Reactions of Arenes (CIE A Level Chemistry)

• 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
• Benzene undergoes a wide range of reactions including combustion - (complete and incomplete) and the following reactions:
  • Halogenation
  • Nitration
  • Friedel-Craft's alkylation
  • Friedel-Craft's acylation
  • Complete Oxidation
  • Hydrogenation

Halogenation

• 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 acts 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

Halogenation of benzene

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, results in the formation of two products

Halogenation of alkylarenes

Alkylarenes are substituted on the 2 or 4 position

• Multiple substitutions occur when excess halogen is used

Halogenation of alkylarenes using an excess of halogen

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

During nitration, a hydrogen atom is replaced by an NO₂ group

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

Nitration of alkylarenes

Alkylarenes are nitrated on the 2 or 4 position

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:
  1. Generating the electrophile
  2. Electrophilic attack on the benzene ring
  3. Regenerating aromaticity of the benzene ring

Examples of Friedel-Crafts alkylation and acylation reactions

During alkylation, an alkyl / R group is substituted on the benzene ring and during acylation, an acyl / RCO group is substituted on the benzene ring

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

Alkylation reactions of benzene follow the 3 steps of electrophile generation, electrophilic attack and regeneration of aromaticity

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 substitutes on the 4 position due to the -CH₃ group on the benzene

Example of a Friedel-Crafts acylation reaction

Acylation reactions of benzene follow the same 3 steps of electrophile generation, electrophilic attack and regeneration of aromaticity

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 affects 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

Oxidation of alkylarenes

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

Hydrogenation of benzene results in a loss of aromaticity

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

Hydrogenation of methylbenzene

Hydrogenation of alkylarenes also results in a loss of aromaticity

Summary of Reactions of Arenes Table