4.1.3 - alkenes

• Alkenes are unsaturated hydrocarbons that contain at least one C=C bond made up of a sigma bond and a pi bond.
• The general formula fro alkenes is CnH2n
• A pi bond is formed when electrons in the adjacent p orbital directly overlap sideways, both above and below the carbon atoms.
• The pi bond restricts rotation of carbon atoms
• The angle and shape of a double bond is

  Trigonal planar with a bond angle of 120 degrees
• Alkenes are more reactive than alkanes because of the high electron density of the double bond and the pi bond is slightly easier to break
• Alkenes only have london forces due to non-polar bonds
• Alkenes are insoluble in water due to them being non-polar and water being polar
• The equation for the complete combustion of pent-2-Ene is

  C5H10 + 7 1/2 O2 ———> 5CO2 + 5H2O
• Alkenes form two types of isomers

  e/z isomers - due to restricted rotation

  cis-trans isomers - if two of the same Substituents are attached to each carbon
• Primary carbocations are the least stable and the carbon is attached to one carbon chain.
• Secondary carbocations are when the carbon is attached to two carbon chains
• Tertiary carbocations are the most stable and the carbon is attached to three carbon chains
• According to markownikoffs law:

  When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen attaches to the carbon on the alkene with the most hydrogens and the least carbon chains.
• The major product is formed from the most stable carbocation (usually a tertiary carbocation)
• The minor product is formed from the least stable carbocation
• the conditions needed for the electrophillic addition of H2O to an alkene is:

  steam in the presence of an acid catalyst, usually phosphoric acid

  reaction is called hydration
• The products of a hydration reaction are an alcohol
• This is the mechanism for the addition of water to ethene

  
• This is the mechanism for HBr and ethene

  
• When a halogen is added to an alkene the reaction is called halogenation
• C=C double bonds interact with non-polar molecules as their high electron density induces a temporary dipole in the halogen molecule making it §+ and the other molecules §- so the §+ molecule is attracted to the double bond
• The reaction mechanism for the reaction of ethene with bromine is

  
• An alkene is converted into an alkane in a reaction called hydrogenation.

  hydrogen gas is added to make it a saturated hydrocarbon.

  The conditions are temperatures above 150 degrees and a nickel catalyst
• Addition polymers are many monomers bonded together via rearrangement of bonds without the loss of any atom or molecule.
• Monomers are molecules which combine to form a polymer.

  They usually have a C=C bond which breaks to leave a repeating pattern
• Polymerisation of ethene is represented by this diagram

  
• Plastics can be disposed by

  landfill

  combustion

  electricity generation

  reuse

  recycle

  organic feedstock
• The disadvantages of recycling are

  plastics must be sorted into different types

  it can be expensive

  it is labour intensive

  required high technology
• Photodegradable polymers break down chemically using energy with wavelengths similar to light.

  Once the break down begins it is not possible to stop the process
• In organic feedstock:

  plastics are separated and broken down into small organic molecules through a series of reactions. The molecules can then be used to produce plastics and in other industries.
• A disadvantage of photodegradable polymers is that they may not be exposed to sufficient light