Directing Effects (OCR A Level Chemistry)

Electron donating and electron withdrawing sides groups

• Side groups on a benzene ring can affect the position on the ring of where substitution reactions will occur
• The side groups on a benzene ring can either be electron-withdrawing or electron-donating groups

• Electron-donating groups, such as -CH₃, -OH and -NH₂, 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 and/or 4 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-bromomethylbenzene and 4-bromomethylbenzene
    • Nitration of methylbenzene will also occur on the 2 and 4 position

Nitration of methylbenzene 

• Electron-withdrawing substituents, such as -NO₂, 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 position
  • 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 position
    • The product is 3-bromonitrobenzene 

• We can use the directing effect of the substitute groups when planning an organic synthesis