3.3.4 Alkenes

• The C=C double bond is formed when two p-orbitals overlap sideways to form a pi (π) bond

  
• Alkenes are unsaturated hydrocarbons meaning they contain a carbon-carbon double bond which is an area of high electron density, making it susceptible to electrophilic attack.
• An alkene contains 4 sigma (σ) bonds and one pi (π) bond
• Why is the bond angle around the alkene functional group not exactly 120°?

  The 4 electrons in the double bond provide slightly more repulsion than a single bond. 
• What is the shape and bond angles around the alkene functional group?

  1. Planar shape
  2. 121° and 118°

  
• The presence of the pi (π) bond above and below the single bond leads to restricted rotation around the double bond. 
• Double bond creates a region of high electron density

  • Attracts electrophiles who attack double bond
  • Electrophiles are electron pair acceptors which are attracted to areas of high electron density
  • They add across a double bond because it is unsaturated
• physical properties of alkenes

  • Only van der waals forces
  • Insoluble in water
  • Combust in a similar way to alkanes, but not usually used as fuels as more useful in other chemical reactions
• What are the Cahn-Ingold-Prelog rules for determining priority?

  The atom which has the higher atomic number is given the higher priority.
• Test for alkenes

  • Alkenes will decolourise bromine water. The colour of the bromine water changes from orange/brown to colourless.  
  • This is because the bromine molecule will add across the double bond of the alkene by electrophilic addition to form a dibromoalkane.

  Ethene + Bromine → 1,2-dibromoethane
• Why are alkenes more reactive than alkanes?

  • high electron density of double bond
  • Bond enthalpy of a double C=C bond is almost 2x of a single C-C bond because the pi (π) bond is weaker
• Describe the mechanism for electrophilic addition

  • Electrophile is attracted to C=C double bond and attacks it due to it being an area of high electron density
  • Double bond is broken and electrophile accepts a pair of electrons from double bond
  • A carbocation is formed
  • A negatively charged ion forms a bond with the carbocation
• A carbocation is an ion with a positively charged carbon atom.
• Mechanism for bromine water test

  • C=C double bond has high electron density so repels electrons in bromine molecule, Br-Br becomes polarised
  • The C=C double bond attracts the Brδ+ and forms a bond
  • This repels electrons in the Br-Br bond further, until it breaks.
  • A carbocation is formed
  • The Br- ion bonds with the carbocation and 1,2-dibromoethane is formed.
• Mechanism for addition with hydrogen halides

  • The Hδ+ is the electrophile, it is attracted to the high electron density of the double bond, which it attacks.
  • The Hδ+ accepts a pair of electrons from the double bond, forming a dative covalent bond.
  • A carbocation is formed
  • The bonding pair of electrons from the hydrogen halide moves to the halogen atom, which will form a halide ion with a lone pair. 
  • The halide ion bonds to the carbocation. 
• Markovnikov’s rule: In unsymmetrical alkenes the halogen will join the carbon with the least hydrogens, while the hydrogen will join to the carbon with the most hydrogens. This will be the major product. 
• Tertiary carbocations are the most stable, followed by secondary and then primary.

  This is because alkyl groups are electron donating, and so they stabilise the carbocation by reducing the positive charge on the carbon.
• When propene reacts with hydrogen bromide, 2-bromopropane is formed as a major product via a secondary carbocation which is more stable than the primary carbocation that 1-bromopropane formed from as the minor product.
• Alkenes react with cold concentrated sulphuric acid in electrophilic addition reactions to form alkyl hydrogen sulfates.
• When water is added to an alkyl hydrogen sulfate, an alcohol is produced and sulphuric acid is regenerated - it acts as a catalyst.
• Addition polymers are formed from alkenes where the C=C double bonds (pi bonds) are broken to join them together to form a repeating unit
• Polymers of alkenes are long chain saturated molecules. The chains are held together by Van der Waals forces so are non-polar, making addition polymers unreactive.
• The conditions for addition polymerisation is high temperature, high pressure and a catalyst.
• Poly(chloroethane) (PVC) is very rigid due to strong intermolecular force, but is made flexible by adding plasticisers.
• Plasticisers get between polymer chains and force them apart. This weakens the intermolecular forces (Van der Waals forces) between the chains.
• Addition polymers are hard to dispose of because of their non-polar C-C bonds (unreactive), so they are not broken down by enzymes - non-biodegradable.
• Addition polymers cannot be burnt as a way of disposal because toxic fumes are produced.
• Poly(chloroethane) (PVC) is typically used for drain pipes and window frames while plasticised PVC is used for electrical insulation and clothing.
• Feedstock recycling is when…

  • plastics are heated to high temperatures that will break the polymer bonds to produce monomers
  • monomers can then go through another chemical reaction to produce new polymers
• Mechanical recycling is when…

  plastics are seperated into different types, washed, ground down, melted and re-moulded.
• What is the problem with recycling?

  Repeated melting and remoulding of plastics degrades plastics over time
• Explain why there is an attraction between a C=C double bond and Br2 (2)

  • C=C is an area of high electron density
  • Br-Br becomes polarised
  • Brδ+ attracted to C=C
• Brittleness is reduced by plasticisers.