3.3.4 Alkenes

• Stereoisomerism arises in alkenes due to restricted rotation about the C=C bond
• Stereoisomers are compounds that have the same molecular and structural formula but their atoms are arranged differently in space
• Electrophile is a species which can accept a pair of electrons
• The Br-Br electrophile is not polar. However when it comes close to the electron rich C=C bond, the electrons in the Br-Br bond are repelled towards the futhest bromine atom. This gives the nearest bromine a delta plus (δ+) and the other bromine delta minus (δ-)charge
• Electrophilic Addition of H2SO4:

  The product of the mechanism can be hydrolysed with water forming an alcohol and so the sulfuric acid acts as a catalyst
• The more stable the carbocation the more likely it is formed. Therefore the most stable carbocation leads to the formation of the major product. The least stable carbocation leads to the formation of the minor product.

  Alkyl groups push electron density towards the positively charged carbon atom via the inductive effect. This stabilises the carbocation.
• Properties of polyalkenes:

  The strength of the polymer is determined by the strength of the intermolecular forces between the chains. Polyalkenes only have Van Der Waals forces between the chains.

  Since the polymer chains are often very long the VDW forces between chains are often very strong and so have relatively high melting and boiling points.

  As the chains are not rigidly held in place by each other, polymer tend to be reasonably soft.
• Properties of polyalkenes:

  Plasticisers are used to make a material like PVC softer and more flexible. Plasticisers are small molecules that can be added to polymers during their manufacture. They push the polymer molecules slightly further apart, weakening the forces between them and making the material softer and more flexible. PVC contains dipole dipole forces between the polymer chain
• Problems with Polymers:

  Biodegradable substance can be broken down by microorganisms such as bacteria or fungi.
• Problems with Polymers:

  Addition polymers are not biodegradable and so persist in the environment. This is due to not have polar bonds and containing strong c-c bond making them unreactive.
• Problems with Polymers:

  Ways to get rid of Polymers

  > Landfill

  ad: Easily available

  disad: Landfill sites are running out. Waste has to be collected then transported making it expensive

  > Incineration

  Ad: Energy released can be used for heating + electricity. reduces need for landfills.

  Disad: CO2 is to produced which contributes to global warming

  > Recycling

  Ad: Reduces amount of crude oil used to make polymers and amount of polymer waste/landfill.

  Disad: Polymer waste has to be collected, transported and sorted out first which makes it expensive.
• Electrophilic Addition - Alkenes

  Conditions: r.t.p

  Reagent: Br2 , HBr , H2SO4 (add water to form alcohol)

  Forms: Dibromoalkane, bromoalkane, akyl hydrogensulfate -> alcohol