4.1.3 - alkenes

Cards (32)

  • 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