Quick Recap — Mechanisms, Directing Effects & Stability

  • Electrophilic substitution on benzene: generate the electrophile, form the arenium (sigma) complex, lose H+H^+.
  • Activating o/p directors: −OH-OH, −NH2-NH_2, −OR-OR, −CH3-CH_3 (halogens are deactivating but o/p directing). Deactivating m directors: −NO2-NO_2, −CN-CN, −COOH-COOH, −SO3H-SO_3H.
  • Alkene stability rises with substitution (hyperconjugation); carbocation in HX addition follows 3∘>2∘>1∘3^\circ>2^\circ>1^\circ.
  • Peroxide effect (free-radical) only for HBr; conformations of ethane: staggered (stable) vs eclipsed.