AQA A Level Chemistry

Revision Notes

7.4.3 Friedel–Crafts Acylation

Friedel–Crafts Acylation

  • 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

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