module 4.1

• heterolytic fission: one bonding atom recieves both electrons from the bonding pair
• homolytic fission: each bonding atom recieves one electron from the bonded pair, forming two radicals
• electrophilic addition: the π bond of the C=C double bond breaks, atoms or groups add across the carbon atoms

  this happens because the π bond in the C=C bond is rich in electrons, making it a target for electrophiles - electron pair acceptors that are attracted to areas with a high electron density
• common electrophiles that add to alkenes include

  • positively charged metal ions like H+
  • polar molecules, where the positive δ+ atom is attracted to the C=C electrons
  • halogens such as chlorine and bromine
• hydrogenation


  alkenes react with hydrogen gas in the presence of a nickel catalyst at 150 degrees, forming alkanes

  the double bond breaks, allowing hydrogen atoms to add across the carbon atoms, resulting in a single C-C bond
• halogenation: halogens react with alkenes in an electrophilic addition reaction to form dihaloalkanes
• halogenation


  step 1 - high electron density of the C=C π bond repels electrons in the bromine molecule, polarising the Br-Br bond

  step 2 - leads to heterolytic fission of the Br-Br bond, with one bromine atom taking the bonding pair of electrons, creating a positively charged bromine which bonds to one of the carbon atoms

  step 3 - a positively charged carbocation intermediate forms. the negatively charged bromide ion then bonds to the other carbon atom

  step 4 - the final product 1,3-dibromoethane, has a bromine atom bonded to each of the original alkene carbon atoms

  
• bromine water, an orange solution of bromine, reacts with an alkene:

  • the alkene reacts with bromine via electrophilic addition
  • this reaction removes the orange colour as the bromine is consumed
  • the product is a colourless dibromoalkane solution
• the rapid decolourisation of orange bromine water indicates the presence of carbon-carbon double bonds, offering a simple test for alkenes
• the major product in hydrogen halide addition to unsymmetrical alkenes can be predicted by carbocation stability
• primary carbocations - these have one alkyl group attached to the positively charged carbon. these are the least stable
• secondary carbocations - these have two alkyl groups at the carbocation centre, offering moderate stability
• tertiary carbocations - these have 3 alkyl groups are the most stable
• alkyl groups stabilise the positive charge through inductive effects, with more groups providing greater stability
• the major haloalkane product forms via the most stable carbocation intermediate following markownikoff's rule:

  "the major product has hydrogen adding to the carbon with the most hydrogens already attached"