Edexcel A Level Chemistry:复习笔记7.4.2 Reactions of Benzene

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