CIE A Level Chemistry (9701) exams from 2022

Revision Notes

7.2.1 Reactions of Arenes

Reactions of Arenes

  • Arenes are very stable compounds due to the delocalisation of π electrons in the ring
    • This is because the negative charge is spread out over the molecule instead of being confined to a small area
  • During chemical reactions such as substitution reactions, this delocalised ring is maintained
  • Addition reactions however, disrupt the aromatic stabilisation
  • Arenes undergo a series of reactions including:
    • Substitution
    • Nitration
    • Friedel-Crafts alkylation
    • Friedel-Crafts acylation
    • Complete Oxidation
    • Hydrogenation

Substitution

  • Halogenation reactions are examples of electrophilic substitution reactions
  • Arenes undergo substitution reactions with chlorine (Cl2) and bromine (Br2) in the presence of anhydrous AlCl3 or AlBr3 catalyst respectively to form halogenoarenes (aryl halides)
    • The chlorine or bromine act as an electrophile and replaces a hydrogen atom on the benzene ring
    • The catalyst is required for the reaction to take place, due to the stability of the benzene structure

 

 

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

Arenes undergo substitution reactions with halogens to form aryl halides

  • Alkylarenes such as methylbenzene undergo halogenation on the 2 or 4 positions
  • This is due to the electron-donating alkyl groups which activate these positions
    • Phenol (C6H5OH) and phenylamine (C6H5NH2) are also activated in the 2 and 4 positions
  • The halogenation of alkylarenes therefore result in the formation of two products

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

Alkylarenes are substituted on the 2 or 4 position

  • Multiple substitutions occur when excess halogen is used

 

Hydrocarbons - Multiple Substitutions, downloadable AS & A Level Chemistry revision notes

In the presence of excess halogen, multiple substitutions occur

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

  • Again, due to the electron-donating alkyl groups in alkylarenes, nitration of methylbenzene will occur on the 2 and 4 position

 

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

Nitration of alkylarenes

Friedel-Crafts reactions

  • Friedel-Crafts reactions are also electrophilic 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

Friedel-Crafts alkylation

  • In this type of Friedel-Crafts reaction, an alkyl chain is substituted into the benzene ring
  • The benzene ring is reacted with a chloroalkane in the presence of an AlCl3 catalyst
  • An example of an alkylation reaction is the reaction of benzene with chloropropane (CH3CH2CH2Cl) to form propylbenzene

Hydrocarbons - Friedel-Crafts Alkylation (1), downloadable AS & A Level Chemistry revision notes Hydrocarbons - Friedel-Crafts Alkylation (2), downloadable AS & A Level Chemistry revision notes

Example of a Friedel-Crafts alkylation reaction

Friedel-Crafts acylation

  • In the Friedel-Crafts acylation reaction, an acyl group is substituted into the benzene ring
    • An acyl group is an alkyl group containing a carbonyl, C=O group
  • The benzene ring is reacted with an acyl chloride in the presence of an AlCl3 catalyst
  • An example of an acylation reaction is the reaction of methylbenzene with propanoyl chloride to form an acyl benzene
    • Note that the acyl group is on the 4 position due to the -CH3 group on the benzene

Hydrocarbons - Friedel-Crafts Acylation (1), downloadable AS & A Level Chemistry revision notes Hydrocarbons - Friedel-Crafts Acylation (2), downloadable AS & A Level Chemistry revision notes

Example of a Friedel-Crafts acylation reaction

Complete oxidation

  • Normally, alkanes are not oxidised by oxidising agents such as potassium manganate(VII) (KMnO4)
  • However, the presence of the benzene ring in alkyl arenes affect the properties of the alkyl side-chain
  • The alkyl side-chains in alkyl arenes are oxidised to carboxylic acids when refluxed with alkaline potassium manganate(VII) and then acidified with dilute sulfuric acid (H2SO4)
    • For example, the complete oxidation of ethylbenzene forms benzoic acid

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

The complete oxidation of alkyl side-chains in arenes gives a carboxylic acid

Hydrogenation

  • The hydrogenation of benzene is an addition reaction
  • Benzene is heated with hydrogen gas and a nickel or platinum catalyst to form cyclohexane

 

Hydrocarbons - Hydrogenation of Benzene, downloadable AS & A Level Chemistry revision notes

Hydrogenation of benzene

  • The same reaction occurs when ethylbenzene undergoes hydrogenation to form cycloethylbenzene

 

Hydrocarbons - Hydrogenation of Methylbenzene, downloadable AS & A Level Chemistry revision notes

Hydrogenation of methylbenzene

Summary of reactions of arenes table

Hydrocarbons - Summary of reactions of arenes table, downloadable AS & A Level Chemistry revision notes

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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