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

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

7.2-Hydrocarbons-Overall-Halogenation

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

7.2-Hydrocarbons-Nitration-of-Arenes

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

     

7.2-Hydrocarbons-Alkylation-vs-Acylation

Examples of Friedel-Crafts alkylation and acylation reactions

 

 

转载自savemyexams

更多Alevel课程
翰林国际教育资讯二维码