Alkenes

• Alkenes


  Unsaturated hydrocarbons
• Alkenes


  • Contain a carbon-carbon double bond somewhere in their structure

  
• Alkenes


  • Ethene
  • Propene
• Numbers need to be added to the name when positional isomers can occur
• C=C double covalent bond


  Consists of one sigma (σ) bond and one pi (π) bond
• π bonds


  • Are exposed and have high electron density
  • They are therefore vulnerable to attack by species which 'like' electrons: these species are called electrophiles
• Formation of π bond in alkenes

  Sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule

  
• π bond


  • Is weaker than the σ bond
• Arrangement of bonds around >C=C<

  • Planar and has the bond angle 120o
• Formation of σ bond in alkenes

  Two sp2 orbitals (one from each carbon) overlap to form a single C-C bond called a sigma σ bond
• Stereoisomers


  Have the same structural formulae but have a different spatial arrangement of atoms
• 1. Z stereoisomerism


  Alkenes can exhibit this type of isomerism due to restricted rotation about the C=C bond

  
• But-1-ene is a structural isomer of But-2-ene but does not show E-Z isomerism

  
• Naming E-Z stereoisomers


  1. Determine the priority groups on both sides of the double bond

  2. If the priority atom is on the same side of the double bond it is labelled Z

  3. If the priority atom is on the opposite side of the double bond it is labelled E
• Cis-trans isomerism


  A special case of E-Z isomerism in which two of the substituent groups are the same
• 1. 1,2-dichloroethene


  Polar molecule with polar C-Cl bonds on the same side, higher boiling point due to dipole-dipole attractions

  
• 1. 1,2-dichloroethene


  Non-polar molecule with polar C-Cl bonds on opposite sides, lower boiling point due to only London forces
• Alkenes


  • Relatively reactive because of the relatively low bond enthalpy of the π-bond
• Addition reaction of alkenes with hydrogen

  Alkene + H2 → Alkane
• Electrophilic addition reaction of alkenes with bromine/chlorine

  Alkene + Br2 → Dihaloalkane
• Reaction of hydrogen bromide with alkenes

  Alkene + HBr → Haloalkane
• Markownikoff's rule


  In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate
• Reaction of alkenes with steam to form alcohols

  Alkene + H2O → Alcohol
• Addition polymers


  Formed from alkenes
• Polymers formed from alkenes

  • Polyethene
  • Polypropene
• Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds
• The formation of polymers from ethene based monomers is a major use of alkenes
• Hydrogenation of unsaturated vegetable oils using hydrogen and a nickel catalyst is an important industrial process
• Ways to deal with waste polymers

  • Separation and recycling
  • Combustion for energy production
  • Feedstock for cracking
• Chemists have developed biodegradable, compostable, soluble and photodegradable polymers