Alkenes

• Alkenes


  Unsaturated hydrocarbons that contain double bonds and have the general formula CnH2n
• Examples of alkenes


  • Ethene
  • Butadiene
• Cycloalkenes


  Alkenes that have a ring structure, with two fewer hydrogen atoms than their straight-chain counterparts
• Alkenes


  • They undergo addition reactions
  • They are attacked by electrophiles due to the high electron density in the double bond
• Electrophile


  An electron pair acceptor, attracted to the double bond of an alkene
• Electrophilic addition reaction mechanism


  1. Electrophile approaches double bond

  2. Electrons from double bond attack electrophile

  3. Bond breaks to form carbocation intermediate

  4. Nucleophile attacks carbocation

  5. Product formed
• Bromine water added to an alkene


  Decolorizes, indicating presence of an alkene
• Addition of bromine to an alkene

  1. Bromine molecule polarizes near double bond

  2. Electrons from double bond attack bromine

  3. Carbocation intermediate formed

  4. Bromide ion attacks carbocation

  5. Dibromoalkane product formed
• Addition of hydrogen bromide to an alkene

  1. Hydrogen bromide polarized, with delta positive hydrogen

  2. Electrons from double bond attack delta positive hydrogen

  3. Carbocation intermediate formed

  4. Bromide ion attacks carbocation

  5. Bromoalkane product formed
• Unsymmetrical alkenes can form two different products when reacting with hydrogen halides
• Carbocation stability


  More alkyl groups attached to carbocation, the more stable it is
• Addition of hydrogen bromide to propene

  1. Hydrogen can add to form primary carbocation (minor product)

  2. Hydrogen can add to form secondary carbocation (major product)
• Addition of sulfuric acid to an alkene

  1. Sulfuric acid polarizes and interacts with double bond

  2. Carbocation intermediate forms

  3. Hydrogen sulfate ion attacks carbocation

  4. Alkyl hydrogen sulfate product formed
• Reaction of alkenes with cold concentrated sulfuric acid

  1. Alkene reacts with sulfuric acid

  2. Forms alkyl hydrogen sulfate intermediate

  3. Intermediate is hydrolysed with cold water

  4. Produces alcohol and reforms sulfuric acid catalyst
• Alkyl hydrogen sulfate


  Intermediate formed in reaction of alkene with sulfuric acid
• Hydrolysis


  Breaking a bond using water
• Sulfuric acid acts as a catalyst in the reaction of alkenes to form alcohols
• Asymmetric alkenes can produce two products in this reaction
• Alkenes


  • Useful for making plastics and polymers
  • Monomers that can be joined together in addition polymerization
• Natural polymers


  • Proteins
  • Natural rubber
• Synthetic polymers


  • Polyethylene
  • Polypropylene
• Charles Goodyear discovered vulcanized rubber by adding chemicals to natural rubber
• Synthetic polymers


  • Polyethylene
  • Nylon
  • Teflon
• Synthetic polymers have revolutionized the way we live and are increasingly used in electronic gadgets
• Making polypropylene


  1. Propene monomers join together in addition polymerization

  2. Polymer chain has repeat units with trailing bonds extending beyond brackets
• Polyalkenes


  • Nonpolar and stable, making them difficult to degrade
  • Longer chains have higher melting points and are more rigid and strong
  • Branched chains are more flexible and weaker
• Intermolecular forces


  • Van der Waals forces between nonpolar polyalkene chains
  • Dipole-dipole forces in polymers with halogens like PVC
• Plasticizers


  Additives that make polymers more flexible by weakening intermolecular forces between chains
• PVC can be made more flexible by adding plasticizers which push the polymer chains apart