Reactions of Benzene (DP IB Chemistry: HL)

• Arenes are very stable compounds due to the delocalisation of π electrons in the ring
  • This is because the electron density 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 so they are not favoured

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
• 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