• 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

Alkylarenes are substituted on the 2 or 4 position

• Multiple substitutions?occur when?excess?halogen is used

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

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

   

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

   

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

Hydrogenation of benzene

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

Hydrogenation of ethylbenzene

Summary of Reactions of Arenes Table