reactivity of alkenes vs benzene

• alkenes

  • pi bond contains localised electrons (shared between 2 atoms)
  • high electron density
  • able to induce a dipole in the non-polar $Br_2$ molecule
  • slightly positive bromine atom enables $Br_2$ molecule to act as an electrophile
  • electrophilic addition
• benzene


  • pi electrons are delocalised (shared between more than 2 atoms)
  • low electron density
  • unable to induce a dipole in the non-polar $Br_2$ molecule
  • Halogen carrier catalyst required to generate electrophile
  • electrophilic substitution