CIE A Level Chemistry (9701) exams from 2022

Revision Notes

7.3.1 Production of Halogenoarenes

Production of Halogenoarenes

  • Halogenoarenes are arenes which are bonded to halogen atoms
  • They can be prepared from substitution reactions of arenes with chlorine or bromine in the presence of an anhydrous catalyst

Substitution of benzene to form halogenoarenes

  • Chlorine gas is bubbled into benzene at room temperature and in the presence of an anhydrous AlCl3 catalyst to form chlorobenzene
  • The AlCl3 catalyst is also called a halogen carrier and is required to generate the electrophile (Cl+)
  • This electrophile attacks the electron-rich benzene ring in the first stage of the reaction which disrupts the delocalised π system in the ring
  • To restore the aromatic stabilization, a hydrogen atom is removed in the second stage of the electrophilic substitution reaction to form chlorobenzene
  • When this happens, the delocalised π system of the ring is restored
  • The same reaction occurs with benzene and bromine in the presence of an AlBr3 catalyst to form bromobenzene

Halogen Compounds - Formation of Chlorobenzene, downloadable AS & A Level Chemistry revision notes

Halogenoarenes can be formed from the electrophilic substitution reaction of arenes with halogens

Substitution of methylbenzene to form halogenoarenes

  • The electrophilic substitution of methylbenzene with halogens results in the formation of multiple halogenoarenes as products
  • This is because the methyl group (which is an alkyl group) in methylbenzene is electron-donating and pushes electron density into the benzene ring
  • This makes the benzene ring more reactive towards electrophilic substitution reactions
  • The methly group is said to be 2,3-directing and as a result, the 2 and 4 positions are activated
  • Electrophilic substitution of methylbenzene with chlorine and anhydrous AlCl3 catalyst, therefore, gives 2-chloromethylbenzene and 4-chloromethylbenzene
  • The reaction mechanism is the same as the substitution mechanism of benzene

Halogen Compounds - Formation of 2- and 4-Chloromethylbenzene, downloadable AS & A Level Chemistry revision notes

The methyl group on methylbenzene directs the incoming halogen on the 2 and 4 position

 

  • In the presence of excess chlorine, substitution on the 6 position will also occur

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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