ch5

• Alkenes


  Unsaturated hydrocarbons
• Alkenes


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

  
• Alkenes


  • Ethene
  • Propene
  • But-1-ene
  • But-2-ene
• 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
• The π bond is formed by sideways overlap of two p orbitals on each carbon atom forming a π-bond above and below the plane of molecule
• The π bond is weaker than the σ bond
• Formation of σ bond

  One sp2 orbital from each carbon overlap to form a single C-C bond called a sigma σ bond
• Formation of π bond


  There is Restricted rotation about a pi 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
• 1. Z isomers


  • E-but-2-ene
  • Z-but-2-ene
• Naming E-Z stereoisomers


  • Priority group: The atom with the bigger atomic number is classed as the priority atom
  • If the priority atom is on the same side of the double bond it is labelled Z
  • If the priority atom is on the opposite side of the double bond it is labelled E
• Cis-trans isomerism is a special case of E/Z isomerism in which two of the substituent groups are the same
• Skeletal formulae can also represent E-Z isomerism
• Addition reaction


  A reaction where two molecules react together to produce one
• Electrophilic addition


  Reactions of alkenes where the π bonds are more accessible to electrophilic attack by electrophiles
• Reaction of alkenes with hydrogen


  1. Reagent: hydrogen

  2. Conditions: Nickel Catalyst

  3. Type of reaction: Addition/Reduction
• Reaction of alkenes with bromine/chlorine


  1. Reagent: Bromine (dissolved in organic solvent)

  2. Conditions: Room temperature (not in UV light)

  3. Mechanism: Electrophilic Addition

  4. Type of reagent: Electrophile, Br+

  5. Type of Bond Fission: Heterolytic
• Reaction of hydrogen bromide with alkenes


  1. Reagent: HCl or HBr

  2. Conditions: Room temperature

  3. Mechanism: Electrophilic Addition

  4. Type of reagent: Electrophile, H+

  5. Type of Bond Fission: Heterolytic
• If the alkene is unsymmetrical, addition of hydrogen bromide can lead to two isomeric products
• The major product in electrophilic addition to alkenes is formed via the more stable carbocation intermediate
• Reaction of potassium manganate(VII) with alkenes


  1. Reagent: KMnO4 in an acidified solution

  2. Conditions: Room temperature

  3. Type of reaction: Oxidation

  4. Observation: purple colour of MnO4- ion will decolourise to colourless
• Reaction of bromine water with alkenes


  1. Reagent: Bromine dissolved in water

  2. Conditions: Room temperature

  3. Type of reaction: Addition

  4. Observation: Orange colour of bromine water will decolourise to colourless
• Hydration of alkenes to form alcohols


  1. Reagent: Water

  2. Conditions: High temperature 300 to 600°C, High pressure 70 atm, Catalyst of concentrated H3PO4
• Addition polymers


  Formed from alkenes
• Addition polymers


  • Poly(ethene)
  • Poly(propene)
• Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds
• Addition polymerisation


  The process where 'n' monomers become 'n' repeating units in the polymer
• You should be able to draw the polymer repeating unit for any alkene
• Methods of disposal of waste polymers


  • Incineration
  • Recycling
  • Feedstock for cracking
• Chemists have designed ways to remove toxic waste products like HCl before they are emitted into the atmosphere
• Chemists have also develop biodegradable and compostable polymers