via electrophilic addition is not possible
• Therefore aromatic compounds will react with halogens in the presence of a?metal halide carrier
  • iron(III) bromide
  • aluminium chloride
• The reaction of the metal halide carrier acts as catalyst and creates the electrophile, X+(where X represents a halogen atom)
• At the end of the reaction it is regenerated

AlCl3?+ Cl2?→ AlCl4-?+ Cl+

FeBr3?+ Br2?→ FeBr4-?+ Br+

• The overall equation for halogenation is

C6H6?+ X2?→ C6H5X + HX

Or with Br2?in the presence of a AlBr3

C6H6?+ Br2?→ C6H5Br?+ HBr

Bromination of benzene

• Remember that one hydrogen atom on the benzene ring has been substituted for one halogen atom, therefore HX will be a product

Nitration?

• Another example of a substitution reaction is the?nitration?of arenes
• In these reactions, a nitro (-NO2) group replaces a hydrogen atom on the arene
• The benzene is reacted with a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) at a temperature between 25 and 60?oC

Nitration of benzene

Friedel-Crafts Reactions

• Friedel-Crafts reactions are also?substitution?reactions
• Due to the aromatic stabilisation in arenes, they are often?unreactive
• To use arenes as?starting materials?for the synthesis of other organic compounds, their structure, therefore, needs to be changed to turn them into more reactive compounds
• Friedel-Crafts reactions can be used to substitute a hydrogen atom in the benzene ring for an?alkyl group?(Friedel-Crafts alkylation) or an?acyl group?(Friedel-Crafts acylation)
• Like any other electrophilic substitution reaction, the Friedel-Crafts reactions consist of three steps:
  • Generating the electrophile
  • Electrophilic attack on the benzene ring
  • Regenerating aromaticity of the benzene ring

   

Examples of Friedel-Crafts alkylation and acylation reactions