Alkanes (properties and reactions)

Cards (24)

  • Boiling point of alkanes

    Depends on size and shape
  • Smallest alkanes

    • Gases at room temperature and pressure, have very low boiling points
  • Larger alkanes
    • Liquids, have higher boiling points
  • Alkane bonding

    1. Covalent bonds inside molecules
    2. Induced dipole-dipole interactions (London forces) between molecules
  • Longer carbon chain

    Stronger induced dipole-dipole interactions
  • Longer molecules
    More energy required to overcome induced dipole-dipole interactions, boiling point rises
  • Branched-chain alkane

    Lower boiling point than straight-chain isomer
  • Alkanes burn completely in oxygen
  • Close packing of branched-chain alkanes

    Not possible, reduced surface contact between molecules, reduced induced dipole-dipole interactions
  • Combustion of alkanes
    Alkanes + oxygen → carbon dioxide + water
  • Smaller alkanes
    Turn into gases more easily, burn more easily
  • Larger alkanes
    Release more energy per mole when burned
  • Alkanes make excellent fuels because they release so much energy when burned
  • Burning alkanes in limited oxygen produces carbon monoxide and water
  • Carbon monoxide is poisonous, binds to haemoglobin better than oxygen
  • Lack of oxygen from carbon monoxide can lead to oxygen deprivation and be fatal
  • Complete combustion of ethane
    C2H6 + 3.5O2 → 2CO2 + 3H2O
  • Homolytic fission

    Bond breaks evenly, each bonding atom receives one electron, forms two radicals
  • Heterolytic fission

    Bond breaks unevenly, one atom receives both electrons, forms a cation and an anion
  • Radicals are very reactive due to unpaired electron
  • Photochemical reaction of alkanes with halogens
    Hydrogen atom substituted by halogen (chlorine or bromine)
  • Mechanism of halogen-alkane photochemical reaction

    1. Initiation (free radicals produced)
    2. Propagation (free radicals used up and created in chain reaction)
    3. Termination (free radicals joined to form stable molecules)
  • Excess methane
    Reduces chance of by-products forming
  • Free radical substitution can occur at any point along carbon chain, producing mixture of isomers