13.4 Electrophilic addition of alkenes

• Electrophilic addition reactions

  • the double bond in an alkene represents a region of high electron density because of the presence of pi electrons
  • the high electron density of the pi electrons attracts electrophiles
• Electrophile - an electron pair acceptor
• An electrophile is an atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair. It is a positive ion or a molecule containing an atom with a partial positive charge
• Curly arrow shows the movement of a pair of electrons
• Markownikoff's rule:

  • the major product forms via the most stable carbocation intermediate
• Primary carbocation:

  • the positive charge is on a carbon atom with one carbon attached
  • This is the least stable carbocation
• Secondary carbocation:

  • the positive charge is on a carbon atom with two carbons attached
  • This has the middle stability
• Tertiary carbocation:

  • the positive charge is on a carbon atom with 3 carbons attached
  • This is the most stable carbocation
• The major product is the product that is formed the most, and is made from the most stable carbocation intermediate, whereas the minor product is made less and is from the less stable carbocation intermediate