Electrophilic addition in alkenes

• The mechanism in which alkenes perform addition reactions to form saturated compounds is called electrophilic addition.
• The double bond in an alkene represents a region of high electron density because of the presence of the pi electrons. The higher electron density attracts electrophiles.
• An electrophile is an atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair. An electrophile is usually a positive ion or a molecule containing an atom with a partial positive charge.
• The mechanism for the reaction with a bromine molecule

  1. The interaction causes polarisation of the Br-Br bond forming Br δ+ and Br δ- dipoles. This is an induced dipole.
  2. The electron pair in the pi Bond is attracted to the Br δ+ end of the molecule causing the C=C bond to break
  3. A bond has now been formed between one of the carbon atoms from the C=C bond and a bromine atom
  4. The Br-Br bond breaks by heterolytic fission with the electron pair going to the Brδ- end of the molecule
  5. A bromide ion and a carbocation are formed
  6. The Br– reacts with the carbocation to form a product.
• The mechanism for the reaction with hydrogen bromide

  1. Bromine is more electronegative than hydrogen so hydrogen bromide is polar and contains the dipole Hδ+ - Brδ-
  2. The electron pair in the pi bond is attracted to the partially positive hydrogen atom causing the double bond to break
  3. A bond forms between the hydrogen atom of the H-Br molecule and a carbon atom that was part of the double bond
  4. The H-Br bond breaks by heterolytic fission with the electron pair going to the bromine atom
  5. A bromide ion and a carbocation are formed
  6. The Br– ion reacts to the carbocation to form the product.
• Carbocations of classified by the number of alkyl groups attached to the positively charged carbon atom. Tertiary carbocations (with three R groups) are the most stable, and primary carbocations are the least stable. Addition of a hydrogen halide or water to an unsymmetrical alkene forms the major product via the most stable carbocation.
• Markownikoff’s rule

  When a hydrogen halide reacts with an unsymmetrical alkene the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with a greater number of hydrogen atoms and the smaller number of carbon atoms.