Benzene Reactions (Edexcel International A Level Chemistry)

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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 + Br→ FeBr4- + Br+

  • The overall equation for halogenation is

C6H+ X2 → C6H5X + HX

Or with Br2 in the presence of a AlBr3

C6H+ Br2 → C6H5Br + HBr

Hydrocarbons - Overall Halogenation, downloadable AS & A Level Chemistry revision notes

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

Hydrocarbons - Nitration of Arenes, downloadable AS & A Level Chemistry revision notes

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

Hydrocarbons - Alkylation vs Acylation, downloadable AS & A Level Chemistry revision notes

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

benzene-sulfonation-equation

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Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.