3.3.4 - Alkenes

• Alkenes are unsaturated hydrocarbons
• General Formula For Alkenes: CnH2n
• Alkenes contain a C=C double bond in their structure
• The arrangement of bonds around C=C in alkenes is planar and has the bond angle 120o 
• Alkenes are vulnerable to attack because:

  • Pi bonds are exposed
  • High electron density
  • Electrophiles attack
• What is an electrophile? an electron pair acceptor
• Describe the reaction of hydrogen halides with Alkenes:

  • Change in functional group :alkene -> halogenoalkane
  • Reagent: HCl or HBr
  • Conditions: Room temperature
  • Mechanism: Electrophilic Addition
  • Type of reagent: Electrophile (H^D+)
• Mechanism for electrophilic addition:

  • Electrophile is attracted to C=C due to electron density
  • Electrophiles are positive and accept a pair of electrons from the double bond
  • A positive ion is formed ( carbocation ) 
  • A negatively charged ion forms a bond with the carbocation
• Markownikoff's rule: In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it
• How do addition reactions change if the alkene is unsymmetrical? It can lead to 2 isomeric products
• Order of stability for carbocations:

  tertiary secondary primary
• What is the role of H2SO4 in the reaction of sulfuric acid with alkenes? It acts as a catalyst as it is generated
• Alkenes undergo addition polymerisation
• Test for alkenes:

  • Add orange bromine water
  • Shake
  • bromine is added across the C=C bond by electrophilic addition
  • This turns the water from orange to colourless
  • A dibromoalkane is formed
• Addition polymers:

  • Made from alkenes
  • The double bond opens up and the monomers join together
  • The polyalkene is a saturated molecule
  • The main carbon chain is non-polar, making polyalkenes unreactive
• Long chain polyalkenes will be strong and rigid due to the strong intermolecular van der Waals forces.

  
  

  Short branched chains will be weaker and more flexible
• Plasticisers are used on polymers to make the polymer more flexible
• Plasticiser molecules fit in the gaps between the polymer chains, pushing them apart

  
  

  So, intermolecular forces between chains are reduced in strength, and so the polymer gets easier to bend
• What is an alkene? Unsaturated hydrocarbons with a C=C double bond
• What is the general formula of an alkene? C n H2n
• Why is there no rotation about the C=C double bond? Due to the π (pi) orbital - electron density above and below the single bond, which holds the carbon atoms in place
• Alkenes are more reactive than alkanes, due to high electron density of double bond and the fact the pi-bond is slightly easier to break
• What intermolecular forces of attraction do alkenes have? Only van der Waals due to non-polar bonds
• Alkenes are not soluble in water as they have no non-polar bonds (van der Waals’ < hydrogen bonding)
• What is an electrophile? Electron deficient atoms/ions which accept a pair of electrons (here, from the C=C double bond)
• Major products will be formed from which kinds of carbocations? Tertiary (or the most stable avaliable)
• What conditions are needed for the electrophilic addition of H2O to an alkene?

  • Acid catalyst, usually phosphoric acid
• What conditions are needed for the electrophilic addition of a hydrogen halide to an alkene? Room temperature
• What is an addition polymer? many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule
• What are monomers? What form do they usually take?

  • Molecules which combine to form a polymer
  • Usually have a C=C bond which breaks to leave a repeating pattern
• This represents the polymerisation of ethene

  
• Give 3 uses of poly(chloroethene):

  • PVC
  • Drainpipes
  • Vinyl Aprons
• What are plasticisers?: Small molecules that get between polymer chains to force them apart and allow them to slide over one another
• Why do things containing mainly C-C and C-H bonds not decompose easily? Bonds are non-polar so are not attacked by enzymes
• What is an alkene? Unsaturated hydrocarbons with a C=C double bond
• Are they more or less reactive than alkanes? Why? More reactive, due to high electron density of double bond and the fact the pi-bond is slightly easier to break
• Mechanism for the reaction of sulfuric acid with ethene.
• Give 3 uses of poly(chloroethene): PVC Drainpipes Vinyl Aprons
• Give two examples of plasticisers esters and phthalates