6.2.3 - Chemical Reactions of Alkanes

• why are alkanes hard to react?

  C-C and C-H bonds have high bond enthalpies

  bands are non-polar so alkanes are unreactive with ionic reagents in water - eg: acids, alkalis, oxidising & reducing agents
• reactions of alkanes

  combustion

  halogenation

  cracking

  involve homolytic bond breaking forming free radicals
• combustion of alkanes

  alkanes are completely oxidised to produce water & carbon dioxide - highly exothermic so that's why they are used as fuels but is the main cause of global warming & the greenhouse effect
• why is combustion of alkanes in the gas phase?

  it involves a free radical mechanism which occurs rapidly in the gas phase
• trends in flammability of hydrocarbons

  bigger hydrocarbons get harder to ignite because they don't vaporise as easily due to more London forces

  if liquid isn't very volatile then only molecules on the surface react with oxygen
• volatile

  how easily liquid turns into a gas
• trends in complete combustion of alkanes

  less complete as number of carbon atoms increases
• flame colour for complete combustion of alkanes

  blue flame
• flame colour if combustion of alkanes is incomplete

  yellow, smoky flame
• what type of reaction is halogenation?

  photochemical reaction

  free-radical substitution
• what's a photochemical reaction?

  reactions started by light

  halogenation: UV light
• free-radical substitution

  replacement of hydrogen atoms in a molecule by halogen atoms in a reaction involving free radicals
• stages in halogenation

  initiation: produces free radicals by homolytic fission of halogen molecule (energy comes from UV)

  propagation: radicals used to form products & more radicals are produced in the process creating a chain reaction

  termination: radicals react together to make a stable molecule so reaction stops - very exothermic
• why does halogenation not happen in the dark?

  molecules don't have enough energy for bonds to break when they collide but in UV light enough energy is provided to cause homolytic fission of halogens into free radicals
• problem with free radical substitution - halogenation

  a mixture of products is produced so low atom economy

  further substitution can take place so you may not get the productyou want

  substitution can happen anywhere along carbon chain so a mixture of structural isomers can be formed
• how to reduce chance of by-products in halogenation

  have excess alkane so there's Q greater chance of a radical colliding with an alkane molecule than a by product molecule