AQA A Level Chemistry复习笔记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
    • Hydrogenation

     

  • 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

7.2-Hydrocarbons-Halogenation-of-Alkylarenes

Alkylarenes are substituted on the 2 or 4 position

 

  • Multiple substitutions occur when excess halogen is used

 

7.2-Hydrocarbons-Multiple-Substitutions

In the presence of excess halogen, multiple substitutions occur

 

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

 

7.2-Hydrocarbons-Nitration-of-Alkylarenes-

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

     

7.2-Hydrocarbons-Alkylation-vs-Acylation

Examples of Friedel-Crafts alkylation and acylation reactions

 

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

     

7.2-Hydrocarbons-Friedel-Crafts-Acylation-1_17.2-Hydrocarbons-Friedel-Crafts-Acylation-2

Example of a Friedel-Crafts acylation reaction

 

Hydrogenation

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

 

7.2-Hydrocarbons-Hydrogenation-of-Benzene-

Hydrogenation of benzene

 

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

7-2-hydrocarbons---hydrogenation-of-methylbenzene

Hydrogenation of ethylbenzene

 

Summary of Reactions of Arenes Table

7.2-Hydrocarbons-Summary-of-reactions-of-arenes-table

 

转载自savemyexams

 

 

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