5.4.2 Benzene Reactions

Reaction with oxygen

• Hydrocarbons will burn in air or oxygen to produce carbon dioxide and water providing sufficient oxygen is available 
• Benzene 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 a 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

Sulfonation

• Another example of a substitution reaction is the sulfonation of arenes
• In these reactions, a sulfonyl group (-SO₃H) replaces a hydrogen atom on the arene
• The benzene is warmed with a fuming sulfuric acid at a temperature of 40 ^oC for around 30 minutes
  • The fuming sulfuric acid is made by dissolving sulfur trioxide in concentrated sulfuric acid