IB DP Chemistry: SL复习笔记10.2.13 Reactions of Benzene

Reactions of Benzene

  • 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

7.2-Hydrocarbons-Overall-Halogenation

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

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

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

7.2-Hydrocarbons-Nitration-of-Arenes

Nitration of benzene

  • 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

 

 

 

 

 

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