CIE A Level Chemistry复习笔记7.2.4 Directing Effects of Substituents on Arenes

• Arenes readily undergo electrophilic substitution of one of their hydrogen atoms with another species
• Substituents that are already present on the arenes can affect where the substitution of the hydrogen atom on the arene takes place
  • These groups are said to direct substitution reactions to different ring positions

Electron-withdrawing & -donating groups

• The substituents on the arenes can either be electron-withdrawing or electron-donating groups
• Electron-withdrawing substituents remove electron density from the π system in the benzene ring making it less reactive
  • These groups deactivate attack by electrophiles and direct the incoming electrophile to attack the 3 and/or 5 positions
  • For example, the nitro group in nitrobenzene is an electron-withdrawing group
    • Upon bromination of nitrobenzene, the bromine electrophile will be directed to the 3 and/or 5 position
    • The products are 3-chloronitrobenzene and 5-chloronitrobenzene
  • Electron-donating substituents donate electron density into the π system of the benzene ring making it more reactive
    • These groups activate attack by electrophiles and direct the incoming electrophile to attack the 2, 4 and/or 6 positions
    • For example, the methyl group in methylbenzene is an electron-donating group
      • Upon bromination of methylbenzene, the bromine electrophile will be directed to the 2 and/or 4 position
      • The products are 2-chloromethylbenzene and 4-chloromethylbenzene

Electron-withdrawing & -donating substituents table