7.2.1 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

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

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

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

Nitration of benzene

• 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 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

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

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

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

Hydrogenation of benzene

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

Hydrogenation of methylbenzene

Summary of Reactions of Arenes Table