Electrophilic addition in alkenes

• a reaction mechanism is a series of steps that shows how a reaction takes place.
• alkenes usually take part in addition reactions to form saturated compounds - electrophilic addition
• • the double bond in an alkene represents a region of high electron density bc of the presence of the Pi-electrons
  • the high electron density of the Pi-electrons attracts electrophiles
• • an electrophile is an atom or group of atoms that is attracted to an electron-rich centre and accepts an electron pair.
  • usually a positive ion or a molecule containing an atom with a partial positive (delta+) charge
• The reaction between but-2-ene and HBr:

  1. bromine is more electronegative than hydrogen so HBr is polar and contains a dipole
  2. the electron pair in the Pi-bond is attracted to the partially positive H 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 - electron pair going to the bromine atom
  5. bromide ion (Br-) and carbocation are formed
  6. Br- ion reacts with carbocation to form addition product
• hydrogen bromide adds to but-2-ene to form a single addition product
• A carbocation contains a positively charged carbon atom
• The reaction between propene and bromine:

  • HBr is a polar molecule, so it is easy to see how it acts as an electrophile in its reactions with the alkenes
  • however, alkenes can also react with non-polar molecules, such as Br2 by electrophilic addition
  • bromine adds to propene to form a single addition product
• bromine is a non-polar molecule. when bromine approaches an alkene, the Pi-electrons interact with the electrons in the Br-Br bond
• 1. this interaction causes polarisation of the Br-Br bond, with one end of the molecule becoming Br delta+ and the other end becoming Br delta- - an induced dipole
  2. the electron pair in the Pi-bond is attracted to the Br delta+ end of the molecule, causing the double bond to break
  3. a bond has now been formed between one of the carbon atoms from the double bond and a bromine atom
  4. the Br-Br bond breaks by heterolytic fission, with the electron pair going to the Br delta- end of the molecule
  5. a Br- (bromide) and carbocation form
  6. Br- ion reacts with the carbocation to form the addition product
• Markownikoff's rule:

  • 2 isomeric products are formed when unsymmetrical alkenes react with a hydrogen halide
  • Markownikoff stated that 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 the greater number of hydrogen atoms and the smaller number of carbon atoms
• • electrophilic addition occurs in 2 steps:
  • in the first step a carbocation is formed
  • in the reaction between propene and an hydrogen halide 2 carbocations are possible - primary and secondary
  • in the primary carbocation the positive charge is on a carbon atom at the end of a chain
  • in the secondary carbocation the positive charge is on a carbon atom with 2 carbon chains attached
• • when propene reacts with HBr, the major product is 2-bromopropane formed from the secondary carbocation
  • the yield of 1-bromopropane is much smaller
• Carbocation stability:

  • classified by the number of alkyl groups attached to the positively charged carbon atom
  • an alkyl group normally represented by the symbol -R
  • tertiary carbocations (3 R groups) = most stable
  • primary = least stable
• • stability is linked to the electron-donating ability of alkyl groups
  • each alkyl group donated and pushes electrons towards the positive charge of the carbocation
  • the positive charge is spread over the alkyl groups
  • the more alkyl groups attached to the positively- charged carbon atom - the more the charge is spread out - making the ion more stable
  • therefore tertiary are more stable
• addition of a hydrogen halide to an unsymmetrical alkene forms the major product via the most stable carbocation
• stability increases:

  • primary
  • secondary
  • tertiary