alkenes -> haloalkanes

• Major products will be formed from which types of carbocations?

  Tertiary, or the most stable available.
• What are the conditions required for the electrophilic addition of of a hydrogen halide to an alkene?

  Hydrogen halide gases must be at room temperature.
• Electrophilic addition of hydrogen halides?

  • A hydrogen halide molecule is polar as the hydrogen and halogen atoms have different electronegativities
  • For example, in a molecule of hydrogen bromide, HBr, the bromine atom has a stronger pull on the electrons in the H-Br bond
  • As a result of this, the Br atom has a partial negative and the H atom a partial positive charge
• • In electrophilic addition reactions with hydrogen halides, the H atom acts as an electrophile by accepting a pair of electrons from the C=C bond in the alkene
  • The H-Br bond breaks heterolytically, forming a Br- ion
  • This results in the formation of a highly reactive carbocation intermediate which reacts with the bromide ion, Br-
• What is Markownikoff's rule?

  • Markownikoff’s rule predicts the outcome of electrophilic addition reactions and states that:
  • In an electrophilic addition reaction of a hydrogen halide (HX) to an alkene, the halogen ends up bonded to the most substituted carbon atom
  • In an electrophilic addition reaction of an interhalogen to an alkene, the most electronegative halogen ends up bonded to the most substituted carbon atom
• • Markownikoff addition applies to electrophilic addition reactions with unsymmetrical alkanes, e.g. propene and but-1-ene
  • Markownikoff addition favours the formation of the major product
  • Anti-Markownikoff addition favours the formation of the minor product